WO2012021681A2

WO2012021681A2 - Intrinsically safe thermal conductivity detector for process gas chromatograph

Info

Publication number: WO2012021681A2
Application number: PCT/US2011/047380
Authority: WO
Inventors: Glen Eugene Schmidt; Leroy David Cordill
Original assignee: Siemens Industry, Inc.
Priority date: 2010-08-11
Filing date: 2011-08-11
Publication date: 2012-02-16
Also published as: WO2012021681A3

Abstract

An apparatus and method for providing an intrinsically safe (IS) thermistor detector circuit for process gas chromatography is disclosed. The IS thermistor detector circuit includes a thermistor, a reference resistor connected to the thermistor, an operational amplifier configured to drive a voltage through the reference resistor onto the thermistor to maintain the thermistor at a reference resistance, and an infallible resistance element connected between the thermistor and a high impedance input of the operational amplifier to limit current entering the thermistor from the amplifier. The IS thermistor detector circuit can be isolated from a power supply using a zener diode barrier that limits the maximum voltage from the power supply onto the IS thermistor detector circuit. An apparatus and method for providing a precision voltage regulator is also disclosed. The precision voltage regulator includes a monolithic voltage regulator device and monolithic voltage reference connected in a feedback circuit of the monolithic voltage regulator device

Description

INTRINSICALLY SAFE THERMAL CONDUCTIVITY DETECTOR FOR PROCESS GAS CHROMATOGRAPH

[0001] This application claims the benefit of U.S. Provisional Application No. 61 /372,634, filed August 1 1 , 2010, the disclosure of which is herein

incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to process gas chromatographs used in hazardous environments, and more particularly to an apparatus and method for providing an intrinsically safe thermal conductivity detector for a process gas chromatograph.

[0003] Gas chromatography is a commonly used process for separating and analyzing gaseous compounds. In the design of conventional process gas chromatographs, various techniques are used to meet the hazardous area requirements, which require that an instrument no contribute to the initiation of an explosion in a flammable gas surrounding the instrument, even in the event of a failure within the instrument. In particular, thermal conductivity detectors used in such process gas chromatographs are typically protected by a flameproof enclosure. This typically involves the use of heavy-walled metal enclosures, with covers having many screw threads or wide flange widths, flameproof passages for all gasses in and out of the detector, and a flameproof electrical seal for the signal wire passage. The problem is further complicated by the common requirement that chromatography detectors reside within a high temperature oven. Thus, the flameproof electrical seal must not only be explosion proof, but also must maintain the explosion proof features and be reliable for many years of service at very high temperatures.

[0004] FIG. 1 illustrates a conventional explosion proof detector for gas chromatography. As illustrated in FIG. 1 , the detector 100 contains an all stainless steel body 102 and cover 104. This is an expensive machined casting that must be robust enough to withstand an internal explosion. The detector also contains an electrical feed through 106, which is a complex assembly for bringing the electrical signals through an epoxy seal that is designed, rated, and evaluated to meet an explosion proof rating. Tubing 108 brings the carrier and measurement gas samples to the detector 100. These tubes 108 must be constructed and connected carefully and be of a minimum length and diameter to prevent flame propagation.

[0005] As new instruments are designed, there exists a market desire to have the instruments smaller, and to be built at lower cost. The use of the flameproof enclosure mentioned above precludes this. Therefore, there exists a need to utilize alternate approaches for the design of the detector while still meeting the hazardous area requirements of the instrument.

BRIEF SUMMARY OF THE INVENTION

[0006] The present invention relates to providing an Intrinsically Safe (IS) detector for a process gas chromatograph. Embodiments of the present invention provide an IS detector that minimizes or eliminates any impact to performance of gas chromatography with respect to noise, wander, and drift. Further,

embodiments of the present invention provide a process gas chromatograph detector that does not require a bulky and expensive explosion proof enclosure.

[0007] In one embodiment of the present invention, an IS thermistor detector circuit includes a thermistor, a reference resistor connected to the thermistor, an operational amplifier configured to drive a voltage through the reference resistor onto the thermistor to maintain the thermistor at a reference resistance, and an infallible resistance element connected between the thermistor and a high impedance input of the operational amplifier to limit current entering the thermistor from the amplifier. The infallible resistance element may be

implemented as a low noise metal film MELF package part. The IS thermistor detector circuit can be isolated from a power supply using a zener diode barrier that limits the maximum voltage from the power supply onto the IS thermistor detector circuit.

[0008] In another embodiment of the present invention, an apparatus for providing an intrinsically safe thermistor detector circuit includes a thermistor detector circuit, means for limiting a maximum voltage on the thermistor detector circuit, and means for limiting a maximum current across the thermistor detector circuit.

[0009] In another embodiment of the present invention, a zener diode barrier is provided to limit a maximum voltage on a thermistor. An infallible resistance element is provided to limit current that can enter the thermistor due to failure of an integrated circuit component. Gas eluting from a mixture of a gaseous compound and a reference gas is passed over the thermistor. Voltage is driven onto the thermistor to maintain the thermistor at a reference resistance, and a detector signal that is a measure of the voltage applied to the thermistor is compared to a reference detector signal that is a measure of voltage applied to a second thermistor over which the reference gas is passed.

[0010] Other embodiments of the present invention provide a precision voltage regulator device. In one embodiment, the precision voltage regulator device includes a monolithic voltage regulator device and a monolithic voltage reference connected in a feedback circuit of the monolithic voltage regulator device. The monolithic voltage regulator device is configured to generate a regulated output voltage based on a reference voltage added to the feedback circuit of the monolithic voltage regulator device by the monolithic voltage reference.

[0011] These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 illustrates a conventional explosion proof detector for gas chromatography;

[0013] FIG. 2 illustrates a traditional thermistor detector circuit;

[0014] FIG. 3 illustrates exemplary thermistor detector bridge circuitry;

[0015] FIG. 4 illustrates an IS thermistor detector for process gas chromatography according to an embodiment of the present invention;

[0016] FIG. 5 illustrates the voltage limiter circuit according to an embodiment of the present invention;

[0017] FIG. 6 illustrates the power system of the circuit of FIG. 4; [0018] FIG. 7 illustrates a simplified fault diagram of the IS circuit of FIG. 4 simplified down to protective elements;

[0019] FIG. 8 illustrates the equivalent circuit for the detector circuit of FIG. 4 for evaluating whether the detector circuit meets IS standards;

[0020] FIG. 9 illustrates a method of operating an IS thermistor detection circuit for gas chromatography according to an embodiment of the present invention

[0021 ] FIG. 10 illustrates voltage constraints for an exemplary operational amplifier IC;

[0022] FIG. 1 1 illustrates an exemplary zener diode shunt barrier;

[0023] FIG. 12 illustrates a zener diode barrier according to an

embodiment of the present invention;

[0024] FIG. 13 shows a data sheet for an LP2951 monolithic voltage regulator;

[0025] FIG. 14 shows a more detailed look at the internal workings of the LP2951 monolithic voltage regulator;

[0026] FIG. 15 illustrates a precision voltage regulator according to an embodiment of the present invention;

[0027] FIG. 16 is a block diagram illustrating the implementation of a precision voltage regulator according to an embodiment of the present invention;

[0028] FIG. 17 illustrates a precision voltage regulator according to another embodiment of the present invention;

[0029] FIG. 18 illustrates two examples for an 18.84V precision regulator circuit according to another embodiment of the present invention;

[0030] FIG. 19 illustrates two possible implementations for regulating an intermediate voltage output according to an embodiment of the present invention;

[0031 ] FIG. 20 illustrates a precision voltage regulator implemented using a monolithic switching voltage regulator according to an embodiment of the present invention; and

[0032] FIG. 21 illustrates a method for precision voltage regulation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present invention relates to providing an Intrinsically Safe (IS) detector for a process gas chromatograph. Intrinsic safety is a protection concept deployed in sensitive and potentially explosive atmospheres. Intrinsic safety relies on equipment designed so that it is unable to release sufficient energy, by either thermal or electrical means, to cause an ignition of a flammable gas. Intrinsic safety can be achieved by limiting the amount of power available to the electrical equipment in a hazardous area to a level below that which will ignite the gases. In order to have a fire or explosion, fuel, oxygen and a source of ignition must be present. An Intrinsically Safe (IS) system that operates in an atmosphere where fuel and oxygen are present is designed such that the electrical energy or thermal energy of a particular instrument loop can never be great enough to cause ignition. There are various IS standards set forth by various certifying agencies for a system to be considered IS. Such standards include International Electrical Commission (IEC) IEC 60079-1 1 , Factory Mutual (FM) 3610, Underwriters Laboratories (UL) UL913, Canadian Standards Association CAN/CSA-C22.2 No. 157-92, etc. While any or all of these standards apply with great similarity, the following sections will only quote the IEC standard, as it is the most harmonized of the standards. This does not preclude the use of the other listed standards. Wherever the term "the standard" or "IS standard" is mentioned, this collectively refers to one or all of the applicable IS standards.

[0034] Performance of thermal conductivity detectors dominates the competitive challenge and defines the technology leadership in the industry between suppliers of process gas chromatographs. Introduction of an Intrinsically Safe thermal conductivity detector for process chromatography even at a reduced performance level is a significant innovative challenge, even for those skilled in the art of chromatography detectors. To reach a level of little or no compromise to signal quality is clearly above and beyond the normal level of development and innovation. Embodiments of the present invention provide an implementation that meets IS standards and removes all of the explosion proof constraints, with no compromise in performance, operation or application over the existing benchmark in the industry. [0035] A chromatographic result, while a chemical measurement, is quite often presented in volts over time, much akin to a time domain spectra. The information is contained within chromatographic voltage "peaks" over time, the areas of which define concentrations of components that are being measured. With measurements that commonly reach part per million (ppm) and even part per billion (ppb) levels of some components, while at the same time, reading percentage levels of other components, it can be seen that a large dynamic range and a large signal to noise ratio (SNR) are mutually required. Thus, short-term noise and wander, or the lack thereof, is an important factor. A very small chromatographic peak may have a signal that is equal to the noise or 0 SNR (db).

[0036] In terms of dynamic range, and SNR, it is common to present a dynamic range of 10V with an additional offset or "baseline zero" of 10 V, for a full 20V of range. This full scale is then equated to 24 bits of A/D converter resolution. In the same application, a standard for a non-IS thermistor detector is to operate down to nominally 5 micro-volts of peak to peak noise (electronic contribution only). This leads to an SNR of 126 dB, or 21 bits of actual, usable resolution.

[0037] It is common for a chromatographic cycle to last from one or more minutes to as much as several hours. Thus, the long-term drift of the system is also quite important. Since a cycle of any duration can extend throughout changes in the environment, such as temperature, minimal temperature effects on the electronic measurement become imperative. With all of these factors, it is consistent that even though a chromatogram is a chemical measurement, the qualitative electrical performance of a chromatography detector/electronics is often measured in terms of all three of the following criteria: (1 ) peak to peak voltage noise over a duration of a few seconds (short term noise); (2) peak to peak and/or baseline change over many seconds without short term noise (wander); and (3) baseline change over several minutes (drift).

[0038] According to one embodiment of the invention, there is provided a circuit in the electronics portion of the process gas chromatograph that limits the energy available in the detector to a level that is incapable of initiating an ignition of a flammable gas; i.e. is Intrinsically Safe. This circuitry still provides adequate energy to the detector elements for their normal function of detecting changes in the thermal conductivity of the various gases eluting from the gas chromatograph column. This Intrinsically Safe technique eliminates the need for large

heavy-walled flameproof enclosure, flameproof passages for all gases, and flameproof electrical seal for the signal wires. According to an embodiment of the present invention, the circuit can be implemented such that it is possible to arrange up to eight (8) total circuits that can act together, be physically close together in a compact detector assembly, and all be Intrinsically Safe, without requiring physical separation that might be required for intrinsic safety of less advantageous implementations. This allows for a simpler mechanical design and intrinsic safety certification, hence lower cost.

[0039] According to an embodiment of the present invention, the circuit can also be implemented such that the signal voltages applied to the thermistor detector "bridge" circuitry are not reduced, such that the signal is not compromised with respect to the noise level of the circuit. The energy levels of the circuit can also be implemented such that there is not a compromise to the startup and operating levels of the thermistor detector element, which can have a non-linear startup characteristic that is compromised at low temperatures and in carrier gases of high thermal conductivity, such as hydrogen.

[0040] According to an embodiment of the present invention, there is a second circuit which acquires the signal from the first circuit, such that there is no interference with the first circuit, and no introduction of noise into the signal. The second circuit must not add energy into the first circuit, lest the Intrinsically Safe nature of the original circuit be compromised. The circuit can also be designed such that the collateral effect of higher power consumption is negated in Intrinsic Safety barrier circuits, while still maintaining the performance, operational and full applicability of the gas chromatograph detector.

[0041 ] Embodiments of the present invention can be applied to any process gas chromatograph which must meet hazardous area requirements. In particular, according to an embodiment of the present invention, an intrinsically safe process gas chromatograph detector can be utilized in a process gas

chromatograph in which the detector is located in the heated oven of the gas chromatograph and electronics controlling the gas chromatograph are located in an adjacent area which is suitable for the use of IS barrier devices.

[0042] FIG. 2 illustrates a traditional thermistor detector circuit. The circuit of FIG. 2 has no inherent fault protection and cannot be considered infallible or "safe" in terms of the IS standards or accepted practices. The circuit of FIG. 2 includes a detector amplifier circuit 200 and a precision difference amplifier circuit 210. The detector amplifier circuit 200 outputs a detector signal that is a measure of the voltage required to keep the thermistor 201 at a certain operating temperature, and thus at a certain reference resistance, as a gaseous compound that is mixed with a carrier gas flows over the thermistor 201 . The precision difference amplifier circuit 210 calculates the difference between the detector signal and a reference signal that is a measure of voltage required to keep an identical thermistor at the operating temperature with only the carrier gas. This results in a signal that measures the changes in the thermal conductivity in the various gases eluting from the gas chromatograph column. This signal is then further processed, for example by an A/D converter, etc., as is well understood in the art.

[0043] As illustrated in FIG. 2, the detector amplifier circuit 200 includes the thermistor 201 , resistor networks 202 and 204, and amplifier 206. The resistor networks 202 and 204 can be shared between detector and reference channels. In particular, the resistor networks 202 and 204 can be shared by eight detector amplifier circuits. Unfortunately, these elements are a weakness for making this circuit IS. The technique of I.S. evaluates failure paths where unconstrained levels of energy may prove to have an unacceptable probability of release into an igniting spark or igniting condition. While a resistor may be an excellent means of limiting energy, these precision components violate IS standards because they have an unacceptable level of probability of being compromised, thus providing no limiting effort against the available energy sources.

[0044] Note that even if the resistors could be trusted, the next problem would be how to define the available energy on the other side of the resistance. What might be acceptable at 20V and 1 K ohms may not be acceptable when the 20V could fault to a higher voltage. Since all system voltages are assumed to start with the power grid, then the limit is significantly higher: 1 15V, 230V, or potentially even 440V, depending upon the power system and the right combination of faults. In a similar fashion, aside from simple semiconductor devices such as diodes, a more complex semiconductor device, i.e., just about any integrated circuit, is considered to fail in the most hazardous manner for the worst case evaluation, whether that be shorted to power sources when thought to isolate from an energy source, or open, when the intended path is to shunt energy. For example, amplifier 206 would be considered a short circuit to the power source and to resistor networks 202 and/or 204. Resistor networks 202 and/or 204 would be considered to fail shorted as well, such that the power source is directly presented to the "outside world", thus providing igniting energy to the hazardous area. Since none of these components are "infallible" or meet the rationale of the IS standard, then all of these faults will be considered to be probable to occur all at once to create a hazardous situation.

[0045] Suppose that elements of the detector amplifier circuit 200 could be made IS. Then where the detector signal connects to the precision difference amplifier 210 may be at question, because there is no limit to its voltage potential, unless proven otherwise. To extend this example one step further, if indeed the precision difference amplifier 210 were now to be made IS, then the question would be whether the A/D converter had a source of energy or failure mode that is unconstrained, and if so, then does the subsequent connection to a processor or communication system to the A D have a limited potential. If this rationale were followed all the way to its logical conclusion, then the complete product and all circuits contained within it would have to be IS, or of a known subset of infallible components and component arrangements. Accordingly, this simply is not practical, nor is it likely to be successful.

[0046] Per the IS standards, certain components can be considered as "infallible". Film and wire wound resistors fall into this category, insofar as there is sufficient physical spacing between conductors, and that the resistor is not misapplied. The standard further requires that under no condition will a resistor exceed 2/3 of the rated wattage for the component for any possible situation. Thus, the component may limit energy properly, but the circuit has to also withstand all possible scenarios of simple wiring faults, reversed connections, etc. [0047] Re-iterating the fact from above, the resistor elements 202 and 204 must hold very low TCR values. Unfortunately, high power dissipation values are counterproductive to the guarantee of low TCR values. Low TCR resistors are intentionally de-rated by the manufacturers in order to meet the performance requirements. Thus, there is a conflict between meeting performance requirements and achieving the IS rating for a compact, cost effective design.

[0048] A key element that dictates the design is the fact that the thermistor device has certain physical requirements that are necessary in order to operate properly. A thermistor is used to sense the thermal conductivity of a known reference gas such as hydrogen or helium and contrast this to the measurement gas, which is mixed with the same reference gas. Thus, the result of the difference between the measured electrical signals is the measurement gas all by itself.

Since the thermistor is sensing thermal changes, and the thermistor is a resistive device, then the resistance of the thermistor becomes very important to the operating point of the detector.

[0049] FIG. 3 illustrates exemplary thermistor detector bridge circuitry. Referring to FIG. 3, for a constant temperature application, for the amplifier circuit to be in equilibrium, the reference resistor 302, at 1 K ohm, forces the thermistor 301 to self heat to an operating temperature for which the resistance of the thermistor 301 is also 1 K ohm, in order to match the reference resistor 302. When the circuit is first turned on, in some dense gases such as hydrogen, the thermistor 301 cannot self heat enough to drop the resistance to 1 K and stabilize, unless at least 18V is present on the bridge. Thus, there is an inherent forced constraint on the IS system; a minimum of 18V must be present, without accounting for loss through any of the control of the bridge.

[0050] Although in normal operation, the 1 K resistor 302 would only dissipate 0.081 watts, that same resistor 302 would have to withstand a full 18V application and a 1 .5 rating factor, or 0.486 watts, to meet the standard. For all (8) resistors to be in one package (in order to meet TCR tracking) then the package dissipation would have to be 8 x 0.486 = 3.9 watts. This is highly impractical for a compact, low cost design. The best practical implementation is a wire wound network that allows 0.25 watts per element. This is all being stated at the minimum required voltage of 18V. In reality, the voltage can be much higher, which exacerbates the issue.

[0051 ] With regards to operating voltage, the circuit of FIG. 2 would, in a practical implementation, require >19V (total) to operate amplifier 206, considering loss through the diode and the potential the amplifier 206 cannot drive all the way to the positive supply. It must be noted that this is a minimum voltage, and the maximum voltage that could exist may violate the IS standard when all infallible resistance sources are taken into consideration. As is well known to those skilled in the art, IS protection techniques allow shunt zener diodes for limiting potential energy sources (resistor-zener diode barriers). It is well known that such devices must be evaluated at the maximum zener tolerance. With a 5% tolerance, and a 19V required circuit minimum, a 20V zener would be required, with a maximum of 21 V when tolerance is included. Referring to the IS standard IEC 60079-1 1 , Table A.1 , 1 .5X safety factor, at 21V, a purely resistive (maximum) short circuit current of 262mA is allowed, or an equivalent resistance of 80 ohms.

[0052] Since there may be eight thermistor circuits, and all eight must be considered in parallel, then the equivalent resistance (minimum) of each circuit, under all conditions, must be greater than (80 x 8 = 641 ohms). Since each reference resistor is 1 K on its own, this would allow for a minimum of 1 .8K for the cumulative parallel of all other circuit resistances with connect to the detector interface. This leaves little or no margin for additional resistance. Up to this point, the evaluation has been considered to be completely resistive, when, in fact, there is always a certain amount of capacitance in the practical application, especially where electromagnetic interference must be dealt with. The IS standards do not address multi-dimensional application of resistance and capacitance "on paper". These combinations must be tested by the certifying agency. The aforementioned scenario has an elevated risk of failure and little margin for error. Advantageous embodiments of the present invention have been designed to fall cleanly within the guidelines of the IS standards, and therefore can be readily certified as IS.

[0053] FIG. 4 illustrates an IS thermistor detector for process gas chromatography according to an embodiment of the present invention. The embodiment of FIG. 4 does not require any compromise in performance or capabilities of the process gas chromatograph. As illustrated in FIG. 4, a thermistor 402 and a capacitor 404 can be provided in a hazardous area, such as the heated oven of the gas chromatograph. The thermistor 402 can be a negative temperature coefficient (NTC) thermistor and the capacitor 404 can be an electromagnetic capability (EMC) capacitor. The thermistor 402 and the capacitor 404 are connected to detector circuit by IS wiring 406.

[0054] A reference resistor 408, the thermistor 402, and resistors 410 form a bridge circuit, and amplifier 412 drives voltage through the reference resistor 408 and onto the thermistor 402 in order to control the resistance of the thermistor 402 to match the resistance of the reference resistor 408 by maintaining the thermistor 402 at a certain operating temperature. In a possible implementation, the reference resistor 408 may have a resistance of 1 K ohms.

[0055] Power supply 420 provides the voltage that is driven through the reference resistor 408 and onto the thermistor 402. The power supply 420 can be connected to a fuse 421 that provides input reversal protection to the power supply 420. A linear voltage regulator 436 can be connected to the power supply 420 in order to regulate a specific voltage (e.g., 20V). According to an advantageous implementation, the linear voltage regulator 436 can be a precision voltage regulator device that maintains a precise voltage supply that is between a minimum level needed to supply the detection circuit and an upper limit that would exceed the minimum voltage of the IS clamping zener diodes. Such a precision voltage regulator device is described in greater detail below. The voltage from the voltage regulator 436 passes through a zener diode barrier 418 that limits the maximum voltage that can enter the detection circuit. A second linear voltage regulator 438 can regulate a specific voltage (e.g., 20V) for a buffer amplifier 426 and a difference amplifier 428.

[0056] In the embodiment of FIG. 4, the power rating of reference resistor 408 is resolved by providing an 1 1 mA current limiter circuit 414, such that if the thermistor 402 is shorted, only 0.121 watts is dissipated by the reference resistor 408. This makes the selection of the reference resistor 408 in a network or a matched set of wire wound resistors into a more manageable task, while achieving the necessary low TCR. Thus, no compromise to performance occurs as a result. FIG. 5 illustrates the voltage limiter circuit 414 according to an

embodiment of the present invention. As illustrated in FIG. 5, the voltage limiter circuit 414 is a very low voltage drop "brick wall" limiter that readily allows currents up to 1 1 mA, and limits the current to 1 1 mA when the thermistor resistance approaches zero. The voltage loss is 0.4V or less.

[0057] A limiter of this type is an interpretation of I.S. standard IEC 60079-1 1 7.5.3 for series current limiters. In order to make use of the current limited power source, the detector circuit is re-configured to add a low noise PNP transistor 416, as shown in FIG. 4. Part of the re-configuration is to swap the (+) and (-) inputs of the operational amplifier 412, since the transistor 416 provides an inversion. The 2N5087 or equivalent low noise transistor 502, also used in the current limiter 414, has a drop of only 0.1 V in saturation. Thus, the combination of the current limiter 414 and the control transistor 416 arrangement in FIG. 4 demonstrates that a minimum voltage of 18.425V is required to supply the circuit in order to achieve the necessary minimum startup voltage of 18V to the reference resistor 408 at the worst case thermistor startup condition (in hydrogen). The zener diode barrier 418 of 20V can then be used (19V low, 21 V high tolerances) to provide isolation between the power supply 420 and the detection circuit. This is 2 volts lower than the scenario discussed above. Thus, a significant problem can be solved for both the reference resistor 408 power, and the overall voltage.

[0058] Although the embodiment of FIG. 4 is implemented using the current limiter 414 and the transistor 416, as described above, in an alternative embodiment, these elements are not necessary to make the circuit IS. Where the reference resistor 408 is rated sufficiently and properly de-rated to the IS standard at 1 .5x for any possible condition of the IS voltage and output short, etc, it is possible to exclude the current limiter circuit 414 noted above. However, the challenging requirements for the reference resistor 408 are increased, as well as a penalty will be paid for size of the design. In this case, the remainder of the circuit may be retained, or, alternatively, by using a "rail-to-rail" operational amplifier 412, and swapping the "+" and "-" inputs. However, such simplifications have issues of their own, as those skilled in the art would recognize when incorporating a low noise rail-to-rail operational amplifier. [0059] Referring to FIG. 4, amplifier 412 is a semiconductor integrated circuit that must be treated as fallible. According to an advantageous embodiment of the present invention, an infallible resistance element 422 is placed in front of the high impedance input of the amplifier 412. The infallible resistance element 422 limits the current that can enter the thermistor 402 from the amplifier 412 in the event of a failure of the amplifier 412. The infallible resistance element 422 has a resistance value (e.g., 4.99K) such that the infallible resistance element 422 has no appreciable effect on the performance of the system and a minimal effect on the IS power limiting. Resistance element 422 has input bias currents that could generate unwanted drift error, however, resistance element 422 has a low enough value to merit the I x R product as insignificant. The choice of resistance element 422, however is important. Where this element 422 sits directly on the input of a low noise amplifier 412, there stands the risk of injecting unwanted noise into the system. It is difficult to find a high value resistor of sufficient physical spacing (2 mm) to meet the IS standard, and still be a low noise resistor. Such resistors, in surface mount "2512" or larger packages, are constructed from an undesirable type of film technology from the perspective of noise performance. Placing a poorly chosen resistor at this point in the circuit would add unwanted noise to the circuit. Thus, according to an advantageous implementation, a low noise metal film "MELF" package part is used as the resistance element 422. It can be noted that the use of a "MELF" package on a high performance surface mount design is highly contrary to what is viewed as state of the art manufacturing, as MELF packages are often discouraged on small, surface mount designs, due to mounting concerns. However, the present inventors have recognized that this is a small price to pay for the aversion of inserted noise and a complete solution to the problem presented.

Alternatively, those skilled in the art would recognize that this resistor could be a larger "thru hole" part of equivalent metal film technology, or a bulk metal foil part, being very low noise, but which is an order of magnitude higher in cost than what is presented here. Thus, it is possible, but may be undesirable due to cost.

[0060] As gases eluting from a gaseous compound that is mixed with a reference gas flow over the thermistor 402, a detector signal that is a measure of the voltage required to keep the thermistor 402 at the reference resistance is sent to difference amplifier 428 through a buffer amplifier 426. The detector signal voltage is transmitted through a second infallible resistance element 424. Infallible resistance element 424 is placed in front of the high impedance input of the buffer amplifier 426 and serves a similar role with respect to the buffer amplifier 426 as resistance element 422 with respect to amplifier 412. Resistance element 424 can also be implemented as a MELF, metal film resistor that is used to draw the voltage from the bridge without contributing power back to the bridge, since buffer amplifier 426 has very high input impedance. However, resistance element 424 has a very different source voltage issue from that of resistance element 422. In the case of resistance element 422, the only voltage available is the IS protected 21 V max voltage. In the case of resistance element 424, the voltage comes from the "rest of the world" so to speak. Thus, the voltages are not well defined.

[0061 ] It should be recognized that the process analyzer has a certifying agency rated 24V isolated power supply as the power source. When chosen properly, this can be evaluated at a worst case of 30V. Thus, the areas marked at 20V can fault to 30V, and must be evaluated as such. This allows the rest of the design circuitry, processor, communication, etc., to all work from the 24V supply, given that no other outside voltage sources are available in a fault scenario. The process analyzer is a closed box that is not allowed to be operated "safe" with the enclosure open, without special permits that declare the area to be non-hazardous. Thus, it is a reasonable and normally certifiable scenario to allow 24V, faulted to 30V to be the highest voltage present. However, there is one additional, potentially hazardous voltage present.

In practical implementation, the A/D circuit 430 and difference amplifier 428 require a negative voltage. FIG. 6 illustrates the power system of the circuit of FIG. 4. Referring to FIGS. 4 and 6, a buck-boost converter 434 provides a negative 5V that is linearly regulated (by linear voltage regulator 610) to negative 3V, which passes through a zener diode barrier 419. However, since an energy storing inductor 602 is involved with fallible components (integrated circuits) one must assume the worst, and that is that the inductor 602 can "fly back" to almost any voltage. The combination of limiting resistance 604 and an infallible combination of schottky diodes 606 and zener diodes 608 produce an end limit of voltage for the right side of resistance element 424 in FIG. 4. This means that resistance element 424 will receive no more than 30V and no less than -6.49V.

[0062] This creates a new problem, which is resolved by schottky diodes 432 in FIG. 4. If resistance element 424 or any subsequent circuit is under fault, the +30V to -6.49V driven into resistance element 424 could add unwanted energy to the IS portions of the circuit. Ordinarily, the 4.99K impedance of resistance element 424 could not overcome the lower impedances of the other resistances; however, there is a large value of capacitance shown at the PNP transistor 416, as well as in other unspecified parts of the circuit, such as bypass capacitance for the integrated circuits. Given that the integrated circuits and the PNP transistor 416 are fallible, then it is a possible scenario that all of the capacitance is charged to +30V or -6.49V through resistance element 424. The potential of (8) 1 K reference resistors in parallel to a faulted 30V potential is not safe, per the IS standard tables.

[0063] In order to avert this situation, schottky diodes 432 are placed in an infallible recognized triple shunt arrangement to "dump" any potential above 21V or below ground potential. Since the diodes are reverse biased in normal operation and have no ability to influence the low impedance driving circuit, there is no effect on system performance.

[0064] In order to clarify, FIG. 7 illustrates a simplified fault diagram of the IS circuit simplified down to protective elements. As illustrated in FIG. 7, resistors 408 and 422 are in parallel, so the minimum resistance equivalent of resistors 408 and 422 is equal to 1 K(-0.1 %) | 4.99K(-0.1 %) = 832 Ohms. This represents one of eight identical circuits that are not separated, and therefore assumes that all eight thermistor circuits are paralleled. Thus, the minimum resistance equivalent for all eight thermistor circuits is equal to 832 Ohms / 8 = 104 Ohms and the maximum capacitance equivalent for all eight thermistor circuits is equal to (1 OOpf + 5%) x 8 = 840pF. For the worst case IS evaluation of +30V to -6.49V from the buffer amplifier, the schottky diodes 432 shunt any negative current into resistance element 424 from ground, thus clamping to a -0.5V potential. The schottky diodes 432 shunt over to 21V clamped voltage for a total maximum voltage of 21V + 0.5V = 21 .5V max. Accordingly, a voltage of 21 .5V max and -0.5V min is properly limited per IS standards, but not limited to IS current. The result is that the IS circuit is evaluated at one schottky diode drop higher than the IS limiting voltage of 21V. However, this is not of consequence, as is shown below in FIG. 8.

[0065] FIG. 8 illustrates the equivalent circuit for the detector circuit of FIG. 4 for evaluating whether the detector circuit meets IS standards. This simple model is all that is needed to evaluate the available energy against the requirements of the I.S. standards. As illustrated in FIG. 8, for eight thermistor circuits, the equivalent circuit has a maximum voltage of 21 .5V, a minimum voltage of -0.5V a resistance equivalent of 104 Ohms min, and a capacitance equivalent of 840pF. This results in a maximum current of 207mA. The I.S. standard IEC 60079-1 1 table A.1 , allows 262mA for a safety factor of 1 .5x for Group IIC apparatus. This equivalent circuit is at 207mA, which is approximately 20% below the maximum of 262mA. Therefore, the equivalent circuit meets IS requirements.

[0066] Regarding capacitance, IEC 60079-1 1 table A.1 is a purely resistive chart. However, IEC 60079-1 1 table A.2 addresses a purely capacitive scenario. In this case, 0.176uF (1 .76 x 10⁴ pF) is allowed at 21 .5V. This is 2095x more capacitance than the 840pF that is actually present. Combinations of resistance and capacitance are multi-dimensional and most often, an actual spark ignition test must be performed with various combinations of explosive gasses. However, testing is arguably not necessary when the capacitance is several orders of magnitude away from the purely capacitive limit, as in this case. In addition, this parallel capacitance only applies if somehow, statistically, all (8) detectors are paralleled, and then somehow all shorted with maximum potential.

[0067] Based on the above description, the embodiment of FIG. 4 meets intrinsic safety in all regards without a reduction in signal, without the addition of noise additive components, without a reduction in the number of thermistor circuits (8), without separation of subsets of thermistor circuits, and without a reduction in applications, carrier gasses, or operating range of the process gas chromatograph.

[0068] While this above description has been based upon a 24V system that has a 30V limit, it is to be understood that a system of a different voltage source may be presented, where an additional traditional barrier, most likely involving a fuse, limiting resistor and shunt zener diodes are involved. For example, 1 15V or 230V mains could be incorporated in conjunction with the power supply, such that a higher voltage than 30V is allowed. In this case, the barrier power ratings need only to be adjusted, and/or a fuse added in order to prevent exceeding the rating of the resistor and/or zener diodes. Alternatively, for higher voltages mains such as 1 15V or 230V, the entire difference amplifier 428, A D converter 430 and associated circuitry could be supplied by the IS protected voltages, as a first stage, without IS current limiting, and the communication interface protected with traditional barrier techniques. In such a case, the total design may require a more significant packaging effort, however, diodes 432, resistor 424, and buffer amplifier 426 can then be simplified or eliminated. This alternative approach does not change the needs and requirements for resistors 408 and 422 and as thoroughly discussed above as key aspects to the novelty of the IS thermistor detector design.

[0069] FIG. 9 illustrates a method of operating an IS thermistor detection circuit for gas chromatography according to an embodiment of the present invention. As illustrated in FIG. 9, at step 902, a zener diode barrier is provided to limit a maximum voltage on a thermistor detector circuit. At step 904, one or more infallible resistance elements are provided to limit current that can enter the thermistor due to failure of integrated circuit components. In particular, a first infallible resistance element can be provided that limits current that can enter the thermistor due to failure of an operational amplifier that drives voltage onto the thermistor. A second infallible resistance element can be provided that limits current that can enter the thermistor due to failure of one or more operational amplifiers that are used to process a detector signal that is a measure of voltage applied to the thermistor.

[0070] At step 906, the thermistor is heated to an operating temperature that causes the resistance of the resistor to match a reference resistance. In particular, an operational amplifier drives voltage from a power supply that is isolated by the zener diode barrier through an infallible reference resistor onto the thermistor in order to heat the thermistor to the operating temperature. At step 908, gas eluting from a mixture of a gaseous compound and a reference gas is passed over the thermistor. At step 910, the operational amplifier drives voltage through the reference resistor onto the thermistor to maintain the thermistor at the reference resistance. At step 912, a detector signal that is a measure of the voltage applied to the thermistor is compared to a reference detector signal through an infallible resistor that is a measure of voltage applied to a second thermistor over which the reference gas is passed. This results in a thermal conductivity measurement of a particular gas that was eluted from the gaseous compound.

Precision Voltage Regulator Circuit

[0071 ] As described above, the circuit of FIG. 4 requires that a minimum analog supply voltage be maintained with a voltage regulator while staying below an upper limit that exceeds allowed IS energy levels. If the voltage tolerance is too high, then the energy clamping elements would be active in normal operation, thus remaining IS, but restricting the circuits from having sufficient power for proper operation. According to embodiments of the present invention, a precision voltage regulator circuit is provided that can maintain a voltage that tightly fits within both the upper and lower parameters. Furthermore, precision voltage regulators of this type may have applications in many types of electronic equipment, and are not just limited to the IS circuit described above. Some subset of those applications requires that the voltage supply or supplies must be as accurate as possible in order to allow for the highest possible voltage compliance without encroaching upon the available unregulated voltage low tolerance. Thus, embodiments of the present invention are relevant to a large field of applications, where cost, size, standard components and minimal design effort are required.

[0072] The majority of electrical and electronic circuits rely on the premise that there is some limit to the power being supplied them, i.e., a voltage limit, current limit, or both. More precise and/or higher performing circuits typically demand a very exact set of limits, predominantly on the voltage. For example, an amplifier circuit must accept a signal of a certain range, and then amplify the signal to a different level. In order to do so, the circuit must have voltage supplies that are at least the magnitude of the signal itself, and, in practicality, at least 10% greater, in order to account for voltage losses in the elements (resistances, silicon junctions) that comprise the fundamental building blocks of the amplifier.

[0073] A precision operational amplifier IC such as the Analog Devices OP1 177 may be rated to +/-18V of supply range, however, it's input may only accept +/-16.5V of signal (within 1 .5V of the supply rails), and the output may only "swing" linearly to +/-17.0V (within 1 V of the supply rails). If the power supply were able to supply exactly +/-18V, then this would be ideal. In the practical application, the power supply itself may have a tolerance of several percent to as much as 5%, thus limiting the available operation range of the amplifier to the minimum tolerance of the supply. Furthermore, to account for another factor, the amplifier cannot tolerate a value greater than it's rating of +/-18V, or 36V total. Thus, under maximum tolerance, the supply cannot exceed +/-18V, or 36V total and under minimum tolerance, the amplifier must have enough "headroom" to work. This is shown in the FIG. 10 below, which illustrates voltage constraints for an exemplary operational amplifier IC.

[0074] The designer thus has to choose a power supply regulation voltage that does not violate the maximum rated voltage of the semiconductors, but in the same fashion, does not overly constrain the signal range. As FIG. 10 intuitively shows, the voltage that the OP AMP really has to work with is reduced by two times the power supply tolerance 1012. Therefore, as shown in FIG. 10, for an OP AMP rated with a +/- 18V limit (1002), the voltage regulator must be set no higher than +/-17.1 V (1004), which in turn, may tolerance down to +/-16.2V (1 006) before application to the amplifier IC. The IC, in turn, can then only reliably accept +/-14.7V (1 010) and output can only reliably swing to +/-15.2V (1 008). Thus, the effective signal range of the amplifier is reduced by +/-10% when the supply to the amplifier has to tolerance to +1-5%.

[0075] This is all in the interest of having a maximum signal path through the amplifier. If a lesser signal range were a constraint, then a more complex approach might be required, such as two amplifier stages. In some cases, it is very important to the signal to noise (SNR) performance to have the maximum signal gain and range in the first stage of the amplifier path, and two stages are not acceptable for performance. This is a case where the greater power supply regulator tolerance works against the available SNR. Typically, the noise floor is the same regardless of the signal range. Thus, it can be understood that a more accurate voltage regulation circuit provides for more signal into the same noise floor, thus providing a higher SNR. One example of this is in a photomultiplier tube preamplifier or in an electrometer circuit, where the signal is unipolar in nature. In this case, a very large unbalanced supply voltage (for example, +3V and -33V) totaling the 36V silicon maximum is used for maximum signal with a constant noise floor. In this case, a very accurate voltage regulator is desirable. Otherwise, the actual signal range will be much less than what is possible. One such example is patent number is described in United States Patent No. 7,554,072, where a very tight tolerance set of voltage supplies is quite desirable to maximize SNR. United States Patent No. 7,554,072 does not call on this technique of a tighter supply tolerance. However, as United States Patent No. 7,554,072 is directed to an amplifier configuration to maximize signal to enhance SNR, the embodiments of the present invention described herein would help enhance the total SNR in a complementary way.

[0076] Another area in which a tight supply tolerance is desirable is for sensor circuitry that has a minimum operating voltage, with a maximum operating ceiling. For example, as described above, the chromatography thermistor detector has definite minimum operating voltage in order to be useful in a wide variety of process chromatography applications. However, that same sensor may be subject to maximum voltage limits, such as in the case of the Intrinsically Safe (IS) circuit, where the maximum energy to the circuit is closely limited. In that case, there is a fine line between the minimum voltage for a circuit, and the maximum voltage allowed by Intrinsic Safety. This can be the case for a great variety of circuits, and not just limited to chromatography detector circuits.

[0077] Where power is needed from an IS circuit, a zener diode shunt barrier is most commonly involved. FIG. 1 1 illustrates an exemplary zener diode shunt barrier. As illustrated in FIG. 1 1 , a power supply 1 102 is connected to a protection fuse 1 104, a current limiting resistor 1 106, and zener diodes 1 108. However, the zener diodes 1 108 create the same issue as a broad tolerance voltage regulator. Accordingly, in the example of FIG. 1 1 , assuming a 24V power supply, the useful signal voltage that can be planned for is derived from 19 volts. Furthermore, the circuit has an issue in that it continuously consumes power, regardless of the load. This is because the 24V will always put the 20V zener diodes 1 108 into conduction. The protection fuse 1 104 would open if it were not for the current limit resistor 1 106. However, there is price to pay for the current limit resistor 1 106, in that it provides an undesirable voltage drop to the supplied circuit if that circuit needs current that causes the output voltage to drop well below the zener voltage due to the I x R voltage drop. This current can be significant, but still below that of the fuse rating.

[0078] FIG. 12 illustrates a zener diode barrier according to an

embodiment of the present invention. Referring to FIG. 12, a power supply 1202 is connected to a protection fuse 1204, a linear voltage regulator 1206, and zener diodes 1208. As shown in FIG. 12, with the incorporation of the linear voltage regulator 1206, the resistor (1 106 of FIG. 1 1 ) is eliminated, and is of no

consequence to the IS design. The regulator 1206 must only function to stay below the 19V minimum tolerance of the zener diodes 1208. However, if for example, this is a 5% voltage regulator, the voltage would be -18V, and the usable supply signal voltage is 5% below that, or 17.1 V, much like the earlier OP AMP example shown in FIG. 10. However, this particular situation would not allow an acceptable voltage range for a process chromatography detector. The 17.1V value is not sufficient for proper operation at all temperatures and measurement scenarios. If, instead the regulator 1208 were of a tighter tolerance, the application of this circuit to a process chromatography detector would be possible. For example, if the desired minimum operational voltage is 18.5V, then a regulator of 18.75V +/- 1 .3% would be required. This requires a higher class of regulator than is commonly available at a higher voltage.

[0079] There are many product offerings of monolithic voltage regulator solutions available today, such as the LM78xx, LM79xx, LM137, LM337, LM317, LT1764, or LP2951 just to name a few. Companies such as Fairchild

Semiconductor, National Semiconductor, Texas Instruments, Linear Technology, Analog Devices, Maxim Semiconductor or OnSemi all produce such devices. However, only a limited set of these offerings can accept and regulate voltages greater than 12V, and the list grows very short when the requirement deals with 24-30V of input power. Of that subset, there are a good set of choices that provide a very simple, low cost regulation circuit with one IC, two resistors, and two capacitors, including the LP2951 monolithic voltage regulator. An arrangement of this type can cost as little as $0.30 with all components. FIG. 13 shows a data sheet for an LP2951 monolithic voltage regulator from National Semiconductor. FIG. 14 shows a more detailed look at the internal workings of the LP2951 monolithic voltage regulator, which shows a very complex arrangement with very desirable features, such as external soft start/shut down, built in current limiting, thermal shut down and error detection.

[0080] A search for pricing on the LP2951 shows that it can be purchased for $0.21 in production quantities. However, the price trade off is with tolerance of the supply. It is possible to achieve 3% in some cases, but anything closer requires a departure from the standard solution.

[0081 ] It is quite possible to create a regulation circuit from discrete components (OP Amps, voltage references, transistors, diodes, resistors and capacitors) as shown in block diagram of FIG. 14, while incorporating more precise components, for the purpose of a precision regulator. Unfortunately, these types of circuits have a large component count and tend to be expensive as a result. Since each of the building blocks in FIG. 14 constitutes a separate IC or combination of discrete components, the total cost will be $5-$10 when built and tested on a production basis, not to mention additional development cost. There is a development risk, since discrete implementations of regulator control circuits have to deal with the significant stability problem of large capacitive loads. An analog designer, highly skilled in the art would find such a task to be non-trivial in nature. The factors that must be considered are circuit stability, power up behavior, supply voltages for the internal circuits (since they cannot be powered by the regulator circuit itself), and the potential of input voltages which are beyond the silicon ratings. All of these factors are already encapsulated and accounted for in the inexpensive monolithic voltage regulator designs.

[0082] Embodiments of the present invention provide a novel approach for applying an industry standard low cost monolithic regulator circuit in a manner that provides a low cost, low risk, full featured approach to implement a precision circuit. The approach may be particularly applicable to higher voltage circuits (5 volts and above), but is not limited or constrained to higher voltages. In order to implement this approach, a low cost monolithic voltage reference is utilized to account for the majority of the regulated voltage. The monolithic voltage reference is placed in the feedback circuit of the monolithic regulator device, which has a much wider tolerance. The tolerance improvement will be approximately equivalent to the ratio between the added reference voltage and the internal reference of the monolithic regulator.

[0083] FIG. 15 illustrates a precision voltage regulator according to an embodiment of the present invention. As illustrated in FIG. 15, the precision voltage regulator 1500 includes a monolithic voltage regulator 1502 and a monolithic voltage reference 1504 disposed in the feedback circuit of the monolithic voltage regulator 1502. The monolithic voltage regulator includes feedback resistors 1506 and 1508 and two capacitors 1510 and 1512. The monolithic voltage regulator 1502 can be implemented using the LP2951A monolithic voltage regulator, as shown in FIG. 15, but the present invention is not limited thereto. As shown in FIG. 15, the LP2951A monolithic voltage regulator 1502 has an internal reference of 1 .235V, however the tolerance on the reference is +/-2.4%. Accordingly, after adding 0.1 % feedback resistors, the supply will never be more accurate than +/-2.6% using the LP2951A alone. By adding the monolithic voltage reference 1504 in the feedback circuit and adjusting the feedback resistor values, the tolerance drops to +/-0.41 % worst case. According to an advantageous implementation, the ratio between the internal feedback (1 .235V) and the monolithic voltage reference 1504 value of 10V can be used to estimate the accuracy improvement. Thus, the tolerance is estimated as:

Improved tolerance (estimated) =

[(Existing Circuit Tolerance) x (internal reference value/added reference value)] + external reference tolerance, where the added reference is at least 75% of the output voltage.

[0084] Thus for the circuit of FIG. 15, Improved tolerance (estimated) = [(2.6%) x (1 .235/10)] + 0.1 % = 0.42%. Note that the 0.42% estimate is very close to the exact calculated result of 0.41 % shown in FIG. 15.

[0085] As stated above, the cost for the monolithic voltage regulator 1502 without the added monolithic voltage reference 1504 is approximately $0.30 in production quantities. The monolithic voltage reference 1504 costs $0.87 in production quantities. While as a percentage this addition is relatively high, the total cost of $1 .17 for a highly precise voltage regulator with all the features of a monolithic design is quite attractive. As a point of reference, in a discrete implementation, as discussed above, just one precision operational amplifier would have a total cost in the $1 .00 to $1 .50 range, and it is just one of many components required for a discrete implementation.

[0086] Embodiments of the present invention have the distinct advantage of having no performance risk and no development time, as the incorporation of the monolithic voltage reference 1504 is simply part of the calculation of the feedback resistance for the monolithic voltage regulator 1502. The LP2951 monolithic voltage regulator 1502 will operate as it is designed to do. With the addition of the monolithic voltage reference 1504, none of the following key features of the LP2951 monolithic voltage regulator 1502 are compromised: stability under capacitive loads, internally current limited, thermally protected, soft starting/external shut down controlled, and provided a self diagnostic error signal.

[0087] The external monolithic voltage reference 1504 can be many possible devices, as there are many low cost voltage references available from many manufacturers. In the example of FIG. 15, the monolithic voltage reference 1504 is a Texas Instruments TL4050, but the present invention is not limited thereto. For example, the Analog Devices ADR520/540/5xx series devices will accomplish the same results, as will many other devices.

[0088] FIG. 16 is a block diagram illustrating the implementation of a precision voltage regulator according to an embodiment of the present invention. As shown in FIG. 16, there are two arrangements 1600 and 1610 for inserting the monolithic voltage reference 1604 into the feedback circuit of the monolithic voltage regulator 1602, based on the topology of the monolithic regulator 1602 in use. In arrangement 1600, the monolithic voltage regulator 1602 references the feedback to ground, such as in the LP2951A device. In this case, the monolithic voltage reference 1604 is inserted in the feedback loop before the first feedback resistor 1606. Further, in this case, a shunt or two terminal regulator is necessary. In arrangement 1610, the monolithic voltage regulator 1602 references the feedback to the output voltage (Vout), such as in the LM317 monolithic voltage regulator device. In this case, the monolithic voltage reference 1604 is inserted in the feedback circuit between the second feedback resistor 1608 and the ground.

[0089] FIG. 17 illustrates a precision voltage regulator according to another embodiment of the present invention. As shown in FIG. 17, the precision voltage regulator 1700 includes a monolithic voltage regulator 1702 (shown as a LM317A monolithic regulator device) and a series of monolithic voltage references 1704a, 1704b, 1704c, and 1704d. By using a series of monolithic voltage references 1704a-1704d, any value can be established for the voltage supplied by the precision voltage regulator. Clearly this has a higher component cost (~$0.87 per added reference), however the benefits far outweigh the cost. In the example shown in FIG. 17, the value of 35.9V +/-0.29% can be achieved for a nominal $3.48 additional cost. The benefit is to a very high performing circuit, including but not limited to the circuit in the above-mentioned U.S. Patent No. 7,554,072, in which maximum signal range is paramount, given that the amplifier itself cannot tolerate voltages greater than 36V. In such cases, the return on the investment of the four regulators comes in the form of a direct analog SNR improvement due to more signal "headroom".

[0090] FIG. 18 illustrates two examples for an 18.84V precision regulator circuit according to another embodiment of the present invention. Such an 18.84V precision regulator circuit can be used with a 20V IS barrier circuit in order to maximize signal head room for the subsequent analog circuit, while still preventing an upstream fuse from opening due to zener conduction. Likewise, a series resistor is no longer required to limit the zener current, thus providing current "headroom" to the circuit. In the first example 1800, the monolithic voltage regulator 1802 is implemented using the LM317 device, and in the second example 1810, the monolithic voltage regulator 1802 is implemented using the LP2951 device. In the examples shown, the LP2951 implementation is the most compact and least expensive, however it has a current/load limitation imposed by the controller of the LP2951 regulator. The LM317A implementation represents a much higher current application; however, it is physically larger and has more components as normally applied that may not add to this embodiment. Note that the monolithic voltage references 1804a and 1804b are applied two different ways, as the LP2951 device uses ground referencing feedback, and the LM317 device uses output referencing feedback. In example 1800, the diodes 1806 and 1807 are necessary, per the application information for the LM317 device, when a higher voltage regulated output is created. Note that for both examples 1800 and 1810, two series shunt references 1804a and 1804b are used, as a single 10V reference is the highest commonly available reference value. The total incremental cost is $1 .74.

[0091 ] FIG. 19 illustrates two possible implementations for regulating an intermediate voltage output according to an embodiment of the present invention. As illustrated in FIG. 19, examples 1900 and 1910 show implementation of a precision voltage regulator for regulating an intermediate voltage of 12V using, the LM317A voltage regulator and the LP2951 voltage regulator, respectively, for the monolithic voltage regulator 1902. In each example 1900 and 1910, a single 10V monolithic voltage reference 1904 is inserted in the feedback circuit of the monolithic voltage regulator 1902. The monolithic voltage reference 1904 is applied two different ways in examples 1900 and 1910, as the LP2951 device uses ground referencing feedback, and the LM317 device uses output referencing feedback.

[0092] Although the above described embodiments have focused on linear voltage regulators, the same technique can also be applied to monolithic switching voltage regulators as well. Monolithic switching are widely available devices that follow the same principle as a linear regulator, converting one voltage to another, using a closed loop feedback control circuit. The main difference, as is well known, is that a switching regulator uses energy storage elements (inductance/capacitance) to store and re-deliver energy to a different voltage level (higher and/or lower) at a precise voltage, and at a very high efficiency. The main point, however, is that the feedback mechanism is the same as in a linear regulator. Thus, the exact same improvement can be made on these devices. FIG. 20 illustrates a precision voltage regulator implemented using a monolithic switching voltage regulator according to an embodiment of the present invention. As illustrated in FIG. 20, a monolithic voltage reference 2004 is inserted into the feedback circuit of a monolithic switching voltage regulator 2002. As shown in FIG. 20, the monolithic switching voltage regulator 2002 can be implemented using a LM22671 MRE-ADJ/NOPB device, but the present invention is not limited thereto. In the example of FIG. 20, the 2.2% accurate LM22671 monolithic switching voltage regulator 2002 can be made better than 0.4% accurate by inserting the monolithic voltage reference 2004 in the feedback circuit.

[0093] FIG. 21 illustrates a method for precision voltage regulation according to an embodiment of the present invention. Referring to FIG. 21 , at step 2102, a monolithic voltage reference is provided in a feedback circuit of a monolithic regulator device. At step 2104, an unregulated input voltage is received at the monolithic regulator device. At step 2106, a regulated output voltage is generated using the reference voltage added to the feedback circuit by the monolithic voltage reference and an internal reference of the monolithic voltage regulator.

[0094] Although the examples described above have been for positive supply regulators, the present invention is not limited thereto, and the topology exactly applies in the same manner for a negative supply. Although the examples described above have been for voltage supply regulators, the concept remains the same and is also applicable for regulation circuits that control current or a combination of current and voltage.

[0095] While embodiments of the present invention described above apply to a power supply, embodiments of the present invention may also apply to all monolithic component based control circuits involving a reference and feedback element, where the reference tolerance is an issue, and can be improved by application of the above described inventive concepts. Some examples may include closed loop positioners, pressure control loops, and temperature control loops, but the present invention is not limited thereto.

[0096] The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.

Claims

CLAIMS:

1 . An intrinsically safe thermistor detector circuit for process gas chromatograph comprising:

a thermistor;

a reference resistor connected to the thermistor;

an operational amplifier configured to drive a voltage through the reference resistor onto the thermistor to maintain the thermistor at a reference resistance; and

an infallible resistance element connected between the thermistor and a high impedance input of the operational amplifier to limit current entering the thermistor from the amplifier.

2. The intrinsically safe thermistor detector circuit of claim 1 , wherein the infallible resistance element comprises a low noise metal film MELF package part.

3. The intrinsically safe thermistor detector circuit of claim 1 , further comprising:

a power source configured to supply the voltage through the reference resistor onto the thermistor; and

a zener diode barrier connected between the power source and the reference resistor and configured to limit a maximum voltage from the power source.

4. The intrinsically safe thermistor detector circuit of claim 3, further comprising:

a voltage regulator connected between the power source and the zener diode and configured to maintain a voltage from the power supply between a minimum supply voltage and an upper voltage limit.

5. The intrinsically safe thermistor detector circuit of claim 4, wherein the voltage regulator is a precision voltage regulator comprising:

a monolithic regulator device; and a monolithic voltage reference connected in a feedback circuit of the monolithic regulator device.

6. The intrinsically safe thermistor detector circuit of claim 1 , further comprising:

a difference amplifier configured to compare a detector signal that is a measure of the voltage applied to the thermistor to a reference detector signal that is a measure of a voltage applied to a second thermistor.

7. The intrinsically safe thermistor detector circuit of claim 6, further comprising:

a buffer amplifier configured to transmit the detector signal to the difference amplifier; and

a second infallible resistance element connected between the thermistor and the buffer amplifier and configured to limit current entering the thermistor from the buffer amplifier and the difference amplifier.

8. The intrinsically safe thermistor detector circuit of claim 7, wherein each of the infallible resistance element and the second infallible resistance element comprises a low noise metal film MELF package part.

9. The intrinsically safe thermistor detector circuit of claim 1 , further comprising:

a plurality of thermistors;

a plurality of reference resistors, each connected to a respective one of the plurality of thermistors;

a plurality of operational amplifiers, each configured to drive a voltage through a corresponding reference resistor onto a corresponding thermistor; and

a plurality of infallible resistance elements, each connected between one of the plurality of thermistors and a high impedance input of the corresponding one of the plurality of operational amplifiers.

10. The intrinsically safe thermistor detector circuit of claim 9, further comprising:

a power source configured to supply the voltage to each of the plurality of thermistors; and

a zener diode barrier connected between the power source and each of the plurality of reference resistors and configured to limit a maximum voltage from the power source.

1 1 . The intrinsically safe thermistor detector circuit of claim 9, further comprising:

a plurality of difference amplifiers, each configured to compare detector signals from a pair of the plurality of thermistors.

12. The intrinsically safe thermistor detector circuit of claim 10, further comprising:

a shared resistor network connected to each of the plurality of thermistors and configured to form a bridge circuit with each of the plurality of thermistors and the corresponding one of the plurality of reference resistors.

13. The intrinsically safe thermistor detector circuit of claim 1 , further comprising:

a power source configured to supply the voltage through the reference resistor onto the thermistor;

a zener diode barrier configured to limit a maximum voltage from the power source;

a current limiter connected between the zener diode barrier and the reference resistor and configured to limit a maximum current through the reference resistor from the power source; and

a transistor connected between the current limiter and the reference resistor, wherein the transistor is controllable by the operational amplifier to drive the voltage through the reference resistor and onto the thermistor.

14. The intrinsically safe thermistor detector circuit of claim 1 , further comprising: a plurality of schottky diodes configured to shunt any voltage that is negative or above an upper limit.

15. The intrinsically safe thermistor detector circuit of claim 1 , wherein the thermistor is disposed in a heated oven of a gas chromatograph.

16. An apparatus for providing an intrinsically safe thermistor detector circuit comprising:

a thermistor detector circuit;

means for limiting a maximum voltage on the thermistor detector circuit; and

means for limiting a maximum current across the thermistor detector circuit.

17. The apparatus of claim 16, wherein the means for limiting a maximum current across the thermistor detector circuit comprises:

means for limiting current entering the thermistor detector circuit from an integrated circuit component due to failure of the integrated circuit component.

18. The apparatus of claim 17, wherein the means for limiting current entering the thermistor detector circuit from an integrated circuit component due to failure of the integrated circuit component comprises:

means for limiting current entering the thermistor detector circuit from an operational amplifier that drives voltage onto the thermistor detector circuit.

19. The apparatus of claim 17, wherein the means for limiting current entering the thermistor detector circuit from an integrated circuit component due to failure of the integrated circuit component comprises:

means for limiting current entering the thermistor detector circuit from difference amplifier that compares detector signal corresponding to a voltage applied to the thermistor detector circuit to a reference signal and a buffer amplifier that sends the detector signal to the difference amplifier.

20. The apparatus of claim 17, wherein the means for limiting a maximum current across the thermistor detector circuit further comprises:

means for limiting a current across the thermistor detector circuit from a power supply when a resistance of the thermistor detector circuit approaches zero.

21 . The apparatus of claim 16 further comprising:

means for shunting any voltage that is negative or above an upper limit.

22. A method for operating an intrinsically safe thermistor detector circuit comprising:

providing a zener diode barrier to limit a maximum voltage on a thermistor;

providing an infallible resistance element to limit current that can enter the thermistor due to failure of an integrated circuit component;

passing gas eluting from a mixture of a gaseous compound and a reference gas over the thermistor;

driving voltage onto the thermistor to maintain the thermistor at a reference resistance; and

comparing a detector signal that is a measure of the voltage applied to the thermistor to a reference detector signal that is a measure of voltage applied to a second thermistor over which the reference gas is passed.

23. The method of claim 22, further comprising:

driving voltage onto the thermistor in order to heat the thermistor to an operating temperature corresponding to the reference resistance prior to the step of passing gas eluting from a mixture of a gaseous compound and a reference gas over the thermistor.

24. The method of claim 22, wherein the step of providing an infallible resistance element to limit current that can enter the thermistor due to failure of an integrated circuit component comprises: providing a first infallible resistance element to limit current that can enter the thermistor from an operational amplifier that drives voltage onto the thermistor.

25. The method of claim 25, wherein the step of providing an infallible resistance element to limit current that can enter the thermistor due to failure of an integrated circuit component comprises:

providing a second infallible resistance element to limit current that can enter the thermistor from a difference amplifier that compares the detector signal to the reference signal and a buffer amplifier that sends the detector signal to the difference amplifier.

26. The method of claim 22, wherein the step of driving voltage onto the thermistor to maintain the thermistor at a reference resistance comprises: driving the voltage from a power supply that is isolated by the zener diode barrier through a reference resistor having the reference resistance onto the thermistor.

27. A precision voltage regulator device comprising:

a monolithic voltage regulator device; and

a monolithic voltage reference connected in a feedback circuit of the monolithic voltage regulator device, wherein the monolithic voltage regulator device is configured to generate a regulated output voltage based on a reference voltage added to the feedback circuit of the monolithic voltage regulator device by the monolithic voltage reference.

28. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device is configured to generate the regulated output voltage using the reference voltage added to the feedback circuit of the monolithic voltage regulator device by the monolithic voltage reference and an internal reference voltage of the monolithic voltage regulator.

29. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a LP2951 AC monolithic voltage regulator device.

30. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a LM317A monolithic voltage regulator device.

31 . The precision voltage regulator device of claim 27, wherein the monolithic voltage reference comprises a shunt reference.

32. The precision voltage regulator device of claim 27, wherein the monolithic voltage reference comprises a TL4050A5 voltage reference device.

33. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a ground referencing feedback circuit.

34. The precision voltage regulator device of claim 33, wherein the feedback circuit of the monolithic voltage regulator device comprises a first feedback resistor and a second feedback resistor, and the monolithic voltage reference is disposed before the first feedback resistor in the feedback circuit of the monolithic voltage regulator device.

35. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises an output referencing feedback circuit.

36. The precision voltage regulator device of claim 35, wherein the feedback circuit of the monolithic voltage regulator device comprises a first feedback resistor and a second feedback resistor, and the monolithic voltage reference is disposed between the second feedback resistor and a ground in the feedback circuit of the monolithic voltage regulator device.

37. The precision voltage regulator device of claim 27, wherein the monolithic voltage reference comprises a plurality of monolithic voltage references connected in series in the feedback circuit of the monolithic voltage regulator device.

38. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a linear voltage regulator device.

39. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a monolithic switching regulator device.

40. The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a positive supply regulator.

41 . The precision voltage regulator device of claim 27, wherein the monolithic voltage regulator device comprises a negative supply regulator.

42. A method for precision voltage regulation comprising:

providing a monolithic voltage reference in a feedback circuit of a monolithic regulator device;

receiving an unregulated input voltage at the monolithic regulator device; and

generating a regulated output voltage using the reference voltage added to the feedback circuit by the monolithic voltage reference.

43. The method of claim 42, wherein the step of generating a regulated output voltage using the reference voltage added to the feedback circuit by the monolithic voltage reference comprises:

generating the regulated output voltage using the reference voltage added to the feedback circuit by the monolithic voltage reference and an internal reference of the monolithic voltage regulator.

44. The method of claim 42, wherein the monolithic voltage regulator device comprises a LP2951 AC monolithic voltage regulator device.

45. The method of claim 42, wherein the monolithic voltage regulator device comprises a LM317A monolithic voltage regulator device.

46. The method of claim 42, wherein the monolithic voltage reference comprises a shunt reference.

47. The method of claim 42, wherein the monolithic voltage reference comprises a TL4050A5 voltage reference device.

48. The method of claim 42, wherein the monolithic voltage regulator device comprises a ground referencing feedback circuit and the step of providing a monolithic voltage reference in a feedback circuit of a monolithic regulator device comprises:

providing the monolithic voltage reference before a first feedback resistor in the feedback circuit of the monolithic voltage regulator device.

49. The method of claim 42, wherein the monolithic voltage regulator device comprises an output referencing feedback circuit, and the step of providing a monolithic voltage reference in a feedback circuit of a monolithic regulator device comprises:

providing the monolithic voltage reference between a feedback resistor and a ground in the feedback circuit of the monolithic voltage regulator device.

50. The method of claim 42, wherein the step of providing a monolithic voltage reference in a feedback circuit of a monolithic regulator device comprises:

providing a plurality of monolithic voltage references connected in series in the feedback circuit of the monolithic voltage regulator device.

51 . The method of claim 42, wherein the monolithic voltage regulator device comprises a linear voltage regulator device.

52. The method of claim 42, wherein the monolithic voltage regulator device comprises a monolithic switching regulator device.

53. The method of claim 42, wherein the monolithic voltage regulator device comprises a positive supply regulator.

54. The method of claim 42, wherein the monolithic voltage regulator device comprises a negative supply regulator.

55. An apparatus for providing improved precision of a control circuit, comprising:

a monolithic control circuit having a feedback circuit and an internal reference; and

a monolithic reference device connected in the feedback circuit of the monolithic control circuit, wherein the monolithic control circuit is configured to generate a control signal based on a reference added to the feedback circuit of the monolithic control circuit by the monolithic reference device.

56. The apparatus of claim 55, wherein the monolithic control circuit comprises a regulation circuit that controls a current.

57. The apparatus of claim 55, wherein the monolithic control circuit comprises a regulation circuit that controls a combination of current and voltage.

58. The apparatus of claim 55, wherein the monolithic control circuit comprises a closed loop position controller.

59. The apparatus of claim 55, wherein the monolithic control circuit comprises a pressure control loop.

60. The apparatus of claim 55, wherein the monolithic control circuit comprises a temperature control loop.

61 . An method for providing improved precision of a control circuit, comprising:

providing a monolithic reference device in a feedback circuit of a monolithic control circuit; and generating a control signal by the monolithic control circuit based on a reference added to the feedback circuit of the monolithic control circuit by the monolithic reference device.

62. The method of claim 61 , wherein the monolithic control circuit comprises a regulation circuit that controls a current.

63. The method of claim 61 , wherein the monolithic control circuit comprises a regulation circuit that controls a combination of current and voltage.

64. The method of claim 61 , wherein the monolithic control circuit comprises a closed loop position controller.

65. The method of claim 61 , wherein the monolithic control circuit comprises a pressure control loop.

66. The method of claim 61 , wherein the monolithic control circuit comprises a temperature control loop.

67. An apparatus for providing an Intrinsically Safe (IS) thermistor detection circuit, comprising:

a thermistor;

a reference resistor connected to the thermistor;

an infallible resistance element connected between the thermistor and a high impedance input of the operational amplifier to limit current entering the thermistor from the amplifier

a power source; and

a zener diode barrier connected between the power source and the reference resistor and configured to limit a maximum voltage from the power source, the zener diode barrier comprising:

a plurality of zener diodes; and a precision voltage regulator configured to generate a regulated output voltage above a minimum supply voltage required by the thermistor and below a tolerance level of the zener diodes, the precision voltage regulator comprising:

a monolithic voltage regulator device, and

a monolithic voltage reference connected in a feedback circuit of the monolithic voltage regulator device, wherein the monolithic voltage regulator device is configured to generate the regulated output voltage based on a reference voltage added to the feedback circuit of the monolithic voltage regulator device by the monolithic voltage reference.