JPH081678B2 - 2-wire measuring circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a two-wire measuring circuit, and in particular, it is configured as a so-called intelligent transmitter by incorporating a sensor and a microcomputer, and is used for industrial purposes. The present invention relates to a formula measuring circuit.

[Conventional technology]

As an example of a conventional intelligent transmitter, Japanese Patent Laid-Open No. 59-59
There is one disclosed in Japanese Patent Publication No. 114930. According to the conventional circuit disclosed in this document, a plurality of analog input signals sent from the sensor are selected by a multiplexer, the selected input signals are amplified to a predetermined level by a variable gain amplifier, and then the analog signal is digitally converted. In order to convert into a signal, the pulse width obtained by comparing the magnitude of the free-running triangular wave with the output of the variable gain amplifier is time-measured and converted into a digital value. This digital value is corrected and normalized by a built-in microcomputer.

[Problems to be Solved by the Invention]

In the conventional measurement circuit described above, when measuring the pulse width, the voltage level of the triangular wave is compared with the voltage level of the input signal to measure the pulse width. However, there is a problem in that it is easily affected by noise, and A / D conversion cannot be performed accurately. Also, if the signal from the sensor contains noise, the A / D conversion value instantaneously changes in proportion to the instantaneous peak value of the noise. Especially in the 2-wire type measurement circuit, because each circuit unit operates with a very small amount of power, it is limited to suppress noise and improve the measurement accuracy due to the limitation of mounting and the limit of available devices. There was a problem that

A first object of the present invention is to provide a two-wire measurement circuit that has a measurement circuit system resistant to noise and can achieve high measurement accuracy.

A second object of the present invention is to use a well-known integral type A / D conversion circuit and configure it by an optimum bias method.
It is to provide a line type measuring circuit.

A third object of the present invention is to provide a two-wire measuring circuit which can reduce performance requirements for the device, although the two-wire measuring circuit is originally limited in active devices that can be used. To do.

[Means for solving the problem]

The first two-wire type measurement circuit according to the present invention includes at least two or more sensors for measuring various physical quantities,
An excitation circuit that holds these sensors in an active state, a switch circuit that selectively inputs one of the output signals of the plurality of sensors based on a control signal, and a switch circuit that is configured so that the gain can be changed by the control signal. An operational amplifier for amplifying the output signal from the operational amplifier, a summing amplifier (A3) for adding the output voltage of the operational amplifier and the first reference voltage (V1) and shifting the output voltage of the operational amplifier to a required level, The output voltage (VA) of the summing amplifier (A3) is input, the input voltage (VA) is subtracted from the second reference voltage (VR), and after integration for a predetermined time, the voltage (VA) is replaced with the third voltage. Of the reference voltage (V3), subtracts the input voltage (V3) and the second reference voltage (VR) and integrates, and the output voltage of the integrator, which is arranged in the subsequent stage of the integrator. Voltage ratio to compare the second reference voltage (VR) with With a comparator (A5)
Is greater than the second reference voltage (VR) and the second reference voltage (VR) is greater than the third reference voltage (V3).

A second two-wire measuring circuit according to the present invention includes at least two sensors for measuring various physical quantities,
An excitation circuit that holds these sensors in an active state, a switch circuit that selectively inputs one of the output signals of the plurality of sensors based on a control signal, and a switch circuit that is configured so that the gain can be changed by the control signal. Amplifier for amplifying the output signal from the differential amplifier, and an addition amplifier (A3 for adding the output voltage of the differential amplifier and the first reference voltage (V1) to shift the output voltage of the differential amplifier to a required level. ) And the output voltage (VA) of this summing amplifier (A3) are input, the input voltage (VA) and the second reference voltage (VR) are subtracted, integrated for a predetermined time, and then converted to the voltage (VA). The third reference voltage (V3) is input, the integrator that subtracts and integrates the input voltage (V3) and the second reference voltage (VR), and the integrator that is arranged after this integrator Voltage comparison that compares the output voltage with the second reference voltage (VR) With a container (A5), the first
Is smaller than the second reference voltage (VR), and the second reference voltage (VR) is smaller than the third reference voltage (V3).

A third two-wire measuring circuit according to the present invention is the two-wire measuring circuit of the first or second configuration, wherein the second reference voltage (VR), the first reference voltage (V1), and the third Reference voltage (V
3) is the voltage obtained based on the same reference voltage.

A fourth two-wire measuring circuit according to the present invention is configured such that, in the above-mentioned first structure, the pulse width of the voltage comparator is time-measured and the obtained time-series digital value is corrected and calculated by a built-in microcomputer. To be done.

In a fifth two-wire measuring circuit according to the present invention, in the first configuration, the sensor is configured such that the impedance changes depending on the physical quantity of the measurement target.

A sixth two-wire type measurement circuit according to the present invention is the positive power source terminal of the summing amplifier (A3) and a constant voltage source (Vcc) for driving the circuit of the summing amplifier in the first or second configuration.
A resistor (34) is connected in series between and, and a bypass capacitor (35) is connected between the positive power source terminal and the negative power source terminal of the summing amplifier (A3).

[Action]

In the two-wire measuring circuit according to the present invention, a double integration type in which the time until the constant value integral value of the signal voltage and the constant voltage integral value become equal to each other is measured and the digital value is obtained
It uses an A / D converter to improve precision and noise immunity, and to add a summing amplifier (preprocessing amplifier circuit) with a required bias relationship to a variable gain amplifier and an integral A / D converter.
By providing it between the converter and the converter, the integration type A / D converter can be applied to the two-wire measuring circuit.

Further, in the 2-wire type measurement circuit according to the present invention, the reference voltage, the excitation voltage, etc. of each part of the circuit are generated by dividing or amplifying the reference voltage based on a single reference voltage, thereby realizing the 2-wire type measurement circuit. To

〔Example〕

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an embodiment of a two-wire type measuring circuit according to the present invention, and FIG. 2 is a circuit diagram showing in detail a main part of the circuit shown in FIG. In FIG. 1, the solid line indicates the flow of power, the white arrow indicates the flow of analog signals, and the hatched arrow indicates the flow of digital signals.

First, the overall configuration will be described with reference to FIG. In FIG. 1, 1 is a two-wire measuring circuit and 2 is a receiving device. The two-wire measuring circuit 1 and the receiving device 2 are connected by a transmission line 3 formed by two wires. The receiving device 2 includes a receiving circuit 4, a receiving resistor (Rs) 5, and a DC power source (Vs) 6.
The receiving resistor 5, the DC power source 6, and the two-wire measuring circuit are connected in series, and the receiving circuit 4 is connected so as to input the terminal voltage of the receiving resistor 5. The current value Is of the transmission signal flowing through the transmission line 3 is generally a DC constant current of 4 to 20 mA, and
Normally, 24V is used for Vs. As a result, the 2-wire measuring circuit 1 is designed to operate with a small electric power of an applied voltage of about 10 V and a current of 4 mA or less. In this way, the two-wire measurement circuit is made to satisfy the low power consumption. The two-wire measuring circuit 1 is configured as a two-terminal constant current source that operates in response to each sensing operation of a plurality of sensors provided inside, and the receiving circuit 4 outputs from the two-wire measuring circuit 1. The voltage across the terminals of the receiving resistor 5 generated based on the transmission current Is is taken in and measured, thereby obtaining the measured quantity detected by the sensor.

An external communicator 7 is connected to the transmission line 3.

In the two-wire measurement circuit 1, a plurality of, for example, three sensors 8,
9 and 10, these sensors 8, 9 and 10 are excited to a detectable state by an excitation circuit 11. Reference numeral 12 is a constant voltage circuit, and the constant voltage circuit 12 converts the voltage transmitted through the transmission line 3 into a predetermined constant voltage. The output voltage of the constant voltage circuit 12 supplies a stable voltage to each circuit portion of the two-wire measuring circuit, but in FIG. 1, only the supply line to the excitation circuit 11 is shown. 13 is a multiplexer, sensors 8, 9, 10
The output signal from the switch is appropriately selected by a switch based on the control signal, selected, and fetched. Reference numeral 14 is a variable gain amplifier, the gain of which is adjusted to a required value by a control signal. The variable gain amplifier 14 amplifies the sensor output signal selected by the multiplexer 13 and sends it to the double integration type A / D converter 15 in the next stage. The A / D converter 15 converts the sensor detection signal amplified to a predetermined level into a digital value. Circuit configurations and operations of the variable gain amplifier 14 and the A / D converter 15 will be described later in detail with reference to FIG.

Reference numeral 16 is a CPU (microcomputer) having a signal calculation / control processing function, and 17 is a memory attached to the CPU 16. Reference numeral 18 denotes a D / A converter that converts the result calculated / corrected by the CPU 16 into an analog signal again, and the output of the D / A converter 18 is converted into a corresponding current by the voltage / current converter 19.
As another circuit configuration of the two-wire measurement circuit 1, a digital transmission / reception circuit 20 for performing mutual communication with the above-described external communicator 7 is added. Also above CPU16
Includes a multiplexer 13, a variable gain amplifier 14, and an A / D converter 1
The above-mentioned control signals for setting those circuit conditions are given to 5.

In the 2-wire type measurement circuit 1 having the above configuration, the CPU 16
Scans the output signals of the sensors 8 to 10 and other signals, such as the excitation voltage and the input short-circuit state signal of the variable gain amplifier 14, according to the operation program and control data stored in the memory 17, and fetches the measurement data. The analog sensor detection signal selected by the multiplexer 13 is amplified to a required level by the variable gain amplifier 14 set to an appropriate gain under the control of the CPU 16, and then converted into a digital value by the A / D converter 15. To be converted.

As described above, the two-wire measuring circuit 1 is composed of an analog circuit portion and a digital circuit portion, but the circuit configuration of the analog circuit portion particularly determines the measurement limit. That is, it is an important point in making the two-wire measuring circuit 1 that the analog circuit part can operate with a single power supply and can interface well with the digital circuit part.

Next, a detailed circuit configuration of the analog circuit portion in the two-wire measuring circuit 1 will be described with reference to FIG.

In FIG. 2, 8 to 10 are sensors, 11 is an excitation circuit, 12 is a constant voltage circuit, 13 is a multiplexer, 14 is a variable gain amplifier, and 15 is an A / D converter. Each of the sensors 8 to 10 is composed of, for example, two or four resistors, and the excitation circuit 11
The output voltage V _{E of} is applied. The detection output of the sensors 8 to 10 is generated corresponding to the impedance that changes depending on each physical quantity to be measured. The outputs of the sensors 8-10 are input to the multiplexer 13. Constant voltage circuit 12 is constituted by a series circuit of a resistor 31 and a Zener diode ZD, the constant voltage V _R is extracted as the terminal voltage of the Zener diode ZD. The excitation circuit 11 is configured as a non-inverting amplifier circuit using the operational amplifier A7, amplifies the constant voltage V _R to a predetermined level, and generates the voltage V _E. The variable gain amplifier 14 is an operational amplifier
Resistor group 3 consisting of A1 and A2 and multiple resistors connected in ladder form
2 and a plurality of switch elements 33. Second
In the figure, the in-phase component of the sensor output voltage at the output of the variable gain amplifier 14 is shown as V _C. The A / D converter 15 is composed of a preprocessing amplifier circuit 15A and a double integration type A / D converter 15B. The preprocessing amplifier circuit 15A includes an operational amplifier A3 and resistors R1 to R4.
The double integration A / D converter 15B is composed of amplifiers A4 to A6, resistors R5 to R7, switches SW1 to SW3, and an integrating capacitor C. The double integration type A / D converter 15A is already known in principle, and its operation is described in, for example, Japanese Patent Laid-Open No. 42-2509.
No. 0 is disclosed. In Fig. 2, double integral type A
The structure of the control logic part of the / D converter is omitted.

The analog circuit portion having the above-described configuration is designed to operate with a single power source, as described later, in order to operate the two-wire measuring circuit 1. 2-wire measurement circuit 1
Since the analog circuit part that operates with low power and the digital circuit part including the CPU coexist, pulse-like noise is mixed from the digital circuit part to the analog circuit part through the power supply system. Is inevitable. In the present embodiment, the problem of such mixing of pulse noise is solved by employing an integrating A / D converter and designing the circuit as described later.

Next, the operation of the integrator will be described. Here, the output voltage (first reference voltage) V1 of the amplifier A6 is higher than the constant voltage (second reference voltage) VR, and this voltage VR is the switch SW2.
Is set to be higher than the voltage (third reference voltage) V3 supplied to the. Further, since the reference voltage V1 is added to the output of the variable gain amplifier 14 in the operational amplifier (summing amplifier) A3, the output voltage VA of the amplifier A3 is the constant voltage VR.
Will be greater. As a result, the relation of the voltage of each part is VA
>VR> V3 is satisfied. In the first phase, the switches SW1 and SW2 are both off, the switch SW3 is on, and the integrating capacitor C is completely discharged. At this time, the integrating circuit 15B functions as an inverting amplifier,
The value of the output voltage V0 is the same as the voltage VR. In the second phase, switch SW1 is on, switches SW3 and SW2
Is off, and at the same time when the switch SW1 is turned on, a counter (not shown) starts counting the reference clock. In this case, the output voltage V ₀ of the integrating operational amplifier A4 is given by the following equation.

At this time, the voltage V0 operates to drop from the voltage VR.

The output after the counter has counted NS is V0 = VR- (VA-VR) ((NS [tau]) / (R7.C)) when the clock time is [tau].

In the third phase, when the count value reaches a constant value N _S , the switch SW1 is turned off and the switch SW2 is turned on to switch the input of the integrating operational amplifier A4 to the reference voltage V3 given by the reference voltage, and The counter newly starts counting. The output of the integrating operational amplifier A4 after t ′ is Becomes At this time, the voltage V0 rises.

Next, the voltage comparator A5 at the next stage detects the moment when V ₀ = V _R in the relation between the output voltage V ₀ of the integrating operational amplifier A4 and the reference voltage V _R, and counts the counter by N. Assuming that it is stopped at ₀ , the following relational expression holds.

− (VA−VR) (NSτ / (R7 · C)) + VR (N0τ / (R7 · C)) = 0 From this formula, Thus, the analog voltage V _A input to the integrating A / D converter 15B is converted into the digital amount N ₀ .

According to the conversion process of the double integration type A / D converter 15B as described above, the input voltage V _A or the reference voltage V _R is subjected to the integration process, so that pulse noise is mixed into the input voltage and the reference voltage. However, if the existence time of the noise is shorter than the integration time N _S τ, it does not significantly affect the output value, and the error caused by the noise hardly occurs. Moreover, in the integral type A / D converter, the conversion time becomes long, but since the input signal is averaged in analog during that time,
A sufficient S / N ratio can be secured by performing A / D conversion once.

The above operation is performed by another type of A / D converter (for example, R-2R).
Compared with the case of using the successive approximation method using a resistance ladder, the method disclosed in JP-A-59-114930, etc.,
In these types of A / D converters, the timing for determining the digital value is determined by comparing the instantaneous value of the input voltage with a required reference value. Therefore, considering the possibility that pulse-like noise originating from the digital circuit part is mixed, a circuit configuration is required that limits the response band of the variable gain amplifier 14 or repeats measurement to digitally obtain an average value. To be done. Therefore, in the CPU 16, although the processing capability is reduced due to the low power operation, the data acquisition process is frequently requested, and the time for performing the correction calculation process which is the original work is limited. The problem of receiving it occurs. Further, in order to limit the band of the variable gain amplifier 14, every time the output signals from the sensors 8 to 10 are switched by the multiplexer 13, the A / D conversion operation cannot be started until the operation of the variable gain amplifier 14 becomes stable. The problem occurs. Therefore, due to the above problems, it is practically impossible to realize a two-wire measuring circuit when using an A / D converter other than the integral type.

In the two-wire measurement circuit, in order for the circuit to operate normally, it is required that all voltage signals of each part of the circuit have the same polarity with respect to a reference potential (ground potential). The circuit condition for the single power supply operation in the circuit of FIG. 2 is V1> V _R > V3 or V1 <V _R <V3 with respect to the magnitude relationship among the voltages V1, V _R and V3 of each part in the circuit. Therefore, the circuit of this embodiment is designed to satisfy this condition.

However, in the case of V1 <VR <V3, the rise and fall of the voltage V0 in the case of V1>VR> V3 are reversed. In either case, the voltage V0 needs to rise and fall on the basis of the voltage VR, the voltages V1 and V3 have different voltage values, and the voltage VR is between the voltages V1 and V3. Must be the voltage value of.

Therefore, as described above, V1>VR> V3 or V1 <VR <
Must meet one of the V3 requirements.

Under the condition of V1 <VR <V3, in the third phase, the output of the integrating operational amplifier A4 after t ′ seconds is as follows.

In the output signals of the sensors 8 to 10, the common mode voltage with respect to the ground potential is not always equal. In the circuit of FIG. 2, the sensor 8
Common-mode voltage is generated by the excitation circuit regardless of which sensor is selected.
It is set to be about half the output voltage V _E of 11.
The operational amplifiers A1 and A2 in the variable gain amplifier 14 are differential input and differential output amplifiers. Therefore, the variable gain amplifier 14 does not amplify the in-phase voltage component in the output voltage of each sensor, but only the differential voltage component. In this way, the circuit is configured to cancel the influence of the in-phase component of the sensor output to a value that can be practically ignored. Variable gain amplifier 14
The differential voltage component of the sensor output amplified by is amplified to a predetermined level by the pre-processing amplifier circuit 15A that performs level shift addition amplification in the next stage, and is supplied to the stage of the double integration A / D converter 15B. It In the two-wire measurement circuit that requires a single power supply operation, the relationship of R1 / R2 = R3 / R4 is established so that the pre-processing amplifier circuit 15A satisfies the in-phase component cancellation condition. In the double integration type A / D converter 15B, as described above, the summing amplifier A3
After integrating the output V _A for a predetermined time, it is integrated in the reverse direction at the ground potential or constant potential, and the time until the output voltage of the integrator of the A / D converter 15B matches the reference voltage V _R is counted. Is measured and the digital value corresponding to the sensor output is obtained. Also in the insensitive state integration time with respect to the resistor R7 in the integrator, the potential of the positive input terminal of the integrator i.e. integrating operational amplifier A4, it is necessary to match the reference potential V _R of the voltage comparator A5.

As described above, in order for the integrator of the double integration type A / D converter 15B to properly perform the dual slope integration operation, the output _VA of the operational amplifier A3 for addition amplification is the output of the sensor to be measured. Regardless of signal magnitude and polarity, always V _A ≧ V _R or V _A ≦
Must be V _R. That is, it is forbidden to reverse the magnitude relationship between V _A and V _R during measurement.

Therefore, in the present invention, the above V
In order to satisfy A ≧ VR, a summing amplifier A3 is arranged between the variable gain amplifier 14 and the double integration type A / D converter 15B, and V1>
A voltage V1 satisfying VR is added to the input signal of the summing amplifier A3. In order to satisfy the above VA ≦ VR, V1 <
The voltage V1 satisfying VR is added to the input signal of the summing amplifier A3.

When the above-mentioned conditions are satisfied in the two-wire type measurement circuit 1, the entire circuit including the sensor circuit can be operated between a single voltage and the ground voltage, and a single power supply operation can be performed. To give a concrete example of numerical values, V _CC = 6.2V, V1 ＝
_{3.65V, V R = 2.1V, V3} = 0V, R1 = R3 = 68KΩ, R2 = R4 = 92
KΩ and R7 = 420KΩ.

Next, in the 2-wire measuring circuit 1, since the transmission signal Is is 4 mA at the minimum, it is necessary that all circuits operate with power consumption of 4 mA or less. Therefore, each part of the circuit is operated with a current as small as possible. When the operating current is small, the analog circuit portion shown in FIG. 2 is likely to receive noise from the digital circuit portion via the power supply. Therefore, in this embodiment, a countermeasure is taken by inserting a resistor 34 between the power supply terminal of the operational amplifier A3 for summing amplification and the power supply V _CC and forming a bypass path by the capacitor 35 in this. Operational amplifier
If the consumption current of A3 is 100 μA, a resistor of several KΩ can be inserted, and if the bypass capacitor is 1 μF, the filter effect can be expected at about 100 Hz or more. The circuit has a narrow band due to low power consumption. Therefore, the power supply filter needs to be an effective constant even with a resistor. In order to obtain the same effect with only a capacitor or an LC filter as in the conventional case, the element constant becomes extremely large and the practicality is low.

Further, as described above, the circuit of this embodiment is configured to generate a constant single reference voltage by the constant voltage diode ZD. Then, based on this single reference voltage, the non-inverting operational amplifiers A6 and A7 are used to obtain a required predetermined voltage.

In the two-wire measuring circuit of this embodiment, the sensors 8 to 10 are of a type in which the impedance changes depending on the physical quantity to be measured. In such a case, the sensor output ΔVs
Is proportional to V _E , that is, V _R , and ΔVs = kV _R. As described above, the sensor output is proportional to V _R in its A / D conversion value, so k is And the conversion result does not depend on V _R. Therefore, the reference voltage
The circuit of the reference voltage source that generates V _R does not need to be high performance, and low cost devices can be used.

As shown in FIG. 2, the variable gain amplifier 14 uses a ladder resistance circuit to provide a feedback section. According to such a circuit configuration, a large gain variable width can be obtained with a narrow resistance ratio. FIG. 3 shows a detailed circuit configuration of the variable gain amplifier 14. According to this circuit, ladder-connected resistors r1
~ R12 and if the resistors r1 to r12 are all the same resistance, the gain of the variable gain amplifier 14 can be changed up to 3.41 to 575 times. Proper selection of the constant makes it possible to select the gain for each step in any geometric series. If the resistors r8, r9, and r10 do not exist, the required ratio of the maximum value and the minimum value of the resistance reaches several tens of times. According to the configuration of FIG. 3, the variable gain amplifier 14 can be configured with only resistors having extremely small resistance ratios. In monolithic ICs and hybrid ICs, it is desired that the resistance ratio be several times or less in order to obtain a highly accurate resistance ratio and temperature coefficient ratio. Therefore, it is clear that the configuration of FIG. 2 contributes to the performance improvement.

By measuring the short-circuited state of the input terminal of the multiplexer 13 or the constant voltage source instead of the output signals of the sensors 8 to 10, the influence of the sensitivity and offset of the variable gain amplifier 14 can be automatically corrected.

〔The invention's effect〕

As is clear from the above description, according to the present invention, it is possible to configure a two-wire measurement circuit using an integral A / D converter having high measurement accuracy and noise resistance, and use a plurality of various sensors. Since the analog / digital mixed circuit capable of reducing the influence of the external disturbance can be configured, the cost reduction and the high performance of the two-wire measurement circuit can be easily achieved. Further, according to the present invention, since the reference voltage source is commonly used, it is possible to further improve the performance and reduce the cost. Furthermore, since a predetermined resistance and a bypass capacitor are provided between the adding amplifier and the power supply, noise resistance is further improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing the overall configuration of a two-wire type measuring circuit according to the present invention, FIG. 2 is a circuit diagram showing a detailed configuration of the main parts of the two-line type measuring circuit, and FIG. 3 is a variable gain amplifier. 3 is a detailed circuit diagram of FIG. [Explanation of symbols] 1 ... 2-wire type measurement circuit 2 ... Reception device 3 ... Transmission line 8,9,10 ... Sensor 11 ... Excitation circuit 12 ... Constant voltage circuit 13 ... Multiplexer 14 ... Variable Gain amplifier 15 …… A / D converter 15A …… Preliminary amplification circuit 15B …… Double integration type A / D converter 16 …… CPU (microcomputer)

Claims (6)

[Claims] 1. At least two or more sensors each measuring various physical quantities, an excitation circuit for keeping these sensors in an active state, and selectively one of a plurality of output signals of the sensors based on a control signal. And a differential amplifier configured to change the gain by a control signal and amplifying an output signal from the switch circuit, and an output voltage of the operational amplifier and a first reference voltage (V1) are added. Then, the summing amplifier (A3) for shifting the output voltage of the differential amplifier to a required level and the output voltage (VA) of the summing amplifier (A3) are input, and the input voltage (VA) and the second reference After subtracting the voltage (VR) and integrating for a predetermined time, the third reference voltage (V3) is input instead of the voltage (VA), and the input voltage (V3) and the second reference voltage (VR) are input. The integrator that subtracts and integrates And a voltage comparator (A5) that is arranged in a subsequent stage of the voltage comparator and that compares the output voltage of the integrator with the second reference voltage (VR). The first reference voltage (V1) is the second reference voltage (V1). A two-wire measuring circuit characterized in that it is higher than VR) and the second reference voltage (VR) is higher than the third reference voltage (V3). 2. At least two or more sensors each measuring various physical quantities, an excitation circuit for holding these sensors in an active state, and a selective one of the output signals of the plurality of sensors based on a control signal. And a differential amplifier configured to change the gain by a control signal and amplifying an output signal from the switch circuit, an output voltage of the differential amplifier, and a first reference voltage (V1). Is added to shift the output voltage of the differential amplifier to a required level, and the output voltage (VA) of the summing amplifier (A3) is input, and the input voltage (VA) and the second voltage After subtracting the reference voltage (VR) of the above and integrating for a predetermined time, the third reference voltage (V3) is input instead of the voltage (VA), and the input voltage (V3) and the second reference voltage (VR ) And an integrator that subtracts and integrates A voltage comparator (A5) that is arranged in the latter stage of the divider and that compares the output voltage of the integrator and the second reference voltage (VR), and the first reference voltage (V1) is the second reference voltage A two-wire measuring circuit characterized in that it is smaller than (VR) and the second reference voltage (VR) is smaller than the third reference voltage (V3). 3. The two-wire measuring circuit according to claim 1, wherein the second reference voltage (VR) and the first reference voltage (V
1) and the third reference voltage (V3) are voltages obtained based on the same reference voltage, respectively 2
Wire measuring circuit. 4. The two-wire measuring circuit according to claim 1,
A two-wire measuring circuit, characterized in that the pulse width of the voltage comparator is time-measured, and the time-series digital value obtained is corrected and calculated by a built-in microcomputer. 5. The two-wire measuring circuit according to claim 1,
The two-wire measurement circuit, wherein the sensor is configured so that the impedance changes depending on the physical quantity of the measurement target. 6. The two-wire measuring circuit according to claim 1,
A resistor (34) is provided between the positive power supply terminal of the adding amplifier (A3) and a constant voltage source (Vcc) for driving the circuit of the adding amplifier (A3).
Is connected in series and a bypass capacitor (35) is connected between the positive power supply terminal and the negative power supply terminal of the summing amplifier (A3).