GB2298048A - Apparatus for converting capacitance to voltage - Google Patents

Apparatus for converting capacitance to voltage Download PDF

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Publication number
GB2298048A
GB2298048A GB9502315A GB9502315A GB2298048A GB 2298048 A GB2298048 A GB 2298048A GB 9502315 A GB9502315 A GB 9502315A GB 9502315 A GB9502315 A GB 9502315A GB 2298048 A GB2298048 A GB 2298048A
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Prior art keywords
voltage
capacitor
apparatus
capacitance
output
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GB9502315A
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GB9502315D0 (en )
Inventor
Mark Sanders
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Mark Sanders
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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01LMEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L9/00Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material by electric or magnetic means
    • G01L9/12Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material by electric or magnetic means by making use of variations in capacitance, i.e. electric circuits therefor
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01DMEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/14Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
    • G01D5/24Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R27/00Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
    • G01R27/02Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
    • G01R27/26Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
    • G01R27/2605Measuring capacitance

Abstract

The apparatus comprises a pair of analog switches (10,11) connected in series between a stable direct voltage source (14) and earth (15), an oscillator (13) for switching the analog switches (10,11) in anti-phase to each other, a charge buffer amplifier (18) connected through a capacitor (16) (the capacitance of which is to be converted to voltage) to a common terminal between the analog switches (10,11), a feedback capacitor (21) of similar capacitance to capacitor (16) connected in parallel-parallel negative feedback between the output (20) and the input (17) of the charge buffer amplifier (18), and output means (23) connected to the output (20) of the charge buffer amplifier (18) to provide the converted voltage.

Description

APPARATUS FOR CONVERTING CAPACITANCE TO VOLTAGE This invention relates to apparatus for converting capacitance to voltage so that the capacitance of a capacitor can be converted to a corresponding level of voltage, particularly to measure the level of the capacitance either directly or after subsequent processing.

The capacitance of a capacitor is frequently not easily measureable directly, especially if the capacitor is subjected to an external stimulus which affects the value of its capacitance. An example of such a situation is the use of a capacitor in a transducer in which an external stimulus of eg: pressure or humidity, affects the value of the capacitance of the capacitor, and the changes in capacitance level need to be measured or utilised to measure or monitor the pressure or humidity changes.

Consequently it is common in the prior art to convert the changes of capacitance level to changes of voltage which are more readily measured or utilised to measure or monitor the pressure or humidity changes; the apparatus which can carry out this conversion is referred to as a capacitance-to-voltage converter.

The accuracy of a capacitance-to-voltage converter needs to be as high as possible to minimise the introduction of errors by the conversion. The operator has to rely on the converter voltage output to measure the capacitance of the capacitor, and in the example of pressure or humidity changes being monitored or measured by a transducer of which the active part is the capacitor concerned, the accuracy of the capacitance-to-voltage converter is key to the accuracy of measurement of pressure or humidity changes.

It is an object of the present invention to provide apparatus for converting capacitance to voltage with high accuracy.

In accordance with a first aspect of the present invention, apparatus for converting capacitance to voltage comprises a pair of analog switches in series connection through a common terminal and for connection between a stable direct voltage source and earth, digital driving means for switching the analog switches in anti-phase to each other, a charge buffer amplifier for connection to the common terminal through a capacitor of which the capacitance is to be converted to voltage, a feedback capacitor of similar capacitance to that to be converted connected in parallel-parallel negative feedback between the output and the input of the charge buffer amplifier, and output means connected to the output of the charge buffer amplifier to provide the converted voltage.

Preferably the output means is connected to the output of the charge buffer amplifier through decouplin means.

Preferably also the output means is connected to the output of the charge buffer amplifier through an active clamp amplifier.

Preferably further the apparatus as described above is incorporated in an integrated circuit assembly manufactured as a complete stimulus sensor, for example a pressure sensor.

According to another aspect of the invention, there is provided a method of converting a capacitance to a voltage using apparatus as described above.

Other preferred features of the invention will be apparent fr the following description and from the subsidiary claims of the specification.

The invention will now be further described, merely by way of example. by reference to the accompanying drawings, in which Figure 1 15 a circuit diagram of apparatus for converting capacitance to voltage according to a preferred example of the invention, Figure 2 iS a sheet of signal waveforms at varlous parts oz the circuit shown in Figure 1, Figure 3 is an exploded, part-sectioned view of an integrated circuit assembly manufactured as a complete pressure sensor and incorporating the apparatus of Figure 1.

Referring initially to Figure 1, in a preferred example of the apparatus of the invention a pair of analog switches 10 and 11 each having a switch-on resistance of about 50 ohms are connected together in series through a common terminal 12 and are coupled to digital driving means 13 for switching in anti-phase to each other. The digital driving means 13 is provided by an oscillator with complementary outputs. When it is ready for use, the input 14 of the first analog switch 10 is connected to a stable direct voltage source; a precisely regulated direct voltage supply is preferred, with an output typically of between 5 and 10 volts, bt a stabilised, smoothed, mains-derived supply of similar voltage iS acceptable although it will be subject t malns noise and failure.The output 15 of the second analog switch 11 will b earthed.

The common terminal 12 is to be connected through a capacitor 16 of which the capacitance, which is nominally about 200 pico-farads (lpF = 10-12 Farads), is to be converted to voltage, to the negative input 17 of a charge buffer amplifier i8 having a direct voltage gain of magnitude less than unity. The positive input ls of the charge buffer amplifier 18 is earthed and also connected to a screen which protects the negative input 17 against noise, and the output 20 of the charge buffer amplifier 18 is fed back in parallel-parallel negative mode to the negative input 17 through a feedback capacitor 21.The capacitor 21 is of similar capacitance to the capacitor 16 or to its range of capacitance, ie between about one half the minimum capacitance of the capacitor 16, and ten times the maximum capacitance of capacitor 16. A feedback resistor 22 connected in parallel with the feedback capacitor 21 represents the internal resistance of the feedback capacitor 21 and may be an additional resistor as well. The resistance of resistor 22 is typically 10 meg-ohms (1 megohm = 1x10^6 ohms). The charge buffer amplifier 18 produces an output voltage in direct proportion to the capacitance of the capacitor 16.

In the use of the apparatus, the output 20 provides the voltage converted from the capacitance of the capacitor 16 and comprises the output means of the converter. In this preferred example, the output 20 is connected through a decouplIng capacitor 23 of 0.1 micro-f rads to the positive input terminal 24 of an active clamp 25. The input terminal 2 is earthed through a decoupling resIstor 26 of 1 meom; the decoupling capacitor 23 and the decoupling resistor 26 constitute a high pass filter.

The output 27 of the active clamp 25 is connected t pair of oppositely orientated diodes 28, 29 arranged in parallel with each other. Each diode 28 or 29 has its output connected through a corresponding switchable link 30 or 31 to a common terminal 32 so that the closure of either of the links 30 or 31 (with the other link left open) enables the voltage at the common terminal 32 to be either positive or negative depending on the polarity of the rectification produced by the diode concerned.

The common terminal 32 is connected to an output terminal 33 which is connected to a negative input 34 for the active clamp 25 and is also earthed through a resistor 35 and a capacitor 36. The resistor 35 and the capacitor 36 are in parallel. The output terminal 33 constitutes the final output of the capacitance to voltage converter of this example, whereby separate means can be used to measure the voltage on the output terminal 33 either directly or after further processing, and thereby measure the capacltance of the capacitor 16. The active clamp output 33 memorises the peak or maximum level of input 24.

The way in which the circuit of Figure 1 operates will now be described in relation to the signal waveforms of Figure 2, with the capacitor 16 (of which the capacitance is to be converted to voltage and measured) connected as shown in Figure 1, the stable direct voltage source connected to the input 14 and the digital driving means 13 functioning at between 1 and 10 kilohertz, typically at about 5 kilohertz.

In Figure 2 the common vertical axis denotes the variable under consideration, and each horizontal axis denotes time for the waveform concerned; the waveforms A - J are shown.

Waveform A shows the effect of the digital driving means 13 on the first analog switch 10 which is thereby closed at 40 and open at 41 for each cycle of the digital driving means 13 with abrupt changes of state between them.

Waveform B is similar to Waveform A but relates to the second analog switch 11 which is being driven in anti-phase to the first switch 10 and is therefor closed at 42 and open at 43 exactly out of phase with the first switch 10.

Waveform C shows the stable input voltage 44 applied to the input 14. When the first analog switch 10 is closed, the input voltage is connected to the common terminal 12 and cannot leak to earth through the output 15 because the second analog switch 11 is open. Conversely. upon the opening of the first analog switch 10, that voltage source is removed, and the simultaneous closing of the second analog switch 11 connects the terminal 12 to earth.

Waveform D shows the resulting precisely chopped voltage 4* applied to the common terminal 12, and hence to the capacitor 16.

Waveform E shows the pulses 45 of current flow into the capacitor 1 with the maxlmum current t a level determlined by the voltage 44 and the resistance of the first analog switch 10. The capacitor 16 is fully charged on each input pulse 45, and fully discharged by current pulses 46 through the second analog switch 11 whenever that is closed. The charge and discharge pulses 45 and 46 are identical to each other because the resistances of the analog switches 10 and 11 are the same as each other.

Waveform F shows the feedback current pulses 47 from the output 20 through the feedback circuit of the feedback capacitor 21 and feedback resistor 22. This feedback circuit maintains the negatlve input 17 as a virtual earth, and hence the feedback current pulses 47 are identical to the charge and discharge pulses 45 and 46, buffered by the charge buffer amplifier 13.

Waveform G shows the voltage pulses 4S t the output 20.

The maximum voltage levels are directly propcrtional to the capacltance of the capacitor 16, and if that capacitance varies, for example by changing the capacitor 16 or affecting its capacitance if it is acting as a transducer, the height of the voltage pulses 48 rll charge accordingly.

Waveform H shows the effect of the decoupling capacitor 23 and the decoupling resistor 26 by displayl:. the volt z e pulses 49 at the input terminal 24.

The final waveform J shows the voltage 50 at the output 33 which quickly stabilises into a steady direct current voltage. This voltage can be either positive, as shown, resulting from closure of link 30 with link 31 left open, or negative if link 30 is open and link 31 closed. The voltage 50 varies linearly with the changes in capacitance of the capacitor 16 so that its measurement or processing electronics can be simple and inexpensive. Few components are used, increasing the reliability of the converter and minimising costs.

For the highest accuracy the capacitors 16 and 21 are of very similar construction to each other as well as being in a similar environment so that changes in external temperature, humidity etc, affect them equally. Figure 3 shows an example of an integrated circuit assembly GO manufactured as a complete pressure sensor ifl which is achieved, as will be described blow.

The integrated circuit assembly GO comprises a base pate G1, a cover plate 62 through which a port 62 trcv - access for the pressure to be sensed, and an integ@ circuit chip G4 between the plates 61 and 62. The integrated circuit chip 64 includes two identical capacitors 65, 66 whose capacitances vary according to the pressure to which the are subjected with capacitor 65 open to the port 63 and constituting capacitor 16 in the circuit diagram of Figure 1, and capacitor 66 constituting capacitor 21 of the circuit diagram of Figure 1, and sealed from external pressure with the rest of the chip 64 by a cover plate 67.The identical construction of the capacitors 65 and 66, and the way in which they are equally affected by all internal and external factors that could alter their capacitances, with the exception of the external pressure through the port G3 changing the capacitance of capacitor 65, maximises the precision of the conversion of capacitance to voltage. The base plate 61 also mounts the external leads 68 for the power supply to the input 1t, for the output 2C, for the output 24, for the output 33 and for earth.The leads for the output 32 and earth are connected to external voltage measuring means directly or after further processing. The incurpsratiûr. of the sensor in an integrated circuit further increases reliability, and reduces costs and weight.

In modifications of the preferred examples c r te invention described above, the capacitor te can be external to the apparatus, and that apparatus can be used as meter. There are many industrial applicarions for the precise measurement of capacitors in the pico-tarad rang including 1. Capacitive transducer calibration ( i.e pressure, humidity or displacement transducer 2. Component checking ( i.e Checking capacitance variations of manufactured batches 3. Radio frequency engineering ( i.e To measure the capacitance of lumped components or the changing values of variable capacitors in tuned circuits, matching networks, phase-locked loops etc.

4. Digital engineering ( i.e Checking timng capacitors in logic circuit oscillators, monostables and filtering circuits 5. Analog engineering ( i.e Operational amplifier circuits, including filters.

feedback networks and control system circuits

Claims (14)

  1. C L A I M S 1. Apparatus for converting capacitance to voltage comprising a pair of analog switches in series connection through a common terminal and for connection between a stable direct voltage source and earth, digital driving means for switching the analog switches in anti-phase to each other, charge buffer amplifier for connection to the common terminal through a capacitor of which the capacitance is to be converted to voltage, a feedback capacitor of similar capacitance to that to be converted connected in parallel-parallel negative feedback between the output and the input of the charge buffer amplifier, and output means connected to the output of the charge buffer amplifier to provide the converted voltage.
  2. 2. Apparatus as claimed in claim 1 in which the output means is connected to the output of the charge amplifier through decoupling means.
  3. 3. Apparatus as claimed in claim 1 or 2 In which the means is connected to the output of the charge buffer amplifier through an active clamp amplifier.
  4. 4. Apparatus as claimed in any of the precedIng dais # in which a feedback resistor is connected In parallel with the feedback capacitor.
  5. 5. Apparatus as claimed in any of the preceding claims in which the output means comprises a rectifying diode.
  6. 6. Apparatus as claimed in claim 5 in which the rectifying diode is one of a pair of oppositely orientated diodes arranged in parallel with one another, each diode having a switchable link operable to provide the converted voltage as direct voltage of the corresponding polarity.
  7. 7. Apparatus as claimed in any of the preceding claims incorporated in an integrated circuit.
  8. 8. A stimulus sensor comprising apparatus as claimed In claim 7 and including the capacitor of which the capactance is to be converted to voltage, said capacitor being Incorporated in the integrated circuit, and means enabling the capacitor to sense the stimulus.
  9. 9. .'. sure sensor comprising a stimulus sensor as claimed in claim 8 in which the means enabling the capacitor to sense the stimulus is a port between the capacitor and the atmosphere to enable the capacitor to sanse the pressure of the atmosphere.
  10. 10. Apparatus for converting capacitance to voltage substantially as hereinbefore descrlbed with reference to Figure and 2 of the accompanying drawings.
  11. 11. A pressure sensor substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.
  12. 12. A method of converting a capacitance to a voltage using apparatus or a sensor as claimed in any of the preceding claims.
  13. 13. A method as claimed in claim 12 in whIch the digital driving means is operated at between about 1 and 10 kilohertz.
  14. 14. A method of converting a capacitance to voltage substantially as hereinbefore claimed with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.
GB9502315A 1995-02-07 1995-02-07 Apparatus for converting capacitance to voltage Withdrawn GB9502315D0 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB9502315A GB9502315D0 (en) 1995-02-07 1995-02-07 Apparatus for converting capacitance to voltage

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB9502315A GB9502315D0 (en) 1995-02-07 1995-02-07 Apparatus for converting capacitance to voltage

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GB9502315D0 GB9502315D0 (en) 1995-03-29
GB2298048A true true GB2298048A (en) 1996-08-21

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2014042821A1 (en) 2012-09-17 2014-03-20 Semtech Corporation Capacitance measurement of high voltage device

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3886447A (en) * 1972-05-17 1975-05-27 Iwatsu Electric Co Ltd Capacitance-voltage converter
GB1580335A (en) * 1977-06-27 1980-12-03 Avery Ltd W & T Capacitance to frequency converter
WO1985003358A1 (en) * 1984-01-18 1985-08-01 Transensory Devices, Inc. Capacitive transducer and method

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3886447A (en) * 1972-05-17 1975-05-27 Iwatsu Electric Co Ltd Capacitance-voltage converter
GB1580335A (en) * 1977-06-27 1980-12-03 Avery Ltd W & T Capacitance to frequency converter
WO1985003358A1 (en) * 1984-01-18 1985-08-01 Transensory Devices, Inc. Capacitive transducer and method

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2014042821A1 (en) 2012-09-17 2014-03-20 Semtech Corporation Capacitance measurement of high voltage device
EP2895870A4 (en) * 2012-09-17 2016-07-06 Semtech Corp Capacitance measurement of high voltage device

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Publication number Publication date Type
GB9502315D0 (en) 1995-03-29 grant

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