CN111307026A - A charge-discharge capacitive sensor based on diode switch - Google Patents

Abstract

The invention discloses a charge-discharge capacitive sensor based on a diode switch, comprising: a bipolar square wave generator, a switch circuit, a transimpedance amplifier and a differential-single-ended conversion circuit; the bipolar square wave generator is used to generate a bipolar square wave generator. Polar square wave, the switch circuit is a diode switch circuit, and the bipolar square wave controls the charge and discharge of the differentially changed capacitor pair to be measured under the control of the switch circuit; the transimpedance amplifier is used to convert the charge and discharge current into a Equivalent large reverse voltage; differential-single-ended conversion circuit is used to convert a pair of equally large reversed voltages output by the transimpedance amplifier into single-ended voltage and output; in the switching circuit, transimpedance amplifier and differential- Under the mutual cooperation of the single-ended conversion circuits, the relative capacitance of the capacitance pair to be measured is converted into a voltage value that is easy to measure; the precision of capacitance difference detection is improved. The present invention can obtain high sensitivity and high resolution performance, and at the same time has the advantages of relatively simple circuit structure, low power consumption and the like.

Description

A charge-discharge capacitive sensor based on diode switch

technical field

The invention belongs to the field of high-precision capacitance detection, and more particularly, relates to a charge-discharge type capacitance sensor based on a diode switch.

Background technique

As a traditional non-contact sensor, high-precision capacitive displacement sensor has the advantages of high sensitivity and resolution, good linearity, and strong anti-interference ability, and is widely used in inertial measurement devices, such as accelerometers, gyroscopes, and seismometers. Among the precision measurement devices such as , gravimeters, gravity gradiometers, etc., such instruments have a wide range of uses and important positions in space science, earth science and other fields. At present, the transformer-based bridge capacitive displacement sensor is widely used. The resolution of this sensor is extremely high, but the circuit structure is relatively complex, and the post-processing circuit is relatively cumbersome, which is not conducive to the realization of the design requirements of low power consumption.

Capacitance sensors based on the principle of capacitive charging and discharging usually use analog switching devices to control the charging and discharging process (such as Chinese invention patents CN 105652099 B and CN 103888128 B), which require external control signals, have complex structures, and their dynamic performance is easily affected by analog switching devices. Limited by factors such as dynamic response time, on-resistance, etc.

SUMMARY OF THE INVENTION

Aiming at the defects of the prior art, the purpose of the present invention is to provide a charge-discharge capacitive sensor that can realize high-precision capacitance difference detection and has a relatively simple structure.

The invention provides a charge-discharge capacitive sensor based on a diode switch, comprising: a bipolar square wave generator, a switch circuit, a transimpedance amplifier and a differential-single-ended conversion circuit; the bipolar square wave generator is used to generate dual Polar square wave, the switch circuit is a diode switch circuit, and the bipolar square wave controls the charge and discharge of the differentially changed capacitor pair to be measured under the control of the switch circuit; the transimpedance amplifier is used to convert the charge and discharge current into a Equivalent large reverse voltage; the differential-single-ended conversion circuit is used to convert a pair of equally large reverse voltages output by the transimpedance amplifier into single-ended voltages and output; in the switching circuit, the transimpedance amplifier and the differential- Under the mutual cooperation of the single-ended conversion circuits, the relative capacitance of the capacitance pair to be measured is converted into a voltage value that is easy to measure; the precision of capacitance difference detection is improved.

Further, the bipolar square wave generator includes: a crystal oscillator, a counter, a voltage reference source module and an analog switch; after the frequency generated by the crystal oscillator is divided by the counter, under the action of the analog switch, it is compared with the voltage reference. The source modules are combined to form a bipolar square wave whose frequency and amplitude are adjustable within a certain range. The frequency is determined by the crystal oscillator and the counter, and the bipolar voltage amplitude is determined by the reference voltage source.

Wherein, the voltage reference source module can be connected in parallel by a plurality of voltage reference sources, so as to have the effect of noise suppression.

Furthermore, a diode can be selected as a switch control device, and used in conjunction with a bipolar square wave, the capacitor pair to be measured can be charged and discharged automatically, and no external switch control signal is required. Specifically, the switch circuit includes: diode D ₁ , diode D ₂ , diode D ₃ and diode D ₄ . _The anode of diode D1 and the cathode of diode D2 are connected to the non _- common end of _Ca in the capacitor pair to be measured, and the cathode of diode D1 and the anode of diode _D3 are connected to the _first input end of the transimpedance amplifier _; diode D4 The anode of the diode _D3 and the cathode of the diode D3 are connected to the non _- common terminal of _Cb in the capacitor pair to be measured, and the cathode of the diode D4 and the anode of the diode D2 are connected to the _second input terminal of the transimpedance amplifier.

Among them, the diodes D ₁ , D ₂ , D ₃ and D ₄ are of the switching diode type. Preferably, in order to improve the consistency of performance parameters such as the on-voltage, the diodes are selected as dual-channel or quad-channel integrated high-speed switching diodes.

Among them, a bipolar square wave is used as a modulated carrier signal to periodically charge and discharge the capacitor under test. Preferably, the square wave generator should adopt a scheme with adjustable amplitude and frequency and good stability.

Wherein, a transimpedance amplifier is used to convert the charge and discharge currents into a pair of equally large opposite voltages. Preferably, the transimpedance amplifier adopts an operational amplifier with low input noise current.

Among them, the transimpedance amplifier used is preferably an operational amplifier with an equivalent input current noise spectral density near 10Hz and no more than 20fA/Hz ^1/2 , and an equivalent input voltage noise spectral density near 10Hz and no more than 20nV/Hz ^1/2 .

Among them, a differential to single-ended conversion circuit can be selected to convert a pair of equal and large reverse voltages output by the transimpedance amplifier into a single-ended voltage output, thereby increasing the signal-to-noise ratio of the single-ended voltage signal and improving the resolution of the sensing circuit. rate level.

The invention realizes that the relative capacitance of the differential capacitance pair to be measured is converted into an easily measurable voltage value, and simultaneously, the diode switch circuit is used to control the charge and discharge of the capacitance to be measured without the action of an external sequence control circuit. The sensitivity of the present invention only depends on the feedback resistance R _f , the period f of the square wave, the amplitudes V _p and V _n of the square wave, and the conduction voltage V _d of the diode.

The invention utilizes a bipolar square wave with high amplitude stability generated by a switch-mode square wave generator using a parallel reference voltage source, and adopts two-way or four-way integrated high-speed switching diodes to suppress the difference in conduction voltage. In order to obtain high sensitivity and high resolution performance, the present invention also has the advantages of relatively simple circuit structure and low power consumption.

Description of drawings

FIG. 1 is a schematic diagram of the principle of a charge-discharge capacitive sensor based on a diode switch provided by the present invention.

FIG. 2 is a schematic diagram of a specific implementation of a charge-discharge capacitive sensor based on a diode switch provided by the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

The invention specifically relates to a differential capacitive sensor with a simple structure based on the principle of charge and discharge and using a diode as a switch control device, which is suitable for fields requiring high precision, low power consumption and high reliability capacitive sensors, such as inertial instruments such as space inertial sensors or in geophysical measuring instruments. The charge-discharge capacitive sensor based on the diode switch provided by the invention can not only realize high-precision displacement detection, but also ensure that the circuit has the characteristics of simple structure and low power consumption.

The principle diagram of a charge-discharge capacitive displacement sensing circuit based on a diode switch provided by the present invention is shown in Figure 1. The functional units of the circuit are described as follows: 1 is a bipolar square wave generator, which outputs the level of the square wave. The level amplitudes are equal and opposite, respectively +V _p , -V _n ; 2 is the differentially changed capacitance pair to be measured; 3 is a diode switch circuit; 4 is a transimpedance amplifier; 5 is a differential to single-ended conversion circuit.

The principle of the sensing circuit is summarized as follows: the capacitance C _a and C _b to be measured are a pair of capacitances that are sensitive to a certain physical quantity (such as acceleration) and change differentially with it. When the external input physical quantity is zero, C _a and C The size of _b is equal. When the input physical quantity is not zero, the size of the capacitances C _a and C _b changes differentially (one increases, the other decreases, and the changes are equal). Using the bipolar square wave generated by the bipolar square wave generator 1 , the capacitors C _a and C _b are charged and discharged under the control of the diode switch circuit 3 . When the square wave voltage changes from negative to positive, the square wave charges the capacitors C _a and C _b , and diodes D ₁ and D ₄ are turned on at the same time; when the square wave voltage changes from positive to negative, the capacitors C _a and C _b are discharged and reversed Charge, while diodes D ₂ and D ₃ conduct. Calculate the change in the amount of charge of capacitors C _a and C _b during one square wave cycle by the following formula:

Q _a =(V _p +V _n -2V _d )×C _a , Q _b =(V _p +V _n -2V _d )×C _b .

The amounts of electricity entering the two transimpedance amplifiers in one square wave cycle are:

Q _a0 =(V _p +V _n -2V _d )×(C _a -C _b ), Q _b0 =-(V _p +V _n -2V _d )×(C _a -C _b ).

Since the square wave frequency is f, the currents entering the two transimpedance amplifiers are:

I _a =Q _a0 ×f, I _b =Q _b0 ×f.

The gain of the transimpedance amplifier depends on the value of the feedback resistor R _f , and through the action of the differential circuit, the final output voltage is:

V _out = 2R _f x (V _p +V _n -2V _d ) x (C _a -C _b ).

Let ΔC=C _a -C _b , then the sensitivity coefficient of the circuit is:

K= _Vout /ΔC= _2Rf × _f ×( _Vp + _Vn- 2Vd).

The relative change of the capacitor pair can be obtained by measuring the final voltage value of the circuit, and the sensitivity of the circuit depends on the feedback resistance R _f , the period f of the square wave, the amplitudes V _p and V _n of the square wave, and the conduction of the diode voltage _Vd . In order to suppress the influence of the diode conduction voltage V _d on the sensitivity, preferably, the square wave is a bipolar square wave with larger amplitudes V _p and V _n . In order to suppress the influence of the noise of the transimpedance amplifier, preferably, a square wave with a higher frequency is used to improve the sensitivity of the sensing circuit. In order to suppress the effect of the diode conduction voltage and the difference of different diodes on the sensitivity and noise floor, and at the same time improve the switching rate, preferably, the four diodes are all made of dual diodes with low conduction voltage and good consistency of the four diodes. One-way or four-way integrated high-speed switching diode models. Preferably, the amplifier circuit adopts an operational amplifier circuit with low input noise current.

FIG. 2 is a schematic diagram of the specific implementation of the above characteristics, which mainly includes a bipolar square wave generating circuit 1, a pair of differentially changing capacitors 2 composed of C _a and C _b , a diode switch circuit 3, and a transimpedance amplifier 4. , Differential to single-ended conversion circuit 5.

The bipolar square wave generating circuit 1 includes: a crystal oscillator 11, a counter 12, a voltage reference source module 13 and an analog switch 14. After the frequency generated by the crystal oscillator 11 is divided by the counter 12, under the action of the analog switch 14, the The voltage reference source module 13 (which can be implemented by multiple voltage reference sources in parallel) is combined to form a bipolar square wave whose frequency and amplitude are adjustable within a certain range. The frequency is determined by the crystal oscillator 11 and the counter 12. The voltage magnitude is determined by reference voltage source 13 . When the sensitive physical quantity of the capacitive sensor changes, the capacitances C _a and C _b change differentially (one increases, the other decreases, and the amount of change is equal). By using bipolar square waves, the diode switch circuit 3 controls It automatically charges and discharges the pair of capacitors, converts the pair of capacitance signals into charge and discharge current signals, and converts them into a pair of equal and reverse voltage signals under the action of the transimpedance amplifier. The differential-to-single-ended conversion circuit 5 differentiates the voltage signal to obtain a single-ended output voltage, thereby suppressing the low-frequency noise of the transimpedance amplifier. Finally, the voltage signal is collected in real time by the voltage acquisition system, and the output voltage is converted into the input capacitance difference according to the capacitance measurement sensitivity coefficient obtained by calibration, that is, the differential capacitance sensing measurement is realized. For example, the amplitude of the square wave is ±10V, the frequency is 0.625MHz, and the diode is the integrated double diode BAS40 of NXP Semiconductors (NXP). is 0.3V; the transimpedance amplifier adopts the operational amplifier OP297 of Analog Devices (ADI), which has a lower equivalent input voltage noise of 17nV/Hz ^1/2 at 1kHz, and has a lower equivalent input voltage at 10Hz. The input current noise is 20fA/Hz ^1/2 , and the feedback resistance R _f is selected as 1MΩ, the theoretical value of the output sensitivity of the sensing circuit is about 25V/pF.

Those skilled in the art can easily understand that the above are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, etc., All should be included within the protection scope of the present invention.

Claims (7)

1. A diode switch based charge-discharge capacitive sensor comprising: the circuit comprises a bipolar square wave generator (1), a switching circuit (3), a trans-impedance amplifier (4) and a differential-single-ended conversion circuit (5);

the bipolar square wave generator (1) is used for generating bipolar square waves and loading the bipolar square waves at the common end of the capacitor pair (2) to be tested;

the switch circuit (3) is a diode switch circuit, and the bipolar square wave controls the charging and discharging of the differentially changed capacitor pair (2) to be tested under the control of the switch circuit (3);

the trans-impedance amplifier (4) is used for converting the charging and discharging current into a pair of large reverse voltages;

the differential-single-ended conversion circuit (5) is used for converting a pair of equal and opposite voltages output by the trans-impedance amplifier into a single-ended voltage and outputting the single-ended voltage;

under the mutual cooperation of the switch circuit (3), the trans-impedance amplifier (4) and the differential-single end conversion circuit (5), the relative capacitance of the capacitor pair (2) to be measured is converted into an easily-measured voltage value; the precision of capacitance difference detection is improved.

2. The charge-discharge capacitive sensor according to claim 1, characterized in that the bipolar square-wave generator (1) comprises: the device comprises a crystal oscillator (11), a counter (12), a voltage reference source module (13) and an analog switch (14);

after the frequency generated by the crystal oscillator (11) is divided by the counter (12), the frequency is combined with the voltage reference source module (13) to form a bipolar square wave with adjustable frequency and amplitude within a certain range under the action of the analog switch (14), the frequency is determined by the crystal oscillator (11) and the counter (12), and the amplitude of the bipolar voltage is determined by the reference voltage source (13).

3. The charge-discharge capacitive sensor according to claim 2, characterized in that the voltage reference source module (13) is connected in parallel by a plurality of voltage reference sources to achieve a noise suppression effect.

4. A charge-discharge capacitive sensor according to any of claims 1-3, characterized in that the switching circuit (3) comprises: diode D₁Diode D₂Diode D₃And a diode D₄，

Diode D₁Anode of (2) and diode D₂The cathode of the capacitor is connected with C in the capacitor pair (2) to be measured_aIs a non-common terminal, diode D₁Cathode and diode D₃The anode of the voltage regulator is connected with the 1 st input end of the transimpedance amplifier (4);

diode D₄Anode of (2) and diode D₃The cathode of the capacitor is connected with C in the capacitor pair (2) to be measured_bIs a non-common terminal, diode D₄Cathode and diode D₂The anode of the second-path resistor is connected with the 2 nd input end of the transimpedance amplifier (4).

5. The charge-discharge capacitive sensor of claim 4 wherein the diode is selected as a switch for use with a bipolar square wave to autonomously control charge and discharge without the need for external timing control signals.

6. The charge-discharge capacitive sensor of claim 5 wherein the diode D₁、D₂、D₃And D₄The two-way or four-way integrated high-speed switch diode model with relatively good performance consistency is adopted.

7. A charge-discharge capacitance sensor according to any of claims 1-6, characterized in that the charge-discharge current is converted into a pair of largely inverted voltage signals using a transimpedance amplifier (4), the transimpedance amplifier (4) preferably being used for equivalent input current noise spectral densityAt 10Hz and not more than 20fA/Hz^1/2The equivalent input voltage noise spectral density is around 10Hz and not more than 20nV/Hz^1/2The operational amplifier of (1).

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