CN212646814U

CN212646814U - Weak capacitance change measuring circuit

Info

Publication number: CN212646814U
Application number: CN202021290680.2U
Authority: CN
Inventors: 吴远星; 王晓琴; 薛黄琦; 宋军华; 王洪超
Original assignee: Beijing Xiantong Kangqiao Medicine Science & Technology Co ltd
Current assignee: Beijing Xiantong Kangqiao Medicine Science & Technology Co ltd
Priority date: 2020-07-06
Filing date: 2020-07-06
Publication date: 2021-03-02
Anticipated expiration: 2030-07-06

Abstract

The utility model provides a weak electric capacity changes measuring circuit, concretely relates to weak electric capacity signal conversion and detection technology field, include: the device comprises a CV conversion circuit, an amplifying circuit, an analog multiplier, a low-pass filter and a phase-shifting circuit; the CV conversion circuit converts the relative change rate of the capacitance value of the capacitance sensor into the amplitude change of a weak sinusoidal signal; the weak sinusoidal signal is amplified by an amplifying circuit; the amplitude of the output signal of the amplifying circuit is solved by an amplitude demodulation circuit consisting of an analog multiplier and a low-pass filter, and a direct current useful signal which is in direct proportion to the relative change rate of the capacitance of the sensor is output through the low-pass filter; the phase shifter changes the phase difference between the reference signal of the amplitude demodulation circuit and the output signal of the amplifying circuit, and the sensitivity of the circuit is improved.

Description

Weak capacitance change measuring circuit

Technical Field

The utility model relates to a weak electric capacity changes measuring circuit, the less weak electric capacity change of specially adapted sensor basis electric capacity (body electric capacity or static electric capacity) detects belongs to weak electric capacity signal conversion and detection technical field.

Background

The basic principle of a capacitive sensor is to convert some physical quantity, such as displacement, area, dielectric, etc., into capacitance and then indirectly measure the desired physical quantity by measuring the capacitance. Capacitive sensors have a very wide range of applications. Especially in the field of high-precision detection, the capacitive sensor has no alternative position. The capacitance sensor has the advantages of high sensitivity and quick response, but the measuring circuit is complex. In most cases, the capacitance of the capacitive sensor is weak, and the circuit is easily affected by parasitic parameters and environmental changes, which makes the problem more complicated.

Common weak capacitance measurement technologies include a direct current charge and discharge method, an alternating current bridge method, a V/T conversion method and a negative feedback alternating current excitation method based on an operational amplifier; the direct current charging and discharging method adopts direct current excitation, and the measurement precision is easily influenced by offset voltage drift of the amplifier. In addition, the method needs to rapidly charge and discharge the capacitor, an electronic switch is needed, and the circuit precision is easily influenced by the charge injection effect of the electronic switch; the zero setting of the ac bridge method is complicated, and is easily affected by the parasitic capacitance of the circuit, and complex shielding measures need to be implemented in the actual implementation process. The method is difficult to measure the weak change capacitance of the small capacitance; the V/T conversion method is used for measuring capacitance values by measuring the charge-discharge time of capacitors, and the measurement precision is easily influenced by the direct-current voltage drift of a circuit and the injection charge of an electronic switch like a direct-current charge-discharge method; from part of published documents, the circuit with the highest measurement accuracy in the actual application of weak capacitance detection is a negative feedback alternating current excitation method based on an operational amplifier. The method has high resolution and strong parasitic capacitance resistance. In addition, a high-voltage bilateral excitation detection method needs high-frequency high-voltage excitation signals, is only used for specific objects and occasions, and has more limiting conditions.

The above weak capacitance measuring methods also have a common problem that the output of the circuit is proportional to the capacitance value of the capacitor to be measured, rather than the capacitance variation. Many times the desired result is a change in capacitance rather than the capacitance itself. In this case, a controller needs to be introduced, and a balance signal is introduced under the control of a program to balance the basic capacitance of the capacitance sensor to be measured, which introduces digital noise to a certain extent, so that the precision of the measurement system is reduced, and the system becomes more complex. Therefore, a weak capacitance change measuring circuit is required to solve the above problems.

Disclosure of Invention

Technical problem to be solved

The utility model aims at providing a weak electric capacity changes measuring circuit for the technical problem of the weak electric capacity change volume of accurate detection capacitive sensor.

(II) technical scheme

In order to solve the technical problem, the utility model provides a weak electric capacity changes measuring circuit, include: the device comprises a CV conversion circuit, an amplifying circuit, an analog multiplier, a low-pass filter and a phase-shifting circuit; the sine wave of the excitation signal is input to the input ends of the CV conversion circuit and the phase shift circuit, the output end of the CV conversion circuit is connected to the first input end of the analog multiplier, and the output end of the phase shift circuit is connected to the second input end of the analog multiplier; the output end of the analog multiplier is connected with the input end of the low-pass filter; the low pass filter outputs the final useful signal.

The CV conversion circuit converts the relative rate of change of the capacitance value of the capacitive sensor into an amplitude change of a weak sinusoidal signal, which is amplified by the amplification circuit.

Preferably, the CV conversion circuit includes: an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1 and a capacitive sensor CX; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the first capacitor C1 are commonly connected to the non-inverting input end of the operational amplifier U1, and the other end of the third resistor R3 and the other end of the first capacitor C1 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the capacitance sensor CX are commonly connected to the inverting input end of the U1 of the operational amplifier, and the other end of the fourth resistor R4 and the other access end of the capacitance sensor CX are commonly connected to the output end of the operational amplifier U1; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

Preferably, the first capacitance C1 and the capacitance sensor CX access position in the CV conversion circuit can be interchanged.

Preferably, the CV conversion circuit includes: the capacitive sensor comprises an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1, a capacitive sensor CX, a parasitic capacitance compensation capacitor Cs1 and a parasitic capacitance compensation capacitor Cs 2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the first capacitor C1 are commonly connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the third resistor R3 and the other end of the first capacitor C1 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the capacitance sensor CX are commonly connected to the inverting input end of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access end of the capacitance sensor CX are commonly connected to the output end of the operational amplifier U1; one end of a parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs1 is grounded; one end of a parasitic capacitance compensation capacitor Cs2 is connected to the inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs2 is grounded; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

Preferably, the CV conversion circuit includes: an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a capacitive sensor differential capacitance Cx1, a capacitive sensor differential capacitance Cx2, a parasitic capacitance compensation capacitance Cs1, and a parasitic capacitance compensation capacitance Cs 2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the sensor differential capacitor Cx2 are commonly connected to the non-inverting input end of the operational amplifier U1, and the other end of the third resistor R3 and the other end of the sensor differential capacitor Cx2 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the sensor differential capacitor Cx1 are commonly connected to the inverting input end of the U1 of the operational amplifier, and the other end of the fourth resistor R4 and the other access end of the sensor differential capacitor Cx1 are commonly connected to the output end of the operational amplifier U1; one end of a parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs1 is grounded; one end of a parasitic capacitance compensation capacitor Cs2 is connected to the inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs2 is grounded; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

Preferably, the first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitance material and structure and are symmetrically routed, and parasitic capacitances generated by the first capacitor C1 and the capacitive sensor CX are always close to or the same.

The above preferred embodiments of the CV conversion circuit are used in any of the embodiments to which the CV conversion circuit of the present patent is applied, and therefore, are also used in the following embodiments.

The utility model discloses still provide another weak electric capacity change measuring circuit, include: the circuit comprises a CV conversion circuit, an amplifying circuit, an electronic switch multiplier, a low-pass filter, a phase-shifting circuit and a zero-crossing comparator; the sine wave of the excitation signal is input to the input ends of the CV conversion circuit and the phase shift circuit, and the output end of the CV conversion circuit is connected to the first input end of the electronic switch multiplier; the output end of the phase-shifting circuit is connected with the input end of the zero-crossing comparator, and the output end of the zero-crossing comparator is connected with the second input end of the electronic switch multiplier; the output end of the electronic switch multiplier is connected with the input end of the low-pass filter; the low pass filter outputs the final useful signal.

Drawings

Fig. 1 shows a schematic block diagram of a first embodiment of the present invention;

fig. 2 shows a schematic diagram 1 of a CV conversion circuit according to a first embodiment of the present invention;

fig. 3 shows a schematic diagram of a CV conversion circuit of a first embodiment of the present invention 2;

fig. 4 shows a schematic diagram 3 of a CV conversion circuit according to a first embodiment of the present invention;

fig. 5 shows a layout diagram of a capacitor according to a first embodiment of the present invention;

fig. 6 shows a block flow diagram of a second embodiment of the present invention;

fig. 7 shows a schematic diagram of a second embodiment electronic switched multiplier circuit of the present invention;

fig. 8 shows a schematic diagram of a zero-crossing comparator according to a second embodiment of the present invention.

Detailed Description

The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

The embodiments of the present disclosure are described below with specific examples, and other advantages and effects of the present disclosure will be readily apparent to those skilled in the art from the disclosure in the specification. It is to be understood that the described embodiments are merely illustrative of some, and not restrictive, of the embodiments of the disclosure. The disclosure may be embodied or carried out in various other specific embodiments, and various modifications and changes may be made in the details within the description without departing from the spirit of the disclosure. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict. All other embodiments, which can be derived by a person skilled in the art from the embodiments disclosed herein without making any creative effort, shall fall within the protection scope of the present disclosure.

The connection of the utility model refers to the electrical connection, which can be the direct electrical connection or the indirect electrical connection through a certain device.

In particular, the preferred embodiments of the examples apply equally well

Example 1:

fig. 1 shows the circuit schematic block diagram of the first embodiment of the present invention, the present invention provides a weak capacitance variation measuring circuit, including: CV conversion circuit, amplifying circuit, analog multiplier, low-pass filter and phase shift circuit. After the sine wave excitation signal is input to the CV conversion circuit, the CV conversion circuit converts the relative change rate of the capacitance value of the capacitance sensor into the amplitude change of a weak sine signal; the weak sinusoidal signal is amplified by an amplifying circuit; the amplitude of the output signal of the amplifying circuit is solved by an amplitude demodulation circuit consisting of an analog multiplier and a low-pass filter, and a direct current useful signal which is in direct proportion to the relative change rate of the equivalent capacitance of the sensor is output through the low-pass filter; the phase shifter changes the phase difference between the reference signal of the amplitude demodulation circuit and the output signal of the amplifying circuit, and the sensitivity of the circuit is improved.

The principles of the embodied circuits will now be described with reference to specific circuits.

A CV conversion circuit embodiment is shown in fig. 2. The CV conversion circuit converts the relative change rate of the equivalent capacitance value of the sensor into the amplitude change of a weak sine wave. Parameter configuration in a circuit satisfies

The transfer function of the circuit can be derived:

(formula 1)

For input excitation of the angular frequency of a sine wave, let

1/

Get it

Then, 1 in the denominator of the above formula can be ignored, and can be further expressed as:

(formula 2)

Similar to the ac excitation negative feedback method for measuring a minute capacitance, the above circuit utilizes the negative feedback principle, except that the operational amplifier is configured in the form of a symmetrical differential amplification circuit. When the values of the elements at the symmetrical positions in the circuit are the same, the output of the circuit is 0.

Suppose that

As equivalent capacitance of the sensor

Referred to as the balance capacitance,

capacitance value and

the base capacitance is the same.

Taking the sensor base capacitance as

,

，

Specifically, it can be expressed as:

(formula 3)

The right side of the above equation takes the opposite sign when the equilibrium capacitance is interchanged with the sensor position.

From the above equation, the relative rate of change of the capacitance of the sensor equivalent capacitor is proportional to the amplitude of the output sinusoidal signal AS 1.

The CV conversion circuit shown in fig. 3 adds line parasitic capacitance compensation capacitances Cs1, Cs2 in the basic configuration of fig. 2. When the sensor is led out by a lead wire, a parasitic capacitance to the ground is necessarily generated, and at the moment, the circuit cannot be zeroed. This adds parasitic capacitance compensation capacitances Cs1, Cs 2. For convenience of description, Cs1 represents the sum of the artificially added compensation capacitance and the parasitic capacitance that would otherwise exist at the non-inverting input pin of op-amp U1, and Cs2 represents the artificially added compensation capacitance and the parasitic capacitance that would otherwise exist at the inverting input pin of op-amp U1. When the circuit transfer function is

(formula 4)

Get

And then, the above formula is simplified as follows:

(formula 5)

Is the difference between the equivalent capacitance of the sensor and the parasitic capacitance to ground on one side of the first capacitor,

. Adjusting the difference between the compensation capacitors Cs1, Cs2 can zero the circuit. After the circuit is zeroed, the relative rate of change of the capacitance value of the equivalent capacitance of the sensor is proportional to the amplitude of the output sinusoidal signal AS 1.

The circuit of fig. 4 implements the application of differential capacitance. The circuit principle is similar to that of fig. 3, only one of the differential capacitance sensors is replaced by the balance capacitor C1, the sensitivity is doubled, and the rest of the principles are not described again.

The specific implementation of the CV conversion circuit needs to consider the equivalent capacitance magnitude and the parasitic capacitance magnitude of the sensor, select a proper excitation signal frequency according to the two parameters, and select an operational amplifier with enough bandwidth margin according to the excitation frequency. Meanwhile, in order to reduce the influence of parasitic capacitance as much as possible, circuits including electronic components and wiring need to be arranged symmetrically. As shown in fig. 5.

The amplifier is a general purpose amplifying circuit, and the function of the amplifier is to provide the gain of the circuit. The method can be realized by conventional inverting amplifiers, non-inverting amplifiers and other circuits.

The analog multiplier, the phase shifter and the low-pass filter constitute an amplitude demodulation circuit. The analog multiplier can be realized by a universal single-chip integrated analog multiplier; the phase shifter can be realized by a classical active phase shift circuit; the low-pass filter is used for filtering the alternating current signal and can be realized by a conventional low-pass filter.

The basic principle of the circuit is as follows:

the analog multiplier directly provides the product of the signal under test AS2 and the reference signal AS5, and the output AS3 thereof contains a direct current component and a frequency-doubled sine wave:

(formula 6)

In the above formula

To amplify the amplitude of the circuit's output sinusoidal signal AS2,

is proportional to the relative rate of change of the capacitance value of the sensor,

in order to be the amplitude of the excitation signal,

is the gain of the multiplier or the gain of the multiplier,

in order to amplify the gain of the circuit,

is the angular frequency of the excitation signal.

Is the phase difference of signals AS2 and AS 5.

The signal AS3 is low-pass filtered and the ac component is filtered out, resulting in the final useful output of the circuit:

(formula 7)

The phase-shifting circuit can be changed

The size of (1) when

When the sensitivity approaches 0, the circuit sensitivity is highest.

Is the zero error.

The circuit has extremely strong noise suppression capability, and can detect extremely weak sinusoidal signals from a large amount of noise.

Example 2:

to reduce cost, the reference signal can be replaced by a square wave and the analog multiplier replaced by an electronic switched multiplier. The reference signal is used to control the electronic switch so that the output of the CV conversion circuit is alternately multiplied by a coefficient of equal absolute value but opposite sign. The result of the multiplication comprises a series of alternating current signals and a useful direct current signal. The voltage value of the useful direct current signal is proportional to the amplitude of the CV conversion circuit and is also proportional to the cosine value of the phase difference between the reference square wave and the output signal of the CV conversion circuit. The phase shifting circuit functions as described above. In order to save one signal source, the reference square wave signal can be converted from the sinusoidal excitation signal through a zero-crossing comparator. The schematic block diagram of the circuit is shown in fig. 6.

The electronic switching multiplier is shown in fig. 7. The single-pole double-throw electronic switch SW-SPDT1 in the circuit is controlled by a square wave signal DS 0. Under control of the DS0 signal, SW-SPDT1 periodically alternates switching contacts between two stable states, which causes the circuit to periodically switch between two gains, which are equal in magnitude but opposite in sign. Mathematically equivalent to multiplying the input signal by a symmetric square wave. A square wave can be decomposed into a sum of a fundamental sinusoidal signal and harmonics. The result of the multiplication can likewise be decomposed into the sum of the direct current signal and the alternating current signal. The AC component is filtered by a low-pass filter to obtain a useful DC signal:

(formula 8)

The meaning of each variable in the above formula is the same as that in formula 7, and is not described in detail.

The reference square wave signal is converted by the excitation sinusoidal signal. As shown in fig. 8, the circuit outputs a high level when the input sinusoidal signal is in the positive half cycle, and outputs a low level when the input sinusoidal signal is in the negative half cycle, so as to obtain a square wave signal with a 50% duty ratio.

The phase shift circuit, the amplifying circuit and the low-pass filter circuit in the circuit are all general circuits, and the principle is not repeated.

The above description is only a preferred embodiment of the present invention, and should not be taken as limiting the invention, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A weak capacitance change measuring circuit, comprising: the device comprises a CV conversion circuit, an amplifying circuit, an analog multiplier, a low-pass filter and a phase-shifting circuit; the sine wave of the excitation signal is input to the input ends of the CV conversion circuit and the phase shift circuit, the output end of the CV conversion circuit is connected to the first input end of the analog multiplier, and the output end of the phase shift circuit is connected to the second input end of the analog multiplier; the output end of the analog multiplier is connected with the input end of the low-pass filter; the low pass filter outputs the final useful signal.

2. The weak capacitance change measurement circuit according to claim 1, wherein the CV conversion circuit includes: an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1 and a capacitive sensor CX; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the first capacitor C1 are commonly connected to the non-inverting input end of the operational amplifier U1, and the other end of the third resistor R3 and the other end of the first capacitor C1 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the capacitance sensor CX are commonly connected to the inverting input end of the U1 of the operational amplifier, and the other end of the fourth resistor R4 and the other access end of the capacitance sensor CX are commonly connected to the output end of the operational amplifier U1; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

3. The weak capacitance change measurement circuit according to claim 1, wherein the CV conversion circuit includes: the capacitive sensor comprises an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1, a capacitive sensor CX, a parasitic capacitance compensation capacitor Cs1 and a parasitic capacitance compensation capacitor Cs 2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the first capacitor C1 are commonly connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the third resistor R3 and the other end of the first capacitor C1 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the capacitance sensor CX are commonly connected to the inverting input end of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access end of the capacitance sensor CX are commonly connected to the output end of the operational amplifier U1; one end of a parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs1 is grounded; one end of a parasitic capacitance compensation capacitor Cs2 is connected to the inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs2 is grounded; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

4. The weak capacitance change measurement circuit according to claim 1, wherein the CV conversion circuit includes: an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a capacitive sensor differential capacitance Cx1, a capacitive sensor differential capacitance Cx2, a parasitic capacitance compensation capacitance Cs1, and a parasitic capacitance compensation capacitance Cs 2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the sensor differential capacitor Cx2 are commonly connected to the non-inverting input end of the operational amplifier U1, and the other end of the third resistor R3 and the other end of the sensor differential capacitor Cx2 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the sensor differential capacitor Cx1 are commonly connected to the inverting input end of the U1 of the operational amplifier, and the other end of the fourth resistor R4 and the other access end of the sensor differential capacitor Cx1 are commonly connected to the output end of the operational amplifier U1; one end of a parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs1 is grounded; one end of a parasitic capacitance compensation capacitor Cs2 is connected to the inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs2 is grounded; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

5. The weak capacitance variation measuring circuit according to claim 2, wherein the first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitance material and structure, and are symmetrically routed.

6. A weak capacitance change measuring circuit, comprising: the circuit comprises a CV conversion circuit, an amplifying circuit, an electronic switch multiplier, a low-pass filter, a phase-shifting circuit and a zero-crossing comparator; the sine wave of the excitation signal is input to the input ends of the CV conversion circuit and the phase shift circuit, and the output end of the CV conversion circuit is connected to the first input end of the electronic switch multiplier; the output end of the phase-shifting circuit is connected with the input end of the zero-crossing comparator, and the output end of the zero-crossing comparator is connected with the second input end of the electronic switch multiplier; the output end of the electronic switch multiplier is connected with the input end of the low-pass filter; the low pass filter outputs the final useful signal.

7. The weak capacitance change measurement circuit according to claim 6, wherein the CV conversion circuit includes: an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1 and a capacitive sensor CX; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the first capacitor C1 are commonly connected to the non-inverting input end of the operational amplifier U1, and the other end of the third resistor R3 and the other end of the first capacitor C1 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the capacitance sensor CX are commonly connected to the inverting input end of the U1 of the operational amplifier, and the other end of the fourth resistor R4 and the other access end of the capacitance sensor CX are commonly connected to the output end of the operational amplifier U1; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

8. The weak capacitance change measurement circuit according to claim 6, wherein the CV conversion circuit includes: the capacitive sensor comprises an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1, a capacitive sensor CX, a parasitic capacitance compensation capacitor Cs1 and a parasitic capacitance compensation capacitor Cs 2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the first capacitor C1 are commonly connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the third resistor R3 and the other end of the first capacitor C1 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the capacitance sensor CX are commonly connected to the inverting input end of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access end of the capacitance sensor CX are commonly connected to the output end of the operational amplifier U1; one end of a parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs1 is grounded; one end of a parasitic capacitance compensation capacitor Cs2 is connected to the inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs2 is grounded; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

9. The weak capacitance change measurement circuit according to claim 6, wherein the CV conversion circuit includes: an operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a capacitive sensor differential capacitance Cx1, a capacitive sensor differential capacitance Cx2, a parasitic capacitance compensation capacitance Cs1, and a parasitic capacitance compensation capacitance Cs 2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the input signal of the CV conversion circuit, the other end of the first resistor R1 is connected to the non-inverting input end of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input end of the operational amplifier U1; one end of the third resistor R3 and one end of the sensor differential capacitor Cx2 are commonly connected to the non-inverting input end of the operational amplifier U1, and the other end of the third resistor R3 and the other end of the sensor differential capacitor Cx2 are commonly connected to the ground; one end of the fourth resistor R4 and one input end of the sensor differential capacitor Cx1 are commonly connected to the inverting input end of the U1 of the operational amplifier, and the other end of the fourth resistor R4 and the other access end of the sensor differential capacitor Cx1 are commonly connected to the output end of the operational amplifier U1; one end of a parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs1 is grounded; one end of a parasitic capacitance compensation capacitor Cs2 is connected to the inverting input end of the U1 of the operational amplifier, and the other end of the parasitic capacitance compensation capacitor Cs2 is grounded; the output terminal of the operational amplifier U1 is the CV conversion circuit output terminal.

10. The weak capacitance variation measuring circuit according to claim 7, wherein the first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitance material and structure, and are symmetrically routed.