A circuit for measuring weak capacitance changes

Abstract

The invention provides a circuit for measuring weak capacitance change, which specifically relates to the technical field of weak capacitance signal conversion and detection, and 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 a capacitance sensor into the amplitude change of a weak sinusoidal signal; the weak sinusoidal signal is amplified by the amplifying circuit; the amplitude of the output signal of the amplifying circuit is decoded by an amplitude demodulation circuit composed of an analog multiplier and a low-pass filter, and a DC useful signal proportional 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, so as to improve the sensitivity of the circuit.

Description

Technical Field

[0001] The invention relates to a weak capacitance change measurement circuit, which is particularly suitable for detecting weak capacitance changes of sensors with relatively small basic capacitance (body capacitance or static capacitance), and belongs to the technical field of weak capacitance signal conversion and detection. Background Art

[0002] The basic principle of capacitive sensors is to convert a 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 uses. Especially in the field of high-precision detection, capacitive sensors have an irreplaceable position. Capacitive sensors have the advantages of high sensitivity and fast response, but the measurement circuit is relatively complex. In most cases, the capacitance of capacitive sensors is relatively weak, and the circuit is easily affected by its own parasitic parameters and environmental changes, which makes the problem more complicated.

[0003] Common weak capacitance measurement techniques include DC charge and discharge method, AC bridge method, V / T conversion method and negative feedback AC excitation method based on operational amplifier; DC charge and discharge method uses DC excitation, and the measurement accuracy is easily affected by the offset voltage drift of the amplifier. In addition, this method requires the use of electronic switches to quickly charge and discharge the capacitor, and the circuit accuracy is easily affected by the charge injection effect of the electronic switch; the zero adjustment of the AC bridge method is cumbersome and easily affected by the parasitic capacitance of the circuit, and complex shielding measures need to be implemented in the actual implementation process. This method is more difficult to measure the weak capacitance of small capacitors; the V / T conversion method uses the measurement of the charging and discharging time of the capacitor to measure the capacitance value. Like the DC charge and discharge method, the measurement accuracy is easily affected by the DC voltage drift of the circuit and the charge injected by the electronic switch; from some public literature, the circuit with the highest measurement accuracy in the actual application of weak capacitance detection is the negative feedback AC excitation method based on the operational amplifier. This method has a high resolution and strong resistance to parasitic capacitance. There is also a high-voltage bilateral excitation detection method, which requires a high-frequency and high-voltage excitation signal and is only used for specific objects and occasions, with many restrictions.

[0004] Another common problem of the above-mentioned weak capacitance measurement methods is that the output of the circuit is proportional to the capacitance value of the capacitor to be measured, rather than the capacitance change. In many cases, the desired result is the capacitance change, rather than the capacitance value itself. In this case, a controller needs to be introduced to introduce a balancing signal under the control of the program to balance the basic capacitance of the capacitance sensor to be measured, which introduces digital noise to a certain extent, reduces the accuracy of the measurement system, and makes the system more complicated. Therefore, a weak capacitance change measurement circuit is needed to solve the above problems. Summary of the invention

[0005] (I) Technical Problems Solved The purpose of the present invention is to provide a circuit for measuring a weak capacitance change, which is used to accurately detect the technical problem of the weak capacitance change of a capacitance sensor.

[0006] (II) Technical Solution In order to solve the above technical problems, the present invention provides a weak capacitance change measurement circuit, including: a CV conversion circuit, an amplifying circuit, an analog multiplier, a low-pass filter and a phase shifting circuit; the excitation signal sine wave is input to the input ends of the CV conversion circuit and the phase shifting circuit, the output end of the CV conversion circuit is connected to the first input end of the analog multiplier, the output end of the phase shifting circuit is connected to the second input end of the analog multiplier; the output end of the analog multiplier is connected to the input end of the low-pass filter; the low-pass filter outputs the final useful signal.

[0007] The CV conversion circuit converts the relative change rate of the capacitance value of the capacitance sensor into an amplitude change of a weak sinusoidal signal, and the weak sinusoidal signal is amplified by the amplifier circuit.

[0008] 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 CV conversion circuit input signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input terminal 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 terminal 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 an input terminal of the capacitive sensor CX are commonly connected to the inverting input terminal of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access terminal of the capacitive sensor CX are commonly connected to the output terminal of the operational amplifier U1; the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

[0009] Preferably, the connection positions of the first capacitor C1 and the capacitive sensor CX in the above CV conversion circuit can be interchanged.

[0010] 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, a capacitance sensor CX, a parasitic capacitance compensation capacitor Cs1, and a parasitic capacitance compensation capacitor Cs2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the CV conversion circuit input signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input terminal 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 The non-inverting input terminal, 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 an input terminal of the capacitance sensor CX are commonly connected to the inverting input terminal of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access terminal of the capacitance sensor CX are commonly connected to the output terminal of the operational amplifier U1; one end of the parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end is grounded; one end of the parasitic capacitance compensation capacitor Cs2 is connected to the inverting input terminal of the operational amplifier U1, and the other end is grounded; the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

[0011] Preferably, the connection positions of the first capacitor C1 and the capacitive sensor CX in the above CV conversion circuit can be interchanged.

[0012] 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 capacitance sensor differential capacitor Cx1, a capacitance sensor differential capacitor Cx2, a parasitic capacitance compensation capacitor Cs1, and a parasitic capacitance compensation capacitor Cs2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the CV conversion circuit input signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input terminal of the operational amplifier U1; one end of the third resistor R3 and one end of the capacitance sensor differential capacitor Cx2 are commonly connected to the The non-inverting input terminal, the other end of the third resistor R3 and the other end of the capacitance sensor differential capacitor Cx2 are commonly connected to the ground; one end of the fourth resistor R4 and an input terminal of the capacitance sensor differential capacitor Cx1 are commonly connected to the inverting input terminal of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access terminal of the capacitance sensor differential capacitor Cx1 are commonly connected to the output terminal of the operational amplifier U1; one end of the parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end is grounded; one end of the parasitic capacitance compensation capacitor Cs2 is connected to the inverting input terminal of the operational amplifier U1, and the other end is grounded; the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

[0013] Preferably, the first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitor material and structure, and are symmetrically routed, and the parasitic capacitances generated by the two are always close to or the same.

[0014] The preferred solution of the above CV conversion circuit is used in any solution of the CV conversion circuit of this patent, and therefore is also used in the application solution below.

[0015] The present invention also provides another weak capacitance change measurement circuit, comprising: a CV conversion circuit, an amplifying circuit, an electronic switch multiplier, a low-pass filter, a phase shift circuit and a zero-crossing comparator; an excitation signal sine wave is input to the CV conversion circuit and the phase shift circuit input end, and the CV conversion circuit output end is connected to the first input end of the electronic switch multiplier; the phase shift circuit output end is connected to the zero-crossing comparator input end, and the zero-crossing comparator output end is connected to the second input end of the electronic switch multiplier; the electronic switch multiplier output end is connected to the low-pass filter input end; and the low-pass filter outputs a final useful signal.

[0016] 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 CV conversion circuit input signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input terminal 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 terminal 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 an input terminal of the capacitive sensor CX are commonly connected to the inverting input terminal of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access terminal of the capacitive sensor CX are commonly connected to the output terminal of the operational amplifier U1; the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

[0017] Preferably, the connection positions of the first capacitor C1 and the capacitive sensor CX in the above CV conversion circuit can be interchanged.

[0018] 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, a capacitance sensor CX, a parasitic capacitance compensation capacitor Cs1, and a parasitic capacitance compensation capacitor Cs2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the CV conversion circuit input signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input terminal 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 The non-inverting input terminal, 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 an input terminal of the capacitance sensor CX are commonly connected to the inverting input terminal of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access terminal of the capacitance sensor CX are commonly connected to the output terminal of the operational amplifier U1; one end of the parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end is grounded; one end of the parasitic capacitance compensation capacitor Cs2 is connected to the inverting input terminal of the operational amplifier U1, and the other end is grounded; the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

[0019] Preferably, the connection positions of the first capacitor C1 and the capacitive sensor CX in the above CV conversion circuit can be interchanged.

[0020] 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 capacitance sensor differential capacitor Cx1, a capacitance sensor differential capacitor Cx2, a parasitic capacitance compensation capacitor Cs1, and a parasitic capacitance compensation capacitor Cs2; one end of the first resistor R1 and one end of the second resistor R2 are commonly connected to the CV conversion circuit input signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end of the second resistor R2 is connected to the inverting input terminal of the operational amplifier U1; one end of the third resistor R3 and one end of the capacitance sensor differential capacitor Cx2 are commonly connected to the The non-inverting input terminal, the other end of the third resistor R3 and the other end of the capacitance sensor differential capacitor Cx2 are commonly connected to the ground; one end of the fourth resistor R4 and an input terminal of the capacitance sensor differential capacitor Cx1 are commonly connected to the inverting input terminal of the operational amplifier U1, and the other end of the fourth resistor R4 and the other access terminal of the capacitance sensor differential capacitor Cx1 are commonly connected to the output terminal of the operational amplifier U1; one end of the parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end is grounded; one end of the parasitic capacitance compensation capacitor Cs2 is connected to the inverting input terminal of the operational amplifier U1, and the other end is grounded; the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

[0021] Preferably, the first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitor material and structure, and are symmetrically routed, and the parasitic capacitances generated by the two are always close to or the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A circuit diagram of the first embodiment of the present invention is shown;

[0023] Figure 2 The CV conversion circuit principle of the first embodiment of the present invention is shown as follows Figure 1 ;

[0024] Figure 3 The CV conversion circuit principle of the first embodiment of the present invention is shown as follows Figure 2 ;

[0025] Figure 4 The CV conversion circuit principle of the first embodiment of the present invention is shown as follows Figure 3 ;

[0026] Figure 5 The capacitor layout diagram of the first embodiment of the present invention is shown;

[0027] Figure 6 Shows a circuit block flow chart of the second embodiment of the present invention;

[0028] Figure 7 The schematic diagram of the electronic switch multiplier circuit of the second embodiment of the present invention is shown;

[0029] Figure 8 A schematic diagram of a zero-crossing comparator circuit according to a second embodiment of the present invention is shown. DETAILED DESCRIPTION

[0030] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0031] The following describes the embodiments of the present disclosure through specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all of the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present disclosure.

[0032] The connection described in the present invention refers to an electrical connection, which may be a direct electrical connection or an indirect electrical connection through a certain device.

[0033] In particular, the preferred solutions in the embodiments are also applicable to

[0034] Embodiment 1:

[0035] Figure 1 The circuit principle block diagram of the first embodiment of the present invention is shown. The present invention provides a weak capacitance change measurement circuit, including: a CV conversion circuit, an amplifier circuit, an analog multiplier, a low-pass filter and a 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 an amplitude change of a weak sinusoidal signal; the weak sinusoidal signal is amplified by the amplifier circuit; the amplitude of the output signal of the amplifier circuit is decoded by the amplitude demodulation circuit composed of the analog multiplier and the low-pass filter, and the low-pass filter outputs a DC useful signal proportional to the relative change rate of the equivalent capacitance of the sensor; the phase shifter changes the phase difference between the reference signal of the amplitude demodulation circuit and the output signal of the amplifier circuit to improve the sensitivity of the circuit.

[0036] The principle of implementing the circuit will now be described in conjunction with a specific circuit.

[0037] CV conversion circuit implementation example Figure 2 As shown. The CV conversion circuit converts the relative rate of change of the equivalent capacitance value of the sensor into the amplitude change of a weak sine wave. The parameter configuration in the circuit satisfies R3=R4=k·R, R1=R2=R, and the transfer function of the circuit can be derived:

[0038]

[0039] ω is the angular frequency of the input excitation sine wave, let ω0 = (1 / RC x ), when k>>ω0 / ω, the 1 in the denominator of the above formula can be ignored, and it can be further expressed as:

[0040]

[0041] Similar to the AC excitation negative feedback method for measuring tiny capacitors, the above circuit uses the negative feedback principle, but the difference is that the operational amplifier is configured as a symmetrical differential amplifier circuit. When the components in the symmetrical position of the circuit have the same value, the circuit output is 0.

[0042] Assumption C x is the equivalent capacitance of the sensor, C1 is called the balancing capacitance, and the capacitance value of C1 is equal to C x The base capacitance is the same.

[0043] Take the sensor base capacitance as C0, C x =C0+ΔC, C1=C0, which can be specifically expressed as:

[0044]

[0045] When the balancing capacitor and the sensor are swapped, the right side of the above equation takes on the opposite sign.

[0046] According to the above formula, the relative change rate of the capacitance value of the sensor equivalent capacitance is proportional to the amplitude of the output sinusoidal signal AS1.

[0047] Figure 3 The CV conversion circuit shown in Figure 2 The line parasitic capacitance compensation capacitors Cs1 and CS2 are added to the basic configuration. When the sensor is led out with a wire, parasitic capacitance to the ground will inevitably be generated, and the circuit cannot be adjusted to zero at this time. At this time, parasitic capacitance compensation capacitors Cs1 and CS2 are added. For the convenience of expression, Cs1 represents the sum of the artificially added compensation capacitor and the parasitic capacitance originally existing at the in-phase input pin of the operational amplifier U1, and Cs2 represents the sum of the artificially added compensation capacitor and the parasitic capacitance originally existing at the inverting input pin of the operational amplifier U1. At this time, the circuit transfer function is

[0048]

[0049] When k>>ω0 / ω, the above formula is simplified to:

[0050]

[0051] When the balancing capacitor and the sensor are swapped, the right side of the above equation takes on the opposite sign.

[0052] ΔC s is the difference between the equivalent capacitance of the sensor and the parasitic capacitance of the first capacitor to ground, ΔC s =C s2 -C s1 The circuit can be adjusted to zero by adjusting the difference between the compensation capacitors Cs1 and CS2. After the circuit is adjusted to zero, the relative change rate of the capacitance value of the sensor equivalent capacitor is proportional to the amplitude of the output sinusoidal signal AS1.

[0053] Figure 4 The circuit realizes the application of differential capacitors. Figure 3 Similarly, only by replacing the balancing capacitor C1 with one of the differential capacitor sensors, the sensitivity is doubled, and the rest of the principles will not be repeated.

[0054] The specific implementation of the CV conversion circuit needs to consider the equivalent capacitance level and parasitic capacitance level of the sensor, select the appropriate excitation signal frequency based on these two parameters, and then select an operational amplifier with sufficient bandwidth margin based on the excitation frequency. At the same time, in order to minimize the influence of parasitic capacitance, the circuit needs to be arranged symmetrically, including electronic components and routing. Figure 5 shown.

[0055] The amplifier is a general amplifier circuit, and its function is to provide circuit gain, and can be implemented by conventional inverting amplifier, non-inverting amplifier and other circuits.

[0056] The analog multiplier, phase shifter and low-pass filter constitute an amplitude demodulation circuit. The analog multiplier can be implemented by a general monolithic integrated analog multiplier; the phase shifter can be implemented by a classic active phase shift circuit; the low-pass filter is used to filter out AC signals and can be implemented by a conventional low-pass filter.

[0057] The basic principle of the circuit is as follows:

[0058] The analog multiplier directly provides the product of the measured signal AS2 and the reference signal AS5, and its output AS3 contains a DC component and a doubled frequency sine wave:

[0059]

[0060] In the above formula, A2 is the amplitude of the output sinusoidal signal AS2 of the amplifier circuit, the size of A2 is proportional to the relative rate of change of the sensor capacitance value, A0 is the amplitude of the excitation signal, k m is the gain of the multiplier, k is the gain of the amplifier circuit, and ω is the angular frequency of the excitation signal. is the phase difference between signals AS2 and AS5.

[0061] After low-pass filtering, the AC component of signal AS3 is removed, and the final useful output of the circuit is obtained:

[0062]

[0063] The phase shift circuit can change The size of When it approaches 0, the circuit sensitivity is the highest. zero is the zero point error.

[0064] This circuit has extremely strong noise suppression capability and can detect extremely weak sinusoidal signals from a large amount of noise.

[0065] Embodiment 2:

[0066] In order to reduce costs, the reference signal can be replaced by a square wave, and the analog multiplier can be replaced by an electronic switch 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 with equal absolute value but opposite sign. The result of the multiplication contains a series of AC signals and a useful DC signal. The voltage value of the useful DC 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 function of the phase shift circuit is the same as described above. In order to save a signal source, the reference square wave signal can be converted from a sinusoidal excitation signal through a zero-crossing comparator. The circuit principle block diagram is shown in the figure. Figure 6 shown.

[0067] The electronic switch multiplier is as follows Figure 7 As shown. The single-pole double-throw electronic switch SW-SPDT1 in the circuit is controlled by the square wave signal DS0. Under the control of the DS0 signal, SW-SPDT1 periodically switches the switch contacts between two stable states, which causes the circuit to periodically switch between two gains, which are of the same magnitude but opposite signs. Mathematically equivalent to multiplying the input signal by a symmetrical square wave. The square wave can be decomposed into the sum of the fundamental frequency sinusoidal signal and the harmonic wave. The result of the multiplication can also be decomposed into the sum of the DC signal and the AC signal. After the AC component is filtered out by the low-pass filter, a useful DC signal can be obtained:

[0068]

[0069] The meanings of the variables in the above formula are the same as those in Formula 7 and will not be repeated here.

[0070] The reference square wave signal is converted from the excitation sine wave signal. Figure 8 As shown, when the input sinusoidal signal is in the positive half cycle, the circuit outputs a high level, and when the input sinusoidal signal is in the negative half cycle, the circuit outputs a low level, thereby obtaining a square wave signal with a 50% duty cycle.

[0071] The phase shift circuit, amplifier circuit and low-pass filter circuit in the circuit are all universal circuits, and the principles will not be repeated here.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, 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 measurement circuit, comprising: CV conversion circuit, amplifier circuit, analog multiplier, low-pass filter and phase shift circuit; The excitation signal sine wave is input to the input end of the CV conversion circuit and the phase shift circuit, the output end of the CV conversion circuit is connected to the input end of the amplifier circuit, the output end of the amplifier circuit is connected to the first input end of the analog multiplier, 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 to the input end of the low-pass filter; the low-pass filter outputs the final useful signal; 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 CV conversion circuit input signal, the first resistor The other end of R1 is connected to the non-inverting input terminal of the operational amplifier U1, the other end of the second resistor R2 is connected to the inverting input terminal 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 terminal of the operational amplifier U1, 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 an input terminal of the capacitive sensor CX are commonly connected to the inverting input terminal of the operational amplifier U1, the other end of the fourth resistor R4 and the other access terminal of the capacitive sensor CX are commonly connected to the output terminal of the operational amplifier U1, and the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

2. The weak capacitance change measurement circuit according to claim 1, characterized in that: The CV conversion circuit also includes a parasitic capacitance compensation capacitor Cs1 and a parasitic capacitance compensation capacitor Cs2; one end of the parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end is grounded; one end of the parasitic capacitance compensation capacitor Cs2 is connected to the inverting input terminal of the operational amplifier U1, and the other end is grounded.

3. The weak capacitance change measurement circuit according to claim 2, characterized in that: The capacitive sensor CX and the first capacitor C1 in the CV conversion circuit are replaced by the capacitive sensor differential capacitor Cx1 and the capacitive sensor differential capacitor Cx2 respectively.

4. The weak capacitance change measurement circuit according to claim 1, characterized in that: The first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitor material and structure, and have symmetrical routing.

5. A weak capacitance change measurement circuit, comprising: CV conversion circuit, amplifier circuit, electronic switch multiplier, low-pass filter, phase shift circuit and zero-crossing comparator; The excitation signal sine wave is input to the CV conversion circuit and the phase shift circuit input end, the CV conversion circuit output end is connected to the input end of the amplifier circuit, and the output end of the amplifier circuit is connected to the first input end of the electronic switch multiplier; the phase shift circuit output end is connected to the zero-crossing comparator input end, and the zero-crossing comparator output end is connected to the second input end of the electronic switch multiplier; the electronic switch multiplier output end is connected to the low-pass filter input end; the low-pass filter outputs the final useful signal; 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 CV The conversion circuit inputs a signal, the other end of the first resistor R1 is connected to the non-inverting input terminal of the operational amplifier U1, the other end of the second resistor R2 is connected to the inverting input terminal 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 terminal of the operational amplifier U1, 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 an input terminal of the capacitive sensor CX are commonly connected to the inverting input terminal of the operational amplifier U1, the other end of the fourth resistor R4 and the other access terminal of the capacitive sensor CX are commonly connected to the output terminal of the operational amplifier U1, and the output terminal of the operational amplifier U1 is the output terminal of the CV conversion circuit.

6. The weak capacitance change measurement circuit according to claim 5, characterized in that: The CV conversion circuit also includes a parasitic capacitance compensation capacitor Cs1 and a parasitic capacitance compensation capacitor Cs2; one end of the parasitic capacitance compensation capacitor Cs1 is connected to the non-inverting input terminal of the operational amplifier U1, and the other end is grounded; one end of the parasitic capacitance compensation capacitor Cs2 is connected to the inverting input terminal of the operational amplifier U1, and the other end is grounded.

7. The weak capacitance change measurement circuit according to claim 6, characterized in that: The capacitive sensor CX and the first capacitor C1 in the CV conversion circuit are replaced by the capacitive sensor differential capacitor Cx1 and the capacitive sensor differential capacitor Cx2 respectively.

8. The weak capacitance change measurement circuit according to claim 5, characterized in that: The first capacitor C1 and the capacitive sensor CX are symmetrically arranged, have the same capacitor material and structure, and have symmetrical routing.

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