A capacitance detection circuit based on differential operational amplifier

Abstract

The invention discloses a capacitance detection circuit based on a differential operational amplifier, relates to the technical field of detection circuits, and comprises a TIA capacitance detection circuit, a differential amplifier circuit, a secondary amplifier circuit and a filter circuit, wherein the TIA capacitance detection circuit is electrically connected to the differential amplifier circuit, and the differential amplifier circuit is electrically connected to the secondary amplifier circuit; in the present scheme, by arranging the TIA capacitance detection circuit, the differential amplifier circuit, the secondary amplifier circuit and the filter circuit, a fixed capacitance difference can be well detected, and the purpose of detecting a dynamic capacitance difference can be achieved (when two capacitances to be measured change dynamically, J8 inputs a carrier wave, the capacitance to be measured modulates the carrier wave, and the output current signal is converted into a voltage signal through a transimpedance amplifier circuit TIA), and after the signal is demodulated by an FFT signal, a voltage value which is linearly related to the capacitance difference can be obtained, thereby realizing effective detection of the capacitance, and the circuit has a simple structure, low cost, and easy procurement of required components on the basis of realizing high-precision detection, so as to facilitate people's use.

Description

Technical Field

[0001] The present invention relates to the technical field of detection circuits, and more particularly to a capacitance detection circuit. Background Art

[0002] Capacitance, also known as "capacitance", refers to the storage of free charge under a given potential difference, denoted by C, and the international unit is farad (F). Generally speaking, charges will move under force in an electric field. When there is a medium between conductors, the movement of charges is hindered and the charges accumulate on the conductors, resulting in the accumulation and storage of charges. The amount of stored charge is called capacitance. The commonly used method in the prior art is transformer differential detection, but the manufacturing process of high-precision transformers is complex and costly, and it is difficult to find the source of the device. In addition, the parasitic electricity of the transformer can easily affect the measurement results, which affects the measurement accuracy. Summary of the invention

[0003] In view of the deficiencies in the prior art, an object of the present invention is to provide a capacitance detection circuit, which has the characteristic of performing circuit detection on capacitance.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A capacitance detection circuit based on a differential operational amplifier comprises a TIA capacitance detection circuit, a differential amplifier circuit, a secondary amplifier circuit and a filter circuit, wherein the TIA capacitance detection circuit is electrically connected to the differential amplifier circuit, the differential amplifier circuit is electrically connected to the secondary amplifier circuit, and the secondary amplifier circuit is electrically connected to the filter circuit.

[0006] Furthermore, the TIA capacitance detection circuit includes C102, C103, C104 and C105, C102, C103, C104 and C105 form a capacitance bridge and connect two TIA amplifier circuits, and the TIA capacitance detection circuit also includes J1, J2 and J8, C102 and C103 are capacitors to be measured, J1 and J2 are externally connected to the control circuit, and J1 and J2 are input ends of the high-voltage control signal, and J8 is the input end of the carrier.

[0007] By adopting the above technical solution, the function of the first part of the circuit is to convert the rear-end current change caused by the capacitance change into a voltage change to be output from the two TIAs.

[0008] Furthermore, the differential amplifier circuit adopts an AD620 operational amplifier to form a differential amplifier circuit. The transfer function of the AD620 operational amplifier is: V=G(V+-V-), and its gain G depends on its external resistance value Rg, and the calculation formula is G=49.4KΩ / Rg+1, where Rg in this circuit is R 24 .

[0009] By adopting the above technical solution, the circuit functions to subtract and amplify the two voltages of the previous stage, and the output voltage amplitude signal represents the difference between the voltages of the previous stage.

[0010] Furthermore, the secondary amplifier circuit is a proportional operational amplifier, using OP07, and OP07 is connected to a negative feedback resistor R 9 , the gain of the secondary amplifier circuit is: G 3 =1+R 9 / R 8 =16.7, used to perform secondary amplification on the front-end output voltage, with the phase and frequency unchanged.

[0011] Further, the filtering circuit is composed of two consecutive low-pass filters and two high-pass filters, both based on LF411;

[0012] By adopting the above technical solution, the passband: about 3.622kHz-13.246kHz; transfer gain: -19.5db; optimal passband: 7.1khz.

[0013] In summary, the present invention has the following beneficial effects:

[0014] In this scheme, by setting a TIA capacitance detection circuit, a differential amplifier circuit, a secondary amplifier circuit and a filter circuit, the fixed capacitance difference can be well detected, and the purpose of detecting the dynamic capacitance difference can be achieved (when the two capacitors to be measured change dynamically, J8 inputs a carrier, the capacitor to be measured modulates the carrier, and the output current signal is converted into a voltage signal through the transimpedance amplifier circuit TIA). The set C102 and C103 capacitors to be measured change, causing the rear-end current to change, which is converted into a voltage value output through the capacitor bridge and the TIA amplifier; the external power supply of the circuit is ±15V, and the two voltage signals are differentially output through the differential amplifier, and after secondary amplification, the carrier is filtered out by a high-pass and low-pass filter and then output to obtain a variable voltage signal. After the signal is demodulated by the FFT signal, a voltage value that is linearly related to the capacitance difference can be obtained, thereby realizing effective detection of the capacitance, and the circuit structure is simple, the cost is reduced, and the production is simple and convenient for people to use, while having the good characteristics of high-precision detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is the overall circuit diagram of the present invention;

[0016] Figure 2 Schematic diagram of a TIA capacitance detection circuit in the present invention;

[0017] Figure 3 is a schematic diagram of a differential amplifier circuit in the present invention;

[0018] Figure 4 is a schematic diagram of a secondary amplifier circuit in the present invention;

[0019] Figure 5 Schematic diagram of the filter circuit in the present invention. DETAILED DESCRIPTION

[0020] See also Figure 1-5 The present invention provides a technical solution: a capacitance detection circuit based on a differential operational amplifier, including a TIA capacitance detection circuit, a differential amplifier circuit, a secondary amplifier circuit and a filter circuit, the TIA capacitance detection circuit is electrically connected to the differential amplifier circuit, the differential amplifier circuit is electrically connected to the secondary amplifier circuit, and the secondary amplifier circuit is electrically connected to the filter circuit.

[0021] In this embodiment: the function of the capacitor bridge structure is to stabilize the circuit gain and shield the parasitic capacitance.

[0022] The TIA capacitance detection circuit includes C102, C103, C104 and C105, which form a capacitance bridge and are connected to two TIA amplifier circuits. The TIA capacitance detection circuit also includes J1, J2 and J8, C102 and C103 are capacitances to be tested, J1 and J2 are externally connected to the control circuit, and J1 and J2 are input terminals of the high-voltage control signal, and J8 is the input terminal of the carrier;

[0023] In this embodiment: TIA is based on current negative feedback, and the gain of the AC signal TIA circuit mainly depends on the resistance in the amplifier feedback loop. The transfer function and voltage gain of the two-way TIA are as follows:

[0024] Transfer function: U O 1=I f *R f (I f =Ui / Z C )

[0025] Voltage Gain:

[0026] Here, the output voltage and input voltage have a 90° phase change, and the impact on the subsequent amplification value can be ignored. When the capacitance difference △C is generated in the capacitor to be measured (C1 increases △C / 2, C2 decreases △C / 2), at this time, the circuit op amp voltage gains G1 and G2 have changed, that is,

[0027] G11=R f *2π*f*C1,G12=R f *2π*f*C2

[0028] Therefore, the output voltage of the two circuits is: Uo=Ui*G

[0029] Therefore, the function of the first part of the circuit is to convert the rear-end current change caused by the capacitance change into a voltage change and output it from the two TIAs.

[0030] The differential amplifier circuit uses the AD620 operational amplifier to form a differential amplifier circuit. The transfer function of the AD620 operational amplifier is: V = G (V + - V -), and its gain G depends on its external resistance value Rg. The calculation formula is G = 49.4KΩ / Rg + 1, where Rg in this circuit is R 24 ;

[0031] In this embodiment, the function of the circuit is to subtract and amplify the two voltages of the previous stage, and the output voltage amplitude signal represents the difference between the voltages of the previous stage.

[0032] That is: Uo2=G2|U11-U12|=Ui*|G11-G12|*G2

[0033] Uo2=UiR f 2πfG2ΔC.

[0034] The secondary amplifier circuit is a proportional operational amplifier using OP07, which is connected to a negative feedback resistor R 9 , the gain of the secondary amplifier circuit is: G 3 =1+R 9 / R 8 =16.7, used to amplify the front-end output voltage twice, with the phase and frequency unchanged;

[0035] In this embodiment: the output voltage is: Uo3 = UiR f 2πfG2G3ΔC.

[0036] The filter circuit is composed of two consecutive low-pass filters and two high-pass filters based on LF411. The output end of the filter circuit is connected to the input end of J12, and J12 is the output port of the filtered signal.

[0037] In this embodiment: the frequency of the input carrier can be selected and set according to the passband. Due to the change of the capacitance to be measured, the circuit modulates the carrier, and then the carrier is filtered out by the filter circuit to obtain a valid output signal.

[0038] The present invention has the following features: TIA is a transimpedance amplifier, and FFT is a fast Fourier transform;

[0039] The capacitance detection circuit also includes J6 and J7, which are interfaces for externally connecting a 15V voltage.

[0040] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A capacitance detection circuit based on a differential operational amplifier, comprising a TIA capacitance detection circuit, a differential amplifier circuit, a secondary amplifier circuit and a filter circuit, characterized in that: The TIA capacitance detection circuit is electrically connected to the differential amplifier circuit, the differential amplifier circuit is electrically connected to the secondary amplifier circuit, and the secondary amplifier circuit is electrically connected to the filter circuit; The TIA capacitance detection circuit includes C102, C103, C104 and C105, which form a capacitance bridge and connect two TIA amplification circuits; The TIA capacitance detection circuit also includes J1, J2 and J8, C102 and C103 are capacitors to be tested, J1 and J2 are externally connected to the control circuit, and J1 and J2 are input terminals of the high-voltage control signal, and J8 is the input terminal of the carrier; The differential amplifier circuit adopts AD620 operational amplifier to form a differential amplifier circuit; The transfer function of the AD620 operational amplifier is: V = G (V + - V -), and its gain G depends on its external resistance value Rg, and the calculation formula is G = 49.4KΩ / Rg + 1, where Rg is R24 in this circuit, and this circuit refers to a differential amplifier circuit; The secondary amplifier circuit is a proportional operational amplifier, using OP07, and OP07 is connected to a negative feedback resistor R9.

2. A capacitance detection circuit based on a differential operational amplifier according to claim 1, characterized in that: The gain of the secondary amplifier circuit is: G3=1+R9 / R8=16.7, which is used to perform secondary amplification on the front-end output voltage, and the phase and frequency remain unchanged, wherein R8 is a protection resistor, R8=R9 / 15.

7.

3. The capacitance detection circuit based on differential operational amplifier according to claim 1, characterized in that: The filtering circuit consists of two consecutive low-pass filters and two high-pass filters, both based on LF411.

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