Ultra-high-resolution detection circuit and method for differential capacitance based on AC bridge

Abstract

The present invention discloses an ultra-high-resolution detection circuit and method for differential capacitance based on an AC bridge. The circuit comprises an AC bridge, a modulated carrier source connected to the AC bridge, and a circuit module. The circuit module comprises an instrument amplifier, an input bootstrap circuit, and a negative power supply bootstrap circuit. The present invention adopts an ultra-high-resolution detection circuit and method for differential capacitance based on an AC bridge. Taking into account the influences of the asymmetry of the differential capacitance of the accelerometer head and the parasitic capacitance of the circuit, a bootstrap circuit is introduced, and a device with a relatively low resistance is selected to achieve high input impedance, thereby improving circuit reliability and gain stability. By introducing a "bootstrap capacitor Cm" at the amplifier input, the voltage loss of the low-capacitance sensor between the non-inverting terminal and the inverting terminal of the first-stage amplifier is reduced. A negative power supply bootstrap circuit is introduced at the negative power supply terminal of the amplifier to reduce the parasitic capacitance between the non-inverting input terminal of the first-stage amplifier and the negative power supply. At the same time, a low-pass filter is introduced to ensure the stability of the amplifier.

Description

Technical Field

[0001] The present invention relates to a quartz flexible accelerometer differential capacitance readout technology, and in particular to an ultra-high resolution detection circuit and detection method of differential capacitance based on an AC bridge. Background Art

[0002] The quartz flexible accelerometer is a force-balanced closed-loop servo-controlled accelerometer primarily used in high-precision inertial applications. Its operating principle is as follows: an external input acceleration causes the quartz pendulum to swing, resulting in a tiny displacement, which in turn causes the capacitance of the differential capacitor to change. The servo loop measures the differential capacitance, calculates the feedback force, and applies the feedback current to the torque coil to offset the input inertial torque.

[0003] In a servo loop, the essential purpose of the differential capacitance sensor within the instrument is to measure the deflection angle of the pendulum. Therefore, the accuracy of the differential capacitance detection plays a decisive role in the accelerometer's performance indicators, such as resolution, stability, and linearity. Currently, the more mature differential capacitance detection methods include bridge capacitance detection circuits and switch capacitance detection circuits.

[0004] The bridge capacitance detection circuit mainly includes triangle wave current bridge modulation detection circuit, dual carrier bridge modulation detection circuit and single carrier bridge modulation detection circuit.

[0005] The triangular wave current bridge modulation detection method has a simple detection principle, but can only achieve a differential capacitance detection resolution of approximately 0.01pF, which limits the measurement resolution of the quartz flexible accelerometer.

[0006] A dual-carrier bridge-modulated differential capacitance detection circuit uses two high-frequency carrier signals with the same frequency and opposite phases to charge and discharge the two capacitors under test, respectively. A single current detector is used for pre-amplification of the charge. This capacitance detection method has a simple circuit structure and does not require a symmetrical circuit structure, thus avoiding measurement errors caused by inconsistencies in circuit components. However, this method requires that the two carrier signals have exactly equal peak voltages, frequencies, and opposite phases. Achieving this dual-carrier signal is extremely difficult and often produces unsatisfactory results.

[0007] Switched capacitance detection circuits use electronic switches to control the charge and discharge of differential capacitance, converting the differential capacitance change information into an analog voltage or directly into a digital signal output. However, switch-type detection circuits have strict requirements for charge and discharge timing, and the electronic switch easily introduces a lot of noise and interference into the analog circuit during the opening and closing process, making them unsuitable for high-precision differential capacitance detection.

[0008] In summary, the traditional quartz flexible accelerometer differential capacitance detection circuit generally has problems such as low resolution and high noise, which limits the measurement accuracy of the accelerometer and inertial navigation system and makes it difficult to achieve ideal accuracy. Summary of the Invention

[0009] To address the above issues, the present invention provides an ultra-high-resolution differential capacitance detection circuit and method based on an AC bridge. Taking into account the influence of the differential capacitance asymmetry of the accelerometer head and circuit parasitic capacitance, a bootstrap circuit is introduced. By introducing a "bootstrap capacitor Cm" at the amplifier input, the voltage loss of the low-capacitance sensor between the non-inverting and inverting terminals of the first-stage amplifier is reduced. A negative power bootstrap circuit is introduced at the negative power supply of the amplifier to reduce the parasitic capacitance between the non-inverting input of the first-stage amplifier and the negative power supply. A full-bridge circuit is constructed by combining a quartz flexible accelerometer head with the circuit structure. Compared to current-type differential capacitance detection solutions, this method has higher accuracy, higher resolution, and lower noise.

[0010] To achieve the above objectives, the present invention provides an ultra-high-resolution differential capacitance detection circuit based on an AC bridge, comprising an AC bridge, a modulated carrier source connected to the AC bridge, and a circuit module, wherein the circuit module comprises an instrumentation amplifier, an input bootstrap circuit, and a negative power supply bootstrap circuit;

[0011] Modulated carrier source for improving carrier signal for bridge capacitance readout;

[0012] Instrumentation amplifier, used to realize dual-path difference and complete pre-stage amplification;

[0013] Input bootstrap circuit and negative power supply bootstrap circuit are used to suppress the influence of distributed parameters.

[0014] The negative power bootstrap circuit includes a first-stage amplifier, a second-stage amplifier and a low-pass filter, the purpose of which is to reduce the parasitic capacitance between the first-stage amplifier's in-phase input terminal and the negative power supply; the inverting terminal of the first-stage amplifier APM1 is connected to the negative power supply via the bootstrap capacitor C m Connect the modulated carrier source, and the first-stage amplifier APM1 is connected between the in-phase terminal and the inverting terminal through the shunt resistor R dc Connection: The output end of the first-stage amplifier APM1 is connected to the inverting end of the first-stage amplifier APM1, and the output end of the first-stage amplifier APM1 is also connected to the inverting end of the second-stage amplifier APM2 through the filter resistor R of the low-pass filter. lp A low-pass filter capacitor C is connected between the in-phase terminal of the secondary amplifier APM2 and the filter resistor. lp One end of the filter capacitor C lp The other end of the second-stage amplifier is grounded, and the inverting end of the second-stage amplifier is connected to the output end of the second-stage amplifier APM2. The output end of the second-stage amplifier APM2 is connected to the feedback capacitor C nIt is connected to the negative power supply interface of the first-stage amplifier; an output circuit is also connected between the filter resistor and the first-stage amplifier.

[0015] The ultra-high-resolution differential capacitance detection method based on an AC bridge includes the following steps:

[0016] S1. Build and integrate a differential capacitance ultra-high-resolution detection circuit based on an AC bridge:

[0017] Combine the internal structure of the accelerometer and the detection circuit module to build a full-bridge detection component, and integrate the detection circuit with the accelerometer head according to the working principle of the quartz accelerometer;

[0018] S2. Verification of the detection circuit module using a partial testing solution:

[0019] Fixed feedback drive causes the meter to output a constant displacement or capacitance difference. The voltage signal output by the detection circuit is measured and combined with a capacitance reference to evaluate measurement noise and resolution.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1) Combine the quartz flexible accelerometer head and circuit structure to build a full-bridge circuit. Compared with current-type differential capacitance detection solutions, it has higher accuracy, higher resolution, and lower noise (detection resolution reaches the order of 1μg).

[0022] 2) A bootstrap circuit with high anti-interference characteristics is introduced to suppress the influence of parasitic capacitance, including input parasitic capacitance and power supply parasitic effects. Furthermore, low-resistance devices are selected to achieve high input impedance, improving circuit reliability and stability.

[0023] 3) By combining electrical parameter simulation with actual sample integration testing, we can select device parameters that can achieve better suppression effects and higher circuit stability, and verify the detection effect of circuit components. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a circuit diagram of the present invention.

[0025] Figure 2 This is the schematic diagram of the differential capacitance detection circuit based on AC bridge.

[0026] Figure 3 This is the schematic diagram of the bootstrapped high input impedance amplifier circuit.

[0027] Figure 4 Improved schematic diagram of negative power supply bootstrap circuit.

[0028] Figure 5 Negative power supply bootstrap circuit for C p2Inhibition effect diagram.

[0029] Figure 6 This is a diagram showing the assembly process of the differential capacitance detection full-bridge circuit for a digital closed-loop quartz flexible accelerometer.

[0030] Figure 7 This is a simulation test result diagram of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be described in further detail below with reference to the accompanying drawings.

[0032] Figure 1 It is the circuit principle diagram of the present invention, as shown in FIG. Figure 1 As shown, a differential capacitance ultra-high resolution detection circuit based on an AC bridge is characterized by comprising an AC bridge, a modulated carrier source connected to the AC bridge, and a circuit module, wherein the circuit module comprises an instrumentation amplifier, an input bootstrap circuit, and a negative power supply bootstrap circuit;

[0033] Modulated carrier source for improving carrier signal for bridge capacitance readout;

[0034] Instrumentation amplifier, used to realize dual-path difference and complete pre-stage amplification;

[0035] Input bootstrap circuit and negative power supply bootstrap circuit are used to suppress the influence of distributed parameters.

[0036] Considering the influence of the asymmetry of the differential capacitance of the accelerometer head and the parasitic capacitance of the circuit, a bootstrap circuit is introduced. m "Reduce the voltage loss of the low-capacitance sensor between the non-inverting terminal and the inverting terminal of the first-stage amplifier; the negative power bootstrap circuit includes a first-stage amplifier, a second-stage amplifier and a low-pass filter, the purpose of which is to reduce the parasitic capacitance between the non-inverting input terminal of the first-stage amplifier and the negative power supply; in the actual circuit, the introduction of feedback from the negative power supply will cause the amplifier to be unstable, so the low-pass filter is introduced. Specifically, the non-inverting terminal of the first-stage amplifier APM1 is connected to the AC bridge, and the inverting terminal of the first-stage amplifier APM1 is connected to the AC bridge through the bootstrap capacitor C m Connect the modulated carrier source, and the first-stage amplifier APM1 is connected between the in-phase terminal and the inverting terminal through the shunt resistor R dc Connection: The output end of the first-stage amplifier APM1 is connected to the inverting end of the first-stage amplifier APM1, and the output end of the first-stage amplifier APM1 is also connected to the inverting end of the second-stage amplifier APM2 through the filter resistor R of the low-pass filter. lp A low-pass filter capacitor C is connected between the in-phase terminal of the secondary amplifier APM2 and the filter resistor. lp One end of the filter capacitor C lpThe other end of the second-stage amplifier is grounded, and the inverting end of the second-stage amplifier is connected to the output end of the second-stage amplifier APM2. The output end of the second-stage amplifier APM2 is connected to the feedback capacitor C n It is connected to the negative power supply interface of the first-stage amplifier; an output circuit is also connected between the filter resistor and the first-stage amplifier.

[0037] The ultra-high-resolution differential capacitance detection method based on an AC bridge includes the following steps:

[0038] S1. Build and integrate a differential capacitance ultra-high-resolution detection circuit based on an AC bridge:

[0039] Combine the internal structure of the accelerometer and the detection circuit module to build a full-bridge detection component, and integrate the detection circuit with the accelerometer head according to the working principle of the quartz accelerometer;

[0040] S2. Verification of the detection circuit module using a partial testing solution:

[0041] Fixed feedback drive causes the meter to output a constant displacement or capacitance difference. The voltage signal output by the detection circuit is measured and combined with a capacitance reference to evaluate measurement noise and resolution.

[0042] For further explanation, the design process of this embodiment is disclosed:

[0043] According to the internal structure of the quartz accelerometer head, a differential capacitance detection circuit based on AC bridge is designed to ensure the full bridge characteristics. Figure 2 As shown, it is actually a pseudo bridge structure, which mainly consists of two parts: AC bridge and instrument amplifier.

[0044] Ideally, without considering the influence of parasitic capacitance, noise and other factors, the output voltage signal is as follows:

[0045]

[0046] The differential capacitance detection signal obtained is the modulation signal V IN The voltage signal with the same frequency and phase, and V out It is proportional to ΔC, but the influence of external capacitors C1 and C2 is also introduced in this part of the transfer function.

[0047] The noise of the AC bridge differential capacitance detection scheme in quartz flexible accelerometers is better than that of the current-type scheme, but the influence of parasitic capacitance cannot be eliminated. Interference factors exist in the transfer function, affecting the gain and introducing additional detection zeros. Therefore, parasitic capacitance needs to be suppressed. In quartz flexible accelerometers, the carrier signal is applied to the common plate, that is, the yoke. The signal source can largely suppress the interference caused by the yoke part. In traditional analog accelerometer schemes, the common plate is grounded. Similarly, the capacitance in the transmission cable. According to analysis, the typical parasitic capacitance under a 10cm transmission line is about C L = 0.928pF. If long-distance transmission is unavoidable in practical applications, an equipotential shield can be added around the cable to isolate some of the parasitic capacitance. The following discusses optimization methods for high-impedance op amp circuits, in addition to meter connections.

[0048] Ideally, the input impedance of the follower is infinite and has no effect on the front-end AC bridge. In practice, a resistor R is required. dc Providing a DC path, it tends to "shunt" the current in nature, resulting in a loss of output voltage for low capacitance sensors. Figure 3 As shown (taking half bridge as an example), the bootstrap capacitor C m ”.

[0049] The equivalent input resistance of the circuit is calculated as shown in formula (2):

[0050]

[0051] According to formula (2), the input impedance amplification factor is approximately On the one hand, it can reduce R dc The value of can get a larger input impedance and reduce the impact on the detection capacitor. The bootstrap circuit introduces positive feedback, and the circuit is prone to oscillation (instability). Therefore, the capacitor C m The selection is critical and requires careful adjustment through simulation and actual circuit testing. The input capacitance of the op amp may exist in three forms, as shown below:

[0052] 1) The capacitance between the non-inverting input and the inverting input is about a few pF, denoted as C p1 ;

[0053] 2) The capacitance between the non-inverting input and the negative power supply, denoted as C p2 ;

[0054] 3) The capacitance between the non-inverting input and the positive power supply is denoted as C p3 .

[0055] Among them, the capacitor C p1 The influence of can be ignored. For parasitic capacitance C p3 , since the positive power supply is connected, the bootstrap circuit cannot suppress Cp3 For the parasitic capacitance C p2 , introduce a negative power bootstrap circuit, such as Figure 4 (a) shows that the negative power supply is connected to the C n Introducing negative feedback, the AC channel can suppress C due to the equal potential of the in-phase and inverting terminals. p2 In actual circuits, the introduction of feedback from the negative power supply will cause the amplifier to be unstable, so a low-pass filter is introduced, such as Figure 4 As shown in (b), the filter frequency is between the amplifier unity gain bandwidth value and the signal source frequency value.

[0056] Simulation verification of "feedback capacitor C n "For C p2 The inhibitory effect, such as Figure 5 As shown. When C p2 When the parasitic capacitance C is 5pF and 10pF respectively, the output amplitude attenuation is 0.51dB and 1.19dB respectively. After the negative power supply bootstrap circuit is introduced, the signal output does not attenuate. p2 The larger the value, the more the output amplitude is attenuated, which affects the output gain of the detection circuit. Adding a negative power supply bootstrap circuit can suppress C p2 impact.

[0057] Build a digital closed-loop quartz flexible accelerometer differential capacitance detection full-bridge circuit and integrate it. The circuit structure and installation method are as follows: Figure 6 As shown. For the detection circuit module, a "partial" test solution was used to complete the verification. Specifically, the feedback drive was fixed so that the meter output a constant displacement or capacitance difference, the voltage signal output by the detection circuit was measured, and the measurement noise and resolution were evaluated in conjunction with a capacitance benchmark. The test results and curves are shown in the table. Under static conditions, there is an approximate relationship between the acceleration input excitation and the capacitance detection output: 1μg → 492μV. Based on the simulation test results of the fixed high-precision capacitor in the preamplifier section, it can be seen that the background noise of the detection circuit achieves a resolution of 1μg. When the input differential capacitance is equivalent to 0pF, the noise characteristic of the AC bridge solution is equivalent to 0.19μg (1σ); when the input differential capacitance is equivalent to 3pF, due to the residual influence of parasitic capacitance, the measurement background noise of the AC bridge solution reaches 0.56μg (1σ).

[0058] Therefore, the present invention adopts the above-mentioned ultra-high-resolution detection circuit and detection method of differential capacitance based on AC bridge, combines the quartz flexible accelerometer head and circuit structure, and builds a full-bridge circuit, which is suitable for high-resolution detection of ultra-high-precision digital closed-loop quartz flexible accelerometers, and the detection resolution reaches the order of 1μg; combining the transfer function and distributed parameter characteristics, a bootstrap circuit with high anti-interference characteristics is designed, and its inhibitory effect on distributed parameters is verified through simulation; a detection circuit module is designed and integrated into the accelerometer head, and a "partial" detection mechanism is proposed to verify the detection noise and resolution of the module.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A differential capacitance ultra-high resolution detection circuit based on an AC bridge, characterized by: It includes an AC bridge, a modulated carrier source connected to the AC bridge, and a circuit module, wherein the circuit module includes an instrument amplifier, an input bootstrap circuit, and a negative power bootstrap circuit; Modulated carrier source for improving carrier signal for bridge capacitance readout; Instrumentation amplifier, used to realize dual-path difference and complete pre-stage amplification; Input bootstrap circuit and negative power supply bootstrap circuit are used to suppress the influence of distributed parameters; The negative power bootstrap circuit includes a first-stage amplifier, a second-stage amplifier and a low-pass filter, the purpose of which is to reduce the parasitic capacitance between the first-stage amplifier's in-phase input terminal and the negative power supply; the inverting terminal of the first-stage amplifier APM1 is connected to the negative power supply via the bootstrap capacitor C m Connect the modulated carrier source, and the first-stage amplifier APM1 is connected between the in-phase terminal and the inverting terminal through the shunt resistor R dc Connection: The output end of the first-stage amplifier APM1 is connected to the inverting end of the first-stage amplifier APM1, and the output end of the first-stage amplifier APM1 is also connected to the inverting end of the second-stage amplifier APM2 through the filter resistor R of the low-pass filter. lp A low-pass filter capacitor C is connected between the in-phase terminal of the secondary amplifier APM2 and the filter resistor. lp One end of the filter capacitor C lp The other end of the second-stage amplifier is grounded, and the inverting end of the second-stage amplifier is connected to the output end of the second-stage amplifier APM2. The output end of the second-stage amplifier APM2 is connected to the feedback capacitor C n It is connected to the negative power supply interface of the first-stage amplifier; an output circuit is also connected between the filter resistor and the first-stage amplifier.

2. Ultra-high-resolution differential capacitance detection method based on AC bridge, characterized by: The following steps are involved: S1. Build and integrate the differential capacitance ultra-high resolution detection circuit based on the AC bridge as claimed in claim 1: Combine the internal structure of the accelerometer and the detection circuit module to build a full-bridge detection component, and integrate the detection circuit with the accelerometer head according to the working principle of the quartz accelerometer; S2. Verification of the detection circuit module using a partial testing solution: Fixed feedback drive causes the meter to output a constant displacement or capacitance difference. The voltage signal output by the detection circuit is measured and combined with a capacitance reference to evaluate measurement noise and resolution.

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