Performance Investigation Of Demodulation Circuit Based On Combination Of The Analog Switch And Operational Amplifier

Capacitive displacement sensing is one of the key technologies of high-precision space inertial sensors,which mainly adopts the measurement principle of high-frequency modulation and demodulation based on the bridge structure.Since the measurement frequency of the inertial sensors in space gravitational wave detection needs to reach the millihertz,meanwhile,the current research results show that demodulation circuit noise is one of the main influence to the low-frequency performance of capacitive displacement sensors.In this thesis,theoretical and experimental research on the low frequency performance of the demodulation circuit are carried out,and the main results obtained are as follows.First of all,the basic principles and main schemes of the demodulation circuit are introduced systematically.It focuses on the design of six specific implementation schemes based on the electronic switch-type demodulation circuit.The basic principles of the first to fourth demodulation circuit scheme is:the signal is positively and negatively amplified,and then the analog switch is used for detection;the basic principle of the latter two demodulation circuit schemes is:first,the analog switch is used to detect the signal,and then the signal is differentially amplified and filtered by low-pass.We have completed the parameter design and noise analysis of the six schemes,and analyzed the influence of the charge injection effect of the analog switch.The design results show that the circuit combination:analog switch（ADG613）+differential amplifier（op amp is AD8629）+low-pass filter（op amp is AD8629）,which is named the Scheme 6 has a low noise floor,which lays a certain theoretical foundation for follow-up experimental test research.Secondly,the circuit calibration and noise test on the above-mentioned six schemes are conducted.The analysis of experimental data shows that the circuit calibration results are consistent with the theoretical expectations within the error range.The performance of the six schemes are compared with experiments,and the results of Scheme 6 is the best.The equivalent input voltage noise level reaches 0.7μV/Hz^1/2 at 1m Hz,which is consistent with the theoretical analysis.The effect which this scheme achieved largely benefits from the circuit structure and the excellent low-frequency performance of the operational amplifier AD8629.The performance of the sensing circuit can be improved based on this circuit structure in the future.Based on Scheme 6,the analog switch performance impact experiment is carried out,and the result shows that the contribution of the analog switch to the circuit noise is less than 0.4μV/Hz^1/2.Finally,based on the optimal demodulation circuit scheme,a capacitance displacement sensing circuit is built.The sensitivity coefficient calibration and circuit resolution test of the sensing circuit are completed.With the sensitivity coefficient of 10.5V/p F,the test results show that the resolution of sensing circuit can reach 2.5×10^-7p F/Hz^1/2 at 1m Hz,which is nearly 4 times better than the demodulation circuit before optimization.