A Capacitive Sensor Interface Circuit Using T-Network Notch Feedback and Phase Sensitive Detection

This paper presents a capacitive displacement sensor interface circuit based on T-network notch feedback and phase sensitive detection, and verifies the feasibility and advantages of the proposed interface circuit through experiments. A transimpedance amplifier with T-network circuit is innovatively applied to the interface circuit to improve signal quality. The T-network circuit composed of several small value resistors is used instead of a large value resistor, which not only achieves equivalent amplification gain but also avoids the problem of signal distortion caused by the poor precision and thermal stability of large value resistors. Meanwhile, unlike existing solutions that use full-wave detection for signal demodulation, in this paper, phase sensitive detection is used for signal demodulation to reduce noise. The effects on the dynamic characteristics of phase sensitive detection with different types of low-pass filters are compared, and a sixth-order Bessel low-pass filter is selected ultimately. A prototype of a capacitive displacement sensor is developed based on the proposed interface circuit. Experiments show that the measurement range exceeds 80μm, the sensitivity is 78m V/μm, the RMS resolution is 36.2nm, and the linearity is ±2.28 %.