A Dual-Slope-Based Capacitance-to-Time Signal Conditioning Circuit for Leaky Capacitive Sensors

A robust signal conditioning circuit for capacitive sensors, based on the dual-slope and phase-sensitive-detection technique, is presented in this article. The circuit is designed to measure the capacitance of the leaky capacitive sensors. The phase-sensitive-detection technique mitigates the effect of the shunting conductance. In addition, the circuit is designed in such a way that the error due to the parasitic/stray capacitance on the measurement of the sensor capacitance is negligible. Furthermore, an autotuning frequency-independent quadrature phase-shifter circuit is designed for an accurate 90° phase generation. A prototype of the circuit is built and tested. The experimental results show that the designed circuit can measure the sensor capacitance in the range from 65 to 528 pF with accuracy within ±1%. The robustness of the proposed circuit is tested against the variation in the amplitude and frequency of input sinusoidal excitation signal. The results confirm that the proposed circuit is well suited for sensors, such as capacitive humidity sensors, water level detection sensors, and fingerprint sensors.

I. Introduction

Capacitive sensors are one of the most widely used sensors in industrial, scientific, automation, healthcare, environmental, and agriculture applications for detection and measurement of various physical, chemical, and biological parameters, such as pressure, displacement, force, touch, temperature, humidity, gas concentration, body temperature, micro-droplet, respiration rate, soil moisture content, and body fluid [1]–[5].