An Auto-Balancing Capacitance-to-Pulse-Width Converter for Capacitive Sensors

A novel auto-balancing capacitance-to-pulse-width converter (CPC) that uses sinusoidal excitation, and operates in a closed-loop configuration, is presented in this paper. Unlike most of the existing CPCs, the proposed interface circuit is compatible with both single-element and differential capacitive sensors. In addition, it provides a pulse-width modulated (PWM) signal which can easily be digitized using a counter. From this PWM signal, a ratio output is derived when a single-element sensor is interfaced, and a ratiometric output is obtained for a differential sensor. The final digital output is independent of the nominal capacitance of the sensor and has a linear characteristic irrespective of the sensor characteristic being linear or inverse. The CPC is designed such that the PWM output depends on the change in the sensor capacitance alone. It is insensitive to parasitic capacitance and has very low sensitivity to the non-idealities of the components and ICs used. The effects due to some of the non-idealities are automatically corrected by the negative feedback based auto-balancing employed. The effect of component mismatch is significantly reduced by a one-time correction mechanism. These benefits are achieved without the use of any complex or expensive analog building blocks. The prototype exhibits a maximum non-linearity error of less than 0.7%, a resolution of 13.02 effective number of bits (ENOB), a signal-to-noise ratio (SNR) of 80.12 dB, and a rise time of 5 ms. Thus, the proposed simple, yet effective, low-power, low cost, auto-balancing CPC can be used to interface a wide range of existing and new capacitive sensors to digital systems.