An interface circuit for measuring capacitance changes based upon capacitance-to-duty cycle (CDC) converter

Abstract

We present a direct-to-digital capacitive sensor readout circuit that converts capacitance changes of a sensor element to changes of the duty cycle of a square-wave oscillator, which, in turn, is converted to a digital output by a counter. The readout circuit resembles a single-slope analog-to-digital converter structure. There are several advantages of this readout scheme. First, due to its simplicity and low number of components, the power consumption of the circuit is expected to be significantly smaller than in similar digital readout designs. Furthermore, linearization of the output may be achieved using an EEPROM lookup table. Another advantage is the possibility of performing adaptive measurements where the sensor resolution and bandwidth may be changed via the readout circuit software. Finally, we present a theory of the adaptive measurement and an analysis of the design tradeoffs. The capacitance-to-duty cycle readout circuit may achieve large bandwidth and high resolution in a modern low-voltage, low-power CMOS implementation. The performance of a prototype readout circuit built from discrete components is 13-bit effective resolution with a 1-kHz bandwidth.