A Full-Digital Multi-Channel CMOS Capacitive Sensor

Abstract

A full-digital 12-channel, 100-step capacitive sensor is described. The capacitance to be sensed forms an RC-delay line whose delay is compared with that of a reference RC-delay line. The difference of the RC delays is sensed by a simple full-digital time-to-digital converter (TDC). By compensating the parasitic capacitance at power up, the capacitive sensor implemented in a 0.35 mum standard digital CMOS technology shows 30fF sensing resolution. The capacitive sensor consumes 5 muA per channel under 3.3V supply voltage.