Fully digital, low energy capacitive sensor interface with an auto-calibration unit

Abstract

This report presents an all-digital low energy capacitive sensor interface (CSI) withcalibration unit implemented in cadence virtuoso with 0.18mm CMOS technology.The structure consists of two blocks 1) Capacitance to Time Converter (CTC) 2) Time to Digital Converter (TDC). The CTC block uses a Capacitive Controlled Oscillator (CCO) in addition to a Calibration Unit with a dummy capacitor bank for estimating the capacitance value. The TDC consists of a ring delay line andan edge combiner. The CTC generates a pulse width according to the sensing capacitance (Csense) and passes the generated pulse to the TDC. TDC provides the coarse and fine counts according to the pulse width. The range of the Csense is varied from 0.1pF to 36pF that can cover a wide range of applications. Energy efficiency of this circuit is 81.74pJ and average power consumption is 28.03nWwith 5.10 bit accuracy at an operating voltage of 1.8V at 10Hz sampling frequency and can measure a minimum change in capacitance of 503.88 f F. At a lower voltage of 0.3V, the CSI consumes 1.26nW with 7.15 bit accuracy for the same sampling frequency.