Fully digital ultra low power capacitive sensor interface

Abstract

This report presents a fully-digital ultra low power capacitive sensor interface, which directly converts sensor capacitance to its digital value. Capacitive interface is divided into two parts, 1) capacitance to time converter (CTC) and 2) time to digital converter (TDC). This report demonstrates working principle of CTC and TDC and its implementation using UMC 0.18 mm CMOS technology. Firstly, demonstration of capacitive controlled oscillator (CCO) is carried out by keeping sensor capacitance in the range of 5 pF - 25 pF. CCO generates pulse signals according to sensor capacitance. TDC gets input from CCO. TDC works with three major components, i.e. delay line, edge combiner and counter. TDC measures pulse width of pulse signals generated form CCO and gives their digital equivalent value in two forms:- fine timing data and coarse timing data. Finally, control logic unit is implemented to control each unit and to reduce power consumption of TDC design. To overcome threshold variation at process corners and temprature variation, calibration method is built. Trapezoidal method is used to estimate sensor capacitance using sensor and calibration data. With approximate power consumption of 14nW, ENOB of 6.1 and peak acquisition time of 792 mS, peak FOM of 80.82 fJ/conv is obtained at an operating voltage of 0.3V.