A Continuous-Time Capacitance-to-Digital Converter for Floating-Target Displacement Sensing

Abstract

This article presents a high-performance continuous-time (CT) capacitance-to-digital converter (CDC) for floating-target displacement measurement applications. It uses a di fferential first-order RC low-pass filter (LPF) front end to convert parallel-plate capacitance to phase shift, which is further digitized by a high-resolution second-order CT phase-domain (PD) delta–sigma modulator (CT-PD ∆ΣM). The electrical field interference of the sensor, modeled as a capacitive-coupled charge on the common plate, is rejected by di fferential-mode readout and built-in bandpass interference filtering of the proposed architecture. To achieve a high temperature stability, the RC filter’s temperature coe fficient (TC) is compensated by driving it using an RC frequency reference with the same temperature dependency. Fabricated in a standard 180-nm CMOS process, the sensor draws 347 µA from a 1.8-V supply. The sensor o ffers common-mode rejection ratios (CMRRs) of >119 dB near dc and >97 dB around 100 MHz, providing reliable electrical field interference immunity. Furthermore, the CDC achieves a wide dynamic range (DR) of more than 143 dB without any sub-ranging and a competitive TC of <25 ppm /◦C from −40 ◦C to 85 ◦C.