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

This article presents a high-performance continuous-time (CT) capacitance-to-digital converter (CDC) for floating-target displacement measurement applications. It uses a differential first-order <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> 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<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \Sigma $</tex-math> </inline-formula>M). The electrical field interference of the sensor, modeled as a capacitive-coupled charge on the common plate, is rejected by differential-mode readout and built-in bandpass interference filtering of the proposed architecture. To achieve a high temperature stability, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> filter’s temperature coefficient (TC) is compensated by driving it using an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> frequency reference with the same temperature dependency. Fabricated in a standard 180-nm CMOS process, the sensor draws <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$347~\mu $</tex-math> </inline-formula>A from a 1.8-V supply. The sensor offers common-mode rejection ratios (CMRRs) of >119 dB near dc and >97 dB around 100 MHz, providing reliable electrical field interference immunity. Furthermore, the CDC achieves a wide dynamic range (DR) of more than 143 dB without any sub-ranging and a competitive TC of <25 ppm/°C from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 40~^{\circ }$</tex-math> </inline-formula>C to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$85~^{\circ }$</tex-math> </inline-formula>C.