18.2 A 16fJ/Conversion-Step Time-Domain Two-Step Capacitance-to-Digital Converter

Abstract

Capacitive sensors are widely used to measure various physical quantities, including pressure, humidity [1], and displacement [2]. Ultra-low-power capacitance-to-digital converters (CDCs) are required for sensors with limited battery capacity or powered by energy harvesters. A SAR CDC is simple to design and well-suited for low-to-medium resolution applications. However, to reach high resolution, it requires a low-noise comparator [3] or OTA-based active charge transfer [4], resulting in degraded power efficiency. The ΔΣ CDC [1] is suitable for high-resolution applications, but it requires OTAs and repeated charging of the sensing capacitor, leading to high power consumption. The Zoom CDC in [5] achieves high resolution with only one-time charging, but its energy efficiency is still limited by power-hungry OTAs. The open-loop SAR-VCO CDC in [6] achieves low power consumption by eliminating the OTA; however, the VCO gain variation causes inter-stage gain error and requires background calibration, which increases the design complexity and makes it unsuitable for single-shot measurement in sensor node applications due to the long convergence time.