A 13.5 bit 1.6 mW 3rd order CT ΣΔ ADC for integrated capacitance sensor interface

Abstract

An integrated capacitance sensor interface is proposed with three programmable gain stages to measure femto-farad capacitance. The capacitance sensor interface is a differential measurement set-up to measure the change in capacitance over a fixed nominal capacitance. The programmable gain stages are used to change the gain settings to operate for a wide-range capacitance measurement. The implemented continuous time ΣΔ ADC is a third-order cascade of integrators feedforward topology with a signal bandwidth of 10 kHz. The ADC has a measured peak dynamic range of 84.5 dB while consuming 1.6 mW. The measured figure of merit (FoM) is 3.107 pJ-mm2/conversion at a clock frequency of 6.4 MHz having an active area of 0.45 mm2. The ADC was applied in the femto-farad capacitance measurement using a 0.3 pF–1.2 pF variable capacitor typically encountered in MEMS-based sensor applications like pressure/humidity/flow sensing in System-in-Package (SiP) or Systems-on-Chip (SoC).