A 2-in-1 Temperature and Humidity Sensor With a Single FLL Wheatstone-Bridge Front-End

Abstract

This article presents a CMOS 2-in-1 relative humidity (RH) and temperature (Temp) sensor. A single analog front-end (AFE) interfaces two on-chip transducers and converts the resistance ( ${\propto }$ Temp) and capacitance ( ${\propto }$ RH) into a time-based signal. This conversion occurs with high linearity (±10 ppm error) across the industrial temperature range (−40 to +85 °C) through a frequency-locked loop (FLL). An incomplete-settling, switched-capacitor-based Wheatstone bridge is proposed to sense the ${R}$ and ${C}$ transducers in a power-efficient fashion. The FLL output is digitized by a low-power, time-to-digital converter (TDC) that has high resolution due to the inherent quantization noise shaping. Implemented in a 0.18- $\mu \text{m}$ CMOS process, this AFE consumes 15.6 $\mu \text{W}$ achieving a 2-mK temperature resolution over the industrial temperature range and 0.0073% RH resolution from 10 to 95 %RH. This is the first reported ${R}$ & ${C}$ -to-digital converter (RCDC) with a single, unified AFE and achieves excellent resolution figure-of-merits (FOMs) when normalized to temperature (62 fJ $\cdot \text{K}^{2}$ ) and RH [0.83 pJ $\cdot $ (%RH)2].