A 2-in-1 Temperature and Humidity Sensor With a Single FLL Wheatstone-Bridge Front-End Haowei Jiang , Student Member , IEEE, Chih-Cheng Huang , Matthew R. Chan, Student Member , IEEE, and Drew A. Hall , Senior Member , IEEE

Abstract

This article presents a CMOS 2-in-1 relative humid- ity (RH) and temperature (Temp) sensor. A single analog front-end (AFE) interfaces two on-chip transducers and con- verts the resistance ( ∝ Temp) and capacitance ( ∝ 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 fash- ion. 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- µm CMOS process, this AFE consumes 15.6 µWa c h i e v i n ga2 - m K 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 ·K 2)a n dR H [0.83 pJ·(%RH)2].