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

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 (∝ 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 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-μm CMOS process, this AFE consumes 15.6 μW 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·K2) and RH [0.83 pJ·(%RH)2].