Energy-Efficient CMOS Humidity Sensors Using Adaptive Range-Shift Zoom CDC and Power-Aware Floating Inverter Amplifier Array

This article presents an adaptive zoom-capacitance-to-digital converter (CDC)-based CMOS humidity sensor. The humidity sensor is realized by means of two differential capacitors whose dielectrics are sensitive to humidity. The sensing capacitors are interfaced with a zoom CDC, which consists of a successive-approximation-register (SAR) analog-to-digital converter (ADC) and a 3rd-order delta–sigma modulator ( $\Delta \Sigma \text{M}$ ). The SAR ADC eliminates the influence of the baseline capacitance to reduce the input range of the $\Delta \Sigma \text{M}$ . To improve the energy efficiency of the CDC across the full input range, a power-aware floating inverter amplifier (FIA) array is proposed, which is configured based on the conversion results of the SAR logic. In addition, an adaptive range-shift (ARS) zoom CDC is proposed to: 1) resist off-chip parasitics and interference and 2) allow low redundancy and a more energy-efficient FIA-based comparator, thus reducing power consumption. The proposed CMOS humidity sensor is implemented in a 0.11- $\mu \text{m}$ CMOS process. Measurement results show a capacitance resolution of 17.9 aF and an effective number of bits (ENOB) of 14.0 within a conversion time of 1.01 ms. The proposed humidity sensor consumes 1.5 $\mu \text{W}$ of power and exhibits a 0.0094 % relative humidity (RH) resolution and a ±1.5 %RH peak-to-peak accuracy (3 $\sigma $ error of 5.5 %RH) among 12 chips from 20 to 85 %RH, and it achieves a figure of merit (FoM) of 0.135 pJ $\cdot $ %RH², which is more than six times better than the state of the art.

I. Introduction

In the era of the Internet of Things (IoT), an enormous number of sensors need to be deployed at IoT nodes. Energy-efficient sensor interface circuits are the key to the successful application of such sensors due to the limitations of the batteries or energy harvesters of IoT nodes. In addition, good sensor resolution is required to meet the demands for the perception of weak signals, sensor nonlinearity calibration, and temperature compensation. Moreover, to ensure that such a chip can remain stable for a long time in a harsh environment, it must withstand process, voltage, and temperature (PVT) variations as well as unforeseeable parasitics and interference.