A 1.2-V 8.3-nJ CMOS Humidity Sensor for RFID Applications Zhichao Tan, Student Member , IEEE , Roel Daamen, Aurélie Humbert, Y ouri V . Ponomarev, Y

Abstract

This paper presents a fully integrated CMOS hu- midity sensor for a smart RFID sensor platform. The sensing element is a CMOS-compatible cap acitive humidity sensor, which consists of top-metal finger-structure electrodes covered by a humidity-sensitive polyimide laye r. Its humidity-sensitive capac- itance is digitized by an energy-ef ficient capacitance-to-digital converter (CDC) based on a third- order delta-sigma modulator. This CDC employs current-ef ficient operational transconductance amplifiers based on current-starved cascoded inverters, whose limited output swing is accommodated by employing a feedfor- ward loop- filter topology. A programmable offset capacitor is included to remove the sensor’s b aseline capacitance and thus reduce the required dynamic range. To reduce offset errors due to charge injection of the switches, t he entire system is auto-zeroed. The proposed humidity sensor has been realized in a 0.16- m CMOS technology. Measurement results show that the CDC performs a 12.5-bit capacitance-to-digital conversion in a mea- surement time of 0.8 ms, while consuming only 8.6 Af r o ma 1.2-V supply. This corresponds to a state-of-the-art figure-of-merit of 1.4 pJ/conversion-step. Comb ined with the co-integrated hu- midity sensing element, it provides a resolution of 0.05% RH in the range from 30% RH to 100% RH while consuming only 8.3 nJ per measurement, which is an or der-of-magnitude less energy than the state-of-the-art.