A low power read-out circuit with frequency accuracy of 0.2% for capacitive and resistive sensors

Abstract

This paper presents a read-out circuit for capacitive and resistive sensors with large scale which converts capacitance or resistance to frequency. It utilizes a high precision and high speed comparator to control the oscillation. A method using correlated double sampling (CDS) technique in continuous-time comparator is proposed to attenuate the DC offset and low frequency flicker noise. A calibration circuit was adopted for the on-chip resistance used for capacitive sensors to achieve small variation caused by temperature. This design is based on 0.18μm CMOS process. The circuit works well when capacitance values are in the range from 10 pF to 100 pF and resistance values are in the range from 20 KΩ to 150 KΩ. Simulation results show that the variation of oscillation frequency is only 0.2% from −40 □ to 80Ό, and the minimum capacitance and resistance could be detected is 20 fF and 40 Ω respectively. The power consumption of the read-out circuit is about 40.4 μW, which could be easily integrated in a UHF RFID tag.