A Low Power Read-Out Circuit with Frequency Accuracy of 0.2% for Capacitive and Resistive Sensors

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 continuoustime 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 0.18 mu m 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 K Omega to 150 K Omega. Simulation results show that the variation of oscillation frequency is only 0.2% from -40 rectangle to 80 degrees C, and the minimum capacitance and resistance could be detected is 20 fF and 40 Omega respectively. The power consumption of the read-out circuit is about 40.4 mu W, which could be easily integrated in a UHF RFID tag.