A highly sensitive readout circuitry for a wide range thin film capacitive humidity sensors

This paper presents a highly precise and stable electronic readout circuitry for acquiring and processing raw signal data from capacitive sensors. The standalone readout system accurately measures the sensor capacitance in real-time and converts to frequency. The performance of the system was first calculated mathematically and was tested with discrete capacitors. To find the effectiveness of the circuit, thin film capacitive trace moisture sensor has been used in place of discrete capacitors. The theoretical findings are compared with the experimental results obtained with the developed system. It was found the system can accurately measure all types of capacitive sensors within the range of 191.6 pF to 1368 pF with a maximum error of 2.54%. The capacitance to frequency conversion sensitivity of the circuit for discrete capacitance was found as 65 Hz/pF at oscillator frequency of 100 kHz. The whole system is capable of accurately measuring humidity within the accuracy of ±1 ppm.