Impedance-to-Time Converter Circuit for Leaky Capacitive Sensors With Small Offset Capacitance

Capacitive sensors suffer from problems of the parasitic and offset capacitors. Also, in some capacitive sensors, leaky resistors carry information along with the sensor capacitor. An accurate impedance-to-time converter (ITC) circuit for leaky capacitive sensors that is able to deal with these problems and estimate the sensor capacitor and leaky resistor is presented in this article. The proposed circuit is a modified relaxation oscillator, which combines dual slope and charge transfer methods to measure the sensor capacitor and leaky resistor. The output pulses of the oscillator are acquired, and pulsewidth corresponding to high and low pulse are extracted and averaged. The effect of dc errors and jitter are significantly reduced because of the averaging. A prototype of the proposed ITC has been developed and tested. Experimental results show that the proposed ITC can measure low values of the sensor capacitor (minimum 9.6 pF) in the presence of a wide range of leaky resistors (270 kΩ to 9.6 MΩ) and offset capacitors (9.6 pF to 543.4 pF) with a worst-case error of about 1.1%, as compared to the theoretical values. The targeted application for the designed circuit is the humidity sensor with a frequency range between 500 Hz and 5 kHz.