P2.9.5 A Novel OA-Based Oscillating Circuit for Uncalibrated Capacitive and Resistive Sensor Interfacing Applications

In this paper, we propose a novel square-wave oscillator, based on Operational Amplifier (OA), making an impedance-to-period (Z-T) conversion that, instead of a voltage integration typically performed by other solution presented in the literature, is based on a voltage differentiation. This circuit is suitable as first analog front-end for both capacitive (e.g., for relative humidity) and resistive (e.g., for gas) sensors, working also for wide−range variations (e.g., about 6 capacitive variation decades). Circuit sensitivity can be easily set through the setting of passive components. Preliminary experimental measurements, utilizing the fabricated prototype PCB, sample passive components and the commercial capacitive humidity sensor Honeywell HCH−1000, have shown good linearity and accuracy in the estimation of both capacitances, having a baseline or changing their value in the range [pF√µF], as well as for more reduced variations of resistance, ranging from kΩ to MΩ.