A Novel Uncalibrated Read-Out Circuit for Floating Capacitive and Grounded/Floating Resistive Sensors Measurement

We here propose a new uncalibrated read-out circuit suitable for both capacitance and resistance measurements. This solution, employing only two Operational Amplifiers (OAs) as active blocks and some passive components, is based on a square-wave oscillator which, instead of a voltage integration typically performed by other solutions in the literature, operates a voltage differentiation. The circuit, performing an impedance-to-period (Z-T) conversion, results to be suitable as first analog front-end for both wide variation capacitive (e.g., for relative humidity) and resistive (e.g., for gas) sensors. Circuit sensitivity and dynamic range can be easily set through some passive components. A suitable prototype PCB has been fabricated so to perform electrical and humidity measurements, through the use of sample components and a commercial capacitive humidity sensor. Experimentals have shown a linear behaviour and a satisfactory accuracy in the evaluation of both floating capacitive (in the range [pF÷μF]) and grounded resistive (in the range [kΩ÷MΩ]) variations.