Uncalibrated operational amplifier-based sensor interface for capacitive/resistive sensor applications

In this paper, a new configuration of operational amplifier -based square-wave oscillator is proposed. The circuit performs an impedance-to-period (Z-T) conversion that, instead of a voltage integration typically performed by other solutions presented in the literature, is based on a voltage differentiation. This solution is suitable as first analogue uncalibrated front-end for capacitive and resistive (e.g. relative humidity and gas) sensors, working also, in the case of capacitive devices, for wide variation ranges (up to six capacitive variation decades). Moreover, through the setting of passive components, its sensitivity can be easily regulated. Experimental measurements, conducted on a prototype printed circuit board, with sample passive components and using the commercial capacitive humidity sensor Honeywell HCH-1000, have shown good linearity and accuracy in the estimation of capacitances, having a baseline or reaching a value ranging in a wide interval [picofarads-microfarads], as well as, with a lower accuracy, in the evaluation of more reduced variations of resistances, ranging from kiloohms to megaohms, also when compared with other solutions presented in the literature.