CCII-based interface for capacitive/resistive sensors

In this paper, we propose a Current-Mode (CM) square-wave oscillator, based on Second Generation Current Conveyors (CCIIs), operating an impedance-to-period conversion, as first analog front-end for wide range capacitive/resistive sensors. When compared to typical solutions presented in the literature utilizing a voltage integration, the proposed solution is based on a current differentiation. The interface has been designed as an integrated solution in a standard 0.35µm CMOS technology with low voltage (±1V) and low power (430µW) characteristics resulting suitable for on-chip integration in portable sensor applications. Experimental measurements have been performed through a prototype PCB and commercial gas (Figaro TGS series) and humidity (Honeywell HCH series) sensors. The achieved results have shown good linearity and accuracy both for wide variations of floating capacitive sensors, having a baseline or changing their value in the range [pF-nF], as well as for more reduced variations of grounded resistive sensors, ranging from few kΩ to hundreds of kΩ.