A CCII-Based Oscillating Circuit as Resistive/Capacitive Humidity Sensor Interface

In this paper, we propose a Current-Mode (CM) square-wave oscillator, formed by two Second Generation Current Conveyors (CCIIs) and some passive components, operating an impedance-to-period conversion that, instead of other solutions in the literature, is based on a current differentiation. The circuit is suitable, for example, for resistive/capacitive humidity sensor interfacing and works also for a wide oscillation frequency range (corresponding to up to six to seven variation decades of capacitive variations). It is possible to easily set its sensitivity to sensor parameters (resistance or capacitance) through external passive components. The proposed interface has been designed as an integrated solution at transistor level in a standard CMOS technology (AMS 0.35 μm) with low voltage (±1 V) and low power (430 μW) characteristics; this solution is able to properly work with integrable passive component values (resistance ≤100 kΩ and capacitance ≤100 pF), so it is also suitable for integrated portable sensor applications. In order to verify the interface validity, some experimental measurements have been performed implementing the proposed circuit through a PCB prototype utilizing AD844 as CCII, commercial passive sample resistors and capacitors and gas (e.g., TGS Series by Figaro) and humidity (e.g., HCH-1000 Series by Honeywell) sensors. Measurement results have shown good linearity and accuracy both for variations of floating capacitive sensors, having a baseline or changing their value in the range [pF,nF], as well as for variations of grounded resistive sensors, ranging from few kΩ to hundreds of kΩ.