Integrated CMOS resistance-to-period converter with parasitic capacitance evaluation

In this paper a novel and simple CMOS integrated interface is presented. The circuit is suitable for typical gas sensor applications being able to reveal: a) high resistive values over a wide-range (more than five orders of magnitude, ranging from hundreds of kilohm up to tens of gigohm); b) relatively low value capacitances (few picofarad) and their variations. The proposed front-end utilizes, as active blocks, operational amplifiers that have been implemented, at transistor level, as suitable OTA showing low input voltage offset and relatively high slew-rate characteristics. With respect to the most topologies in the literature, the proposed interface, based on an oscillating architecture, is very simple and allows to obtain good performances all over a high dynamic range without any initial calibration. System sensitivity has been set to 0.1 ms/MOmega. Experimental measurements, performed on chip, fabricated in a standard and low-cost CMOS technology (AMS 0.35 mum), have shown a good linearity confirming the front-end validity in a large frequency range.