A CMOS integrated low-voltage low-power time-controlled interface for chemical resistive sensors

In this paper, an innovative integrated interface circuit, suitable for wide range resistive sensors, such as chemical devices for gas sensing, is presented. The key characteristic is the ability to overcome the main limit of the circuits based on the resistance-to-time (R–T) conversion, that is the long measuring time occurring in the evaluation of high-value resistances. The proposed solution is based on a tricky oscillating circuit architecture performing a sort of “compression” of the higher part of the resistive range, thus limiting the measuring time. The interface is oriented to the development of a single chip for resistive chemical sensor applications, thus it is designed to be as simple as possible, utilizing only operational amplifiers (OAs) and passive components. The proposed front-end is capable to estimate both the sensor resistance over a wide range (five decades) and, through the AC excitation voltage of the sensor, the in-parallel parasitic capacitance, for diagnosis purposes or to provide a more complete characterization of the sensor. Preliminary experimental measurements, conducted through a fabricated discrete component prototype PCB and utilizing commercial sample resistors and capacitors to emulate sensor behaviour, have shown the feasibility of the proposed approach in a wide resistive range. Further experimental results, achieved through the fabricated integrated circuit, developed in 0.35 μm standard CMOS technology, have shown good performance both for resistance and capacitance estimations. The on-chip integrated solution, requiring a silicon area of about 0.9 mm2, supplied at 1.8 V and showing a low power consumption (lower than 600 μW), results to be suitable for resistive sensor array configurations and portable applications, as also witnessed by an experimental test with CO gas and a sample commercial sensor.