A 10 ms-readout interface for the characterization of high-value wide-range experimental resistive sensors

Metal oxide gas sensors exhibit resistance values varying over a wide range, from tens of kilohms to tens of gigohms, depending on the chosen oxide and on the excitation parameters (voltage, temperature, gas exposure). Resistance-to-time converters (RTC) are widely used as electronic interfaces for such sensors, thanks to the low-cost, low-noise and high-range characteristics. RTC main limit is in the variable and long measuring time, ranging from microseconds (tens of kilohms) to several seconds (tens of gigohms), impeding a fine analysis of fast transients. This work proposes a new approach based on combination of the RTC method with a new technique based on the least mean square (LMS) algorithm. The implemented prototype allows the sensor resistance to be estimated with a fixed measuring time of 10 ms over the range 10 kΩ–10 GΩ with relative estimation error below 10% (below 1% in the range 47 kΩ–2 GΩ). It can furthermore estimate the parasitic capacitance of the sensor (in parallel with the resistive component), on the order of few picofarads with a linearity error less than 0.5% full scale (FS). Fast thermal transients of a SnO2 nanowire sensor have been finely analyzed using the new interface system, demonstrating the suitability of the proposed method for accurate analysis of new experimental resistive sensors.