Abstract
Metal oxide (MOX) gas sensors, as well as the new nanowire-based sensors, can show resistive values varying over a wide range and could have the baseline value up to tens of gigohms. Some interface circuits use Resistance-to-Time Conversion (RTC) schemes to ensure a good resolution over the whole range. Such solutions suffer from a variable measuring time, which can become very long with high-value resistances. However, especially for the dynamic experimental characterization of new sensors, a fast and constant measuring time is required. The proposed solution merges an RTC scheme with the use of an Analog-to-Digital Converter (ADC), together with a Least Mean Square processing method, to provide a fast measurement of the sensor resistance and parasitic capacitance. The implemented prototype, based on a low-cost 12-bit ADC, allows the sensor resistance estimation with 100 sample/s (Tmeas=10ms) over the range 10kΩ÷10GΩ with relative estimation error below 10% (below 1% in the range 47kΩ÷2GΩ). Fast thermal transients of a SnO2 nanowire sensor have been finely analyzed thanks to the new interface system.
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