A fast-readout interface circuit for high-value and wide-range resistive chemical sensors

Resistive chemical sensors, such as metal oxide (MOX) devices, usually exhibit resistance values within a wide range, from tens of kilohms to tens of gigohms. Electronic interfaces based on the resistance-to-time conversion (RTC) technique are widely used to handle such sensors, thanks to the low-cost, low-noise and wide-range characteristics. The main limit of the RTC-based schemes is the variable and long measuring time, ranging from microseconds (tens of kilohms) to several seconds (tens of gigohms), impeding, for instance, a fine analysis of fast transients. This work proposes a new approach based on the combination of the RTC method with the use of 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 ÷ 100 GΩ with relative estimation error less than 10% (about 1% in the range 100 kΩ ÷ 100 GΩ). In addition, it is able to estimate the parasitic capacitance of the sensor (in parallel with the resistive component in the range 0 ÷ 50 pF) with a linearity error of about 0.3% full scale (FS). Experimental results conducted using a real MOX sensor show the suitability of the proposed system for applications in which fast transients of the sensor need to be analyzed.