Integrated read-out and temperature control interface with digital I/O for a gas-sensing system based on a SnO2 microhotplate thin film gas sensor

A complete gas-sensing system for portable ambient monitoring purpose, consisting of a sensor (with embedded heater and thermometer) and an integrated interface (with read-out channel and closed-loop temperature control circuit) is presented. The micromachined sol-gel tin-oxide sensor features a 450 nm thick silicon nitride/oxide membrane, an active area of 170 mum times 170 mum and a power consumption of 25 mW at 350degC in isothermal operation. The read-out circuit uses a current integration and resistance-to-frequency conversion, providing digital output. The sensor is biased with a constant voltage and its temperature is regulated by an on-off closed-loop control circuit, consisting of a thermometer conditioning network, an A/D converter and a digital control block that drives the on-chip output p-MOS power stage. The circuit can operate with different sensors to grant maximum flexibility. Chemical measurements with different gases have been performed, operating the sensor at different temperatures.