Research on gas sensors aims to improve the performance, minimize size, power consumption and cost of such devices. For implementing gas sensing microsystems with the above mentioned features, the micromodule approach, where the sensors and the read-out electronics are manufactured on different chips and mounted in the same package or on the same substrate, appears well promising and versatile. The proposed system, realized for environmental monitoring purpose, consists of an integrated tin-oxide SnO2 resistive sensor over micromachined substrate followed by a 0.35μm CMOS auto-scaling read-out circuit. The interface circuit, which is actually a trans-resistance variable gain amplifier connected to an incremental A/D converter, after the initial calibration procedure, required to minimize offset and gain inter-scale error, achieves a worst case accuracy of about 0.1% over a range of more than 5 …