A CMOS integrated interface circuit for metal-oxide gas sensors

This paper presents an integrated gas-sensor interface circuit that includes a high-efficiency temperature control loop, with a switching power stage and a digital set-point, as well as a wide-dynamic range read-out circuit. The temperature control loop adjusts the temperature of the sensor over a range of 250 degrees C with an accuracy better than 1.5 degrees C and with a maximum peak-to-peak ripple of 1.0 degrees C. The read-out circuit achieves, without calibration, a precision in sensor resistance measurement of 2.65%, over a range of 5.3 decades, leading to an equivalent dynamic range of 138 dB. The overall system is flexible and can be interfaced to sensors with different fabrication parameters. The interface circuit chip, realized with a 0.35 mu m CMOS technology, includes on the same 10-mm(2) die the high-accuracy read-out circuit and the switching power stage. In spite of the interferences produced by the power stage, the read-out circuit maintains always a dynamic range performance above 130 dB. The proposed interface circuit, together with a micromachined metal-oxide gas sensor, can detect 3 ppm of CO in air.