A new fast-readout front-end for high resistive chemical sensor applications

Abstract

Metal oxide (MOX) gas sensors show a resistive value varying in a wide range, from hundreds of kilohms up to tens of gigohms. For this reason, oscillating circuits are often used to interface these sensors. Simple oscillators need a measuring time which is directly proportional to the resistive value; when large resistive value (gigohms) occur, the output updating time is on the order of seconds. In addition, the sensor is excited with a square wave, but the variable frequency can affect the measurement accuracy. In this paper, we propose a new oscillating circuit having a limited oscillating time regardless the resistive value. This behavior is obtained by means of self-moving thresholds, the slower limiting the maximum measuring time. The proposed circuit has been simulated and experimentally tested with commercial resistors (values between 1 MΩ and 100 GΩ). © 2010 Springer Science+Business Media B.V.