Gas-Sensor Interface Circuit Based on Calibration Free Novel Frequency Measurement Approach with 16-Bit Digital Output

In this paper we present a low-cost CMOS wide-dynamic-range integrated interface circuit for indoor resistive gas sensors based on an enhanced oscillator approach. The state of the art of this measurement method has been improved biasing the sensor with a constant voltage, thus increasing linearity by separating the oscillator circuit from the sensing device. Another important novelty of this circuit is an embedded digital measurement control system that extracts the sensor resistance value by the ratio of a reference counter and a resistance dependent one, actually doubling the measurement range in terms of decades. Test results on a silicon prototype show that the proposed circuit achieves, without calibration, a precision of about 0.4% over a range of 4 decades and better than 0.8% over 5 decades (dynamic range: DR>140dB). After calibration, it reaches a precision of 0.8% over a range of 6 decades (DR>165dB). The circuit may also work over a reduced range of 2-3 decades with a throughput up to 100Hz and a precision of 0.2%. Chemical measurements demonstrate the real effectiveness of this system.