A 296 nJ Energy-per-Measurement Relaxation Oscillator-Based Analog Front-End for Chemiresistive Sensors

An energy-efficient, wide dynamic-range (DR) CMOS analog front-end (AFE) for chemiresistive sensors is presented. The circuit is specifically designed for the Metal Oxide (MOX) gas sensors, a special technology of chemiresistive sensors, broadly diffused in modern portable devices due to their low-cost and simplicity of use. Energy efficiency is mandatory for the AFE in order to prolong the battery life that supply these devices. The proposed circuit implements the resistance-to-time (R-to-T) conversion of the sensor's resistance by adopting a relaxation oscillator-based architecture. A limiting resistor in series with the sensor is introduced for reducing the circuit's energy-per-measurement (EpM), while mitigating the error due to the sensor's parasitic capacitance. The analysis of the circuit is presented with emphasis on the design trade-off between error due to the sensor's parasitic capacitance and power consumption on one side and read-out sensitivity on the other. The chip prototype is realized in AMS 0.35 mu m process and has been tested in the DR between 100 Omega and 4.7M Omega with an accuracy less than 0.1% and a precision less than 0.029%. The efficacy of the presented AFE is proved by adopting the circuit in a real chemical environment with a commercial sensor. The proposed AFE shows a maximum EpM of 296nJ which is three times better than the state of the art.