This paper presents a gas sensor readout IC for amperometric and conductometric electrochemical sensors. The Analog Front-End (AFE) readout circuit enables tracking long term exposure to hazardous gas fumes in diesel and gasoline equipment, which may be correlated to diseases. The AFE reads out the output of eight conductometric sensor arrays and eight amperometric sensor arrays. The IC consists of a low noise potentiostat, and associated 9bit current-steering DAC for sensor stimulus, followed by the first order nested chopped ΣΔ A ADC. The conductometric sensor uses a current driven approach for extracting conductance of the sensor depending on gas concentration. The amperometric sensor uses a potentiostat to apply constant voltage to the sensors and an I/V converter to measure current out of the sensor. The core area for the AFE is 2.65×0.95 mm2. The proposed system achieves 91 dB SNR at 1.32mW quiescent power consumption per channel. With digital offset storage and nested chopping, the readout chain achieves 500 μV input referred offset.