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This thesis presents the design of analog front-end (AFE) circuits for gas sensing applications. In a gas sensing microsystem, the AFE circuit is the most critical building block, since it must amplify weak signal in the presence of large interferences. A PCB prototype circuit and an on-chip front-end TIA circuit are implemented and verified. The first work is a two-stage transimpedance amplifier with auto-calibration. The noise of circuit is minimized by using batteries as voltage source and limiting circuit bandwidth to 1 Hz. The auto-calibration of the TIA is realized by digital control from the microcontroller. A portable CO2 detection device is constructed with the TIA integrated-in, and the slopes of voltage change under different concentrations of CO2 are calculated to extract the gas concentration. It demonstrates a minimum detection concentration of 4%. The second work presents an analog front-end transimpedance amplifier with DC photocurrent cancellation. Chopping technique is utilized in this work to reduce the effect of amplifier offset and flicker noise. A digital calibration loop is implemented to cancel the large DC photocurrent and increase the detection dynamic range. The chip is fabricated with UMC 0.18-μm CMOS process. The core area is 0.28 mm2 and it consumes 0.67 mW from 1.8-V supply when the current DAC is disabled. It achieves a SNR of 62.4 dB under 1-kHz bandwidth with the current DAC disabled. The results prove the feasibility to integrate this circuit in a CO2 detection system, achieving detection resolution of 50 ppm.

Sq represents the root mean square value of ordinate values within the definition area. It is equivalent to the standard deviation of heights.