Integrated low-power interfaces for impedimetric chemical sensors
Su, Jin Jyh
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This thesis presents two interface circuits for impedimetric chemical sensors: one for passive chemical sensors and the other for ChemFETs. Both interfaces were fabricated in 0.35μm BiCMOS technology and provide the same output data rate of 1Hz. The interface for passive impedimetric sensors is reconfigurable for performing either resistance or capacitance measurements and provides a fully digital output with less than 81.8μW power consumption at VDD = 2.5V. The interface features a 176dB resistance dynamic range (31.6Ω-200MΩ, <±0.8% nonlinearity, and >40dB SNR) realized with only two sub-ranges to minimize calibration efforts and a 102dB capacitance dynamic range (0.8-1000pF, <±0.2% nonlinearity, and >40dB SNR). The ChemFET interface is a highly versatile system that can generate a wide range of bias voltages (VG up to 9.74V and VD up to 16.3V depending on the measurement modes) and perform either constant voltage or constant current mode measurement. At maximum rated output (VG = 9.74V, VD = 16.3V, and IDS = 15μA), the interface consumes only 2.02μW at VDD = 3.3V and provides analog readout noise levels of 0.0476μARMS at 10μA and 0.503mVRMS for IDS and VT, respectively. Besides attempting versatile system architectures, detailed noise and efficiency analysis were performed for the passive sensor interface and the ChemFET interface, respectively. The noise analysis suggests that different types of noise (correlated or uncorrelated) dominate the noise performance in different measurement ranges and, thus, noise suppression techniques, such as chopper stabilization, correlated double sampling (CDS), and oversampling/averaging, are applied to adequate parts of the interface system. The efficiency analysis of the boost capacitor charger in the ChemFET interface concludes that applying a moderate pulsewidth (200-300ns) to drive the boost converter yields the best efficiencies for charging a capacitor. Compared to interfaces described in the literature, the proposed interface for passive sensors achieves better versatility and wide dynamic range with less number of sub-ranges and power consumption. The proposed interface for ChemFETs achieves wider voltage supply range at very low power level. In-house fabricated chemical sensors, including passive chemical sensors and ChemFETs, were interfaced with the developed circuits and gas-phase chemical measurements with the systems were demonstrated. The novel passive chemical sensor tested in this thesis employs a multi-functional design, which can be configured into either a chemoresistor or a chemocapacitor; the tested ChemFET employs a bottom-gate TFT structure to allow the semiconducting film to interact with the analytes.