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Continuous-time Â¿Â¿ modulators (CT-DSMs) that use transistor-based Gm-C integrators are simple but sensitive to process and temperature variations. Their performance depends heavily on the input linear range that varies widely. A CT-DSM is implemented using a triode Gm-C integrator and a tri-level switched-capacitor digital-to-analog converter (SCDAC). A self-scaling SCDAC keeps the input signal range constant, and an adaptive input common-mode bias control enables the triode Gm cell to operate always with its constant input linear range. The proposed Gm cell exhibits a tuning range of Â± 15% while maintaining its linear range over the temperature variation from -30Â°C to 85Â°C. A 2-MHz bandwidth, fifth-order CT-DSM in 0.18 Â¿m CMOS samples at 128 MS/s with an over-sampling ratio of 32 (OSR = 32), occupies an active area of 0.4 mm2, and consumes 11 mW at 1.8 V. With a -3 dB input of the full scale (400 mV), the measured dynamic range, second harmonic distortion (HD2), and third harmonic distortion (HD3) are 71, -86, and -91 dB, respectively.
Date of Publication: Sept. 2009