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An automatic RC time constant tuning scheme is proposed for high linearity continuous-time gm-C and active RC circuits in a low power consumption environment. Instead of changing the gm (in gm-C filters), the RC time constant is tuned by discretely varying the integration capacitors to preserve a high linearity. The auto-tuning circuit, consisting of an analog integrator, a voltage comparator, and a digital tuning engine, generates a control word and sets on-chip capacitors to obtain an RC time-constant accuracy of ±2-10%. The proposed scheme is verified by the experimental results of a test chip in a 0.5 μm CMOS technology. It achieves a peak S/(N+D) of 83 dB while a tuning range of over ±40% is accomplished.