This work presents the design and analysis of a compact low-power motion artifact estimator for reducing the input dynamic range in wearable ECG acquisition systems. The estimator employs a novel mixed-signal architecture that performs adaptive filtering on the electrode impedance information to derive a cancellation signal to suppress the motion artifact at the input of the acquisition system. A detailed noise analysis and optimization strategies are also provided to minimize the estimator's noise referred to the acquisition system's input. Fabricated in a 0.18-μm CMOS process and operating from a 1-V supply, the estimator occupies an active area of 0.11 mm and consumes 2.6-3.2 μW of power depending on the final weights used. The low power consumption and small area thus make the estimator suitable for local placement at each recording channel in multi-channel ECG acquisition system.