This article describes an ultra-high input-impedance active electrode (AE) circuit and the system to sense biopotential signals through a capacitively coupled interface. Various techniques from both circuit and system aspects are used to eliminate the input parasitic capacitance of the AE. On-chip parasitic capacitance is compensated by an auto-calibrated positive feedback loop (PFL) without applying any reference signal. A capacitor down-scaling technique combined with SAR-assisted PFL calibration enables femtofarad-level resolution of the capacitor array, alleviating the practical constraints of the conventional PFL to implement a small capacitor below 10 fF. Besides, a dummy input structure ensures that the pad and electrostatic discharge (ESD) capacitances are also canceled by the PFL, while off-chip parasitic capacitance on the printed circuit board (PCB) is nulled by active shielding. Fabricated in a standard 0.18- 1P6M CMOS process, the AE achieves an ultra-high input impedance of 136 at 60 Hz (average of 10 samples). This is equivalent to an input capacitance of 19.5 fF and corresponds to a – improvement over the state-of-the-art. The AE exhibits an input signal range of 700 mVpp and an input-referred noise of (0.5–100 Hz) while consuming from a 1.2-V supply. Each AE integrates an IC and a 3-cm2 copper electrode on the PCB, and the wearable system prototypes successfully measured high-quality electrocardiogram (ECG) and electroencephalogram (EEG) signals from test subjects through capacitively coupled interfaces.