A $136\mathrm{G}\Omega$-Input-Impedance Active Electrode for Non-Contact ECG Using Auto-Calibrated Positive Feedback and Capacitance Scaling in Femtofarad Resolution

This paper presents a $10.46\mu\mathrm{W}, 136\mathrm{G}\Omega$ ultra-high-input-impedance, $700\text{mV}_{\text{pp}}$ linear-input-range active electrode (AE) for non-contact ECG sensing. The femtofarad-resolution calibration against internal parasitic capacitance is realized via an excitation-signal-free auto-calibrated positive feedback loop, as well as the capacitance fine scaling and dummy input structure. The external parasitic capacitance on PCB is bootstrapped by active shielding. As a result, the AE exhibits an equivalent input capacitance of 19.5fF (average of 10 samples), a 2.7x-34x improvement on the state of the art.