An Ultra-High Input Impedance Analog Front End Using Self-Calibrated Positive Feedback

This paper presents circuit techniques for ultra-high input impedance analog front end (AFE). In order to boost input impedance, various on- and off-chip parasitic capacitances are cancelled using an active shield and negative capacitance technique. To maximize the cancellation, a self-calibration scheme with active shield replica is proposed for positive feedback-based negative capacitance, which settles at the boundary between stable and unstable states in calibration mode. A prototype IC fabricated in 0.18- $\mu \text{m}$ CMOS achieves an input impedance of 50 $\text{G}\Omega$ at 50 Hz, equivalent to 60-fF capacitance, while consuming 289 nW from 0.8-V supply. The proposed AFE is applied to heart-rate monitoring using 1-cm 2 dry electrodes over clothes without any straps.