A 3.8-µW 1.5-NEF 15-GΩ Total Input Impedance Chopper Stabilized Amplifier With Auto-Calibrated Dual Positive Feedback in 110-nm CMOS

A capacitively-coupled chopper instrument amplifier (CCIA) for bio-potential monitoring analog front-end (AFE) with auto-calibrated dual positive feedback loops (DPFLs) is presented in this article. The proposed AFE with the DPFL does not only prevent total input impedance degradation by unexpected external and internal parasitic capacitances but also achieves excellent noise efficiency simultaneously. The DPFL consists of a conventional internal positive feedback loop with an attenuator for enhancing calibration resolution and an external positive feedback loop outside of the input chopper to compensate for a large external parasitic capacitance ( $C_{P-EXT}$ ), boosting total input impedance significantly. To precisely determine the feedback factor of the DPFL with respect to parasitic capacitors, an on-chip foreground calibration is also implemented in this work. Fabricated in a 110-nm CMOS process, the prototype AFE consumes 3.83 $\mu \text{W}$ per channel at 1 and 1.5-V supplies and has the input-referred noise of 0.36 $\mu \text{V}_{rms}$ from 0.5 to 300 Hz with the noise efficiency factor of 1.54. The proposed AFE achieves high-input impedances of 15 $\text{G}\Omega $ at 10 Hz and 2 $\text{G}\Omega $ at 50 Hz, even with an additional $C_{P-EXT}$ of 82 pF.