A 3.8-μW/Ch, 15-GΩ Total Input Impedance Chopper Stabilized Amplifier with Dual Positive Feedback Loops and Auto-calibration Scheme

A capacitively-coupled chopper instrument amplifier (CCIA) with dual positive feedback loops (DPFL) is proposed to achieve excellent noise efficiency and high total input impedance (T-Z
IN
) simultaneously. The proposed DPFL consists of a conventional internal positive feedback loop (IPFL) and an external positive feedback loop (EPFL) outside of the input chopper for compensating a large external parasitic capacitance (C
P-EXT
) improving T-Z
IN
significantly. An on-chip background calibration is also proposed to determine the feedback factor of the DPFL to deal with variable parasitic capacitors. The prototype shows the IRN of 0.36 μV
rms
from 0.5 to 300 Hz with the NEF of 1.5 and high input impedances of 15 GΩ at 10 Hz with additional C
P-EXT
of 82 pF.