Sample and Average Common-Mode Feedback in a 101 nW Acoustic Amplifier

We present a Sample and Average Feedback Resistor (SAFR) for accurate and programmable common-mode feedback in capacitively coupled low-power amplifiers. Using only switches, clocks, and capacitors, we reduce the variation across process and temperature by 4.4× and 226x, respectively, compared to a traditional pseudo-resistor implementation. The SAFR was implemented in a low-power audio LNA + PGA + ADC chain, achieving a resistance of 100 TΩ and HP corner programmability from 16 mHz to 4 Hz.