A 20.3 $\mu $ W 1.9G $\Omega $ Input Impedance Capacitively-Coupled Chopper-Stabilized Amplifier for Bio-Potential Readout

This paper presents a low-power chopper-stabilized capacitively-coupled frontend amplifier with auxiliary-path-based input impedance ( Z $_{\mathbf{in}}$ ) boosting. In order to achieve a high Z $_{\mathbf{in}}$ , a low noise and a small chip area, techniques on both system level and circuit level are implemented. On the system level, small capacitors ( C $_{\mathbf{in}}$ $=$ 0.5 pF, C $_{\mathbf{fb}}$ $=$ 25 fF) are used with a biased pseudo-resistor ( R $=$ 2.5 G $\Omega $ ) fed back to an amplifier internal node. As a result, a high achievable Z $_{\mathbf{in}}$ and low high-pass corner frequency are achieved. On the circuit level, an input capacitance shielded current feedback (CSCF) topology achieving effective 10 fF C $_{\mathbf{amp}}$ is proposed as the core of the capacitive feedback amplifier in order not to increase the input noise. Moreover, the design space of auxiliary-path-based boosting is explored to obtain the optimal value of buffer bandwidth and auxiliary capacitor size to save power. The amplifier and its Z $_{\mathbf{in}}$ boosting circuit are implemented in a standard 55 nm CMOS process and characterized experimentally. Measurement results show that the proposed amplifier provides an input noise density of 50 nV/ $\surd$ Hz, and an integrated noise of 0.85 $\mu$ V $_{\mathbf{rms}}$ in 200 Hz band. The Z $_{\mathbf{in}}$ is boosted to 1.92 G $\Omega $ at DC and 1.02 G $\Omega $ at 50 Hz with only 1.0 $\mu$ A in each auxiliary path buffer. The amplifier also archives 77 dB CMRR and 76 dB PSRR while consuming 20.3 $\mu$ W in total.