A 7 mHz-6.29 Hz Configurable High-Pass Analog Front-End With Direct Tunneling Biasing and Output DC-Servo Loop

This paper introduces a capacitively coupled analog front-end featuring an adjustable high-pass cutoff frequency range of 7 mHz to 6.29 Hz. The DC operating point for the input differential pair is set by the dynamic equilibrium of the direct tunneling (DT) current. An output DC-servo loop (O-DSL), utilizing a duty-cycled operational transconductance amplifier (DC-OTA) together with a digital-assistant transconductance amplifier (DA-OTA), is designed at the main amplifier's output nodes. By employing DC-OTA techniques in the O-DSL, an equivalent transconductance as low as 0.18 pA/V has been achieved, enabling the front-end to reach a 7 mHz high-pass cutoff frequency with a 6 pF on-chip integrating capacitor. A DA-OTA technique is used to widen the compensation range for low frequency interference. Additionally, a positive feedback capacitor in conjunction with a dual loop control mechanism is applied to enhance input impedance to 5.2 G$\Omega$ at 50 Hz within 30 ms calibration time. The front-end is fabricated in a standard 180 nm CMOS process, with a total current consumption of 3.01 $\mu$A and 6.37 NEF.