A 15.5-ENOB 335 mVPP-Linear-Input-Range 4.7-GΩ-Input-Impedance CT-ΔΣM Analog Front-End With Embedded Low-Frequency Chopping

This article presents a second-order continuous-time delta-sigma (CT- $\Delta \Sigma $ )-based analog front-end (AFE) for biopotential sensor interfaces. High linearity is achieved by using a current balanced $G_{m,1}$ input stage with gain-boosting and cascode techniques. Low-frequency chopping embedded in gain-boosting OTAs breaks the limitation of chopping frequency in conventional CT- $\Delta \Sigma $ ADCs and mitigates flicker noise without reducing the input impedance. In the second stage, the closed-loop $G_{m,2}$ -OTA-C proportional integrator (PI) relaxes the linearity requirements of the OTA and eliminates the additional active adder. Fabricated in a standard 0.18- $\mu \text{m}$ CMOS technology, this direct-digitization AFE achieves 94.9-dB peak SNDR, $335 \rm mV_{pp}$ linear input range, and 4.7- $\text{G}\Omega $ input impedance at 50 Hz with $64\times $ reduction in the chopping frequency.