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.