A 6.4GΩ-Input-Impedance 104.5dB-CMRR 96dB-DR DD-AFE With Tri-Level IDAC for Small-Diameter Dry-Electrode Interfaces

Abstract

This article presents a direct-digitization analog front end (DD-AFE) with enhanced input-impedance, common-mode rejection ratio (CMRR), and dynamic range (DR) for wearable biopotential (ExG) signal acquisition, especially for small-diameter dry electrodes. The DD-AFE employs a second-order continuous-time delta-sigma modulator (CT-ΔΣM) and multiple circuit techniques to support direct-digitization readouts. These include 1) A high input-impedance input feedforward (FF), embedded in a 4-input 4-bit successive approximation register (SAR) quantizer. This allows two integrators to adopt a compact and energy-efficient G_m-C structure, and improves stability and linearity, resulting in a 6.6dB increase in DR, 42dB increase in SQNR at peak input and a unity-gain signal transfer function (STF) with a gain flatness of 0.04%. 2) A fixed-voltage dead-band assisted tri-level current-steering DAC (IDAC). It not only increases the DR and CMRR of the DD-AFE but also eliminates the harmonic distortion induced by tri-level dynamic element matching (DEM). 3) A high-gain two-stage G_m-boosting inverter-based OTA with embedded low-frequency chopping. The former largely improves linearity and CMRR, while the latter mitigates 1/f noise without compromising the input impedance. Fabricated in a 0.18-µm CMOS process, this DD-AFE achieves 6.4GΩ input impedance and 104.5dB CMRR at 50Hz, as well as 90.4dB peak SNDR, 96dB DR, and up to 425mV_PP linear input range.