Analog Front-End Circuit Techniques for Wearable ExG, BioZ, and PPG Signal Acquisition: A Review

Wearable platforms that concurrently acquire multiple physiological signals enable comprehensive health monitoring but impose stringent requirements on front-end circuit design. The reliable extraction of low-amplitude and low-frequency biosignals is hindered by electrode offset, noise, motion artifacts, and environmental interference. Recent efforts have advanced analog front ends (AFEs) for biopotential (ExG), bioimpedance (BioZ), and photoplethysmography (PPG) sensing, with emphasis on optimizing key metrics such as noise efficiency, input impedance, dynamic range, CMRR, and power consumption. In addition, digitally assisted calibration and direct-digitization schemes have emerged as alternative design directions, offering enhanced robustness and scalability while introducing trade-offs in complexity and energy efficiency. This review surveys these circuit techniques, analyzes their design trade-offs, and outlines future opportunities for next-generation wearable biomedical interfaces.