Ultralow-Power Photoplethysmography (PPG) Sensors: A Methodological Review

Photoplethysmography (PPG) sensors are used to accurately, instantaneously, and noninvasively measure vital signs to provide a real-time indication of overall physical health and long-term well-being. Achieving long-term continuous monitoring is an important requirement to increase user safety and diagnostic accuracy. PPG sensors need a light-emitting diode (LED) with sufficient output power to detect the PPG signal, which consumes tens of milliwatts. On the other hand, low ac/dc ratios of <0.1%-4%, ambient light, motion artifacts, and semiconductor noise greatly affect the signal-to-noise ratio (SNR), dynamic range (DR), and signal quality. Specialized circuit blocks are needed to cancel these interferences, further increasing power consumption. Several ultralow-power designs, circuit techniques, and sampling schemes have been proposed in the literature to extend PPG sensors' lifetime. This article reviews, analyzes, and critiques these solutions to provide designers with comprehensive design considerations for achieving ultralow power consumption while achieving the required SNR and DR in a PPG sensor design.