Advances in PPG Sensors Data Acquisition with Light-to-Digital Converters: A Review

Light-to-digital converters (LDCs) are essential components in photoplethysmography (PPG) readout chains. Over the past decade, PPG sensors have gathered increased interest due to their non-invasiveness and employment in a wide variety of applications. Among these are cardiovascular monitoring, brain mapping, glucose sensing, skin cancer detection, ozone pollutant sensing and commercial fitness tracking devices. This article offers a review of recent developments in PPG monitoring, with a specific focus on LDCs. The critical challenges, such as LED power consumption, low perfusion index, ambient light interference, and motion artefacts, are discussed in detail. Different sampling methods (uniform aggressive, compressive and event-driven) are analyzed to provide an overview of the mitigation techniques of the LED power consumption issue. Moreover, specific circuit solutions to overcome the remaining challenges are explored within the power consumption and signal quality trade space. Recent advances in LDCs employing on-chip photodetectors and DC cancellation circuits are discussed as they significantly improve the noise performance of the readout and provide a considerable overall power reduction. The article also evaluates state-of-the-art circuit design techniques, providing essential considerations and intuitions. Lastly, it outlines future development trends and insights, charting a course for continued innovation in this rapidly evolving field.