26.1 A 4.5mm² Multimodal Biosensing SoC for PPG, ECG, BIOZ and GSR Acquisition in Consumer Wearable Devices

Abstract

Recently, biosensors have become widely deployed in consumer wearable devices, e.g., smart watches and wrist bands, to track user health conditions during sport, sleep, and daily activity. However, unlike biosignal acquisition in well-controlled medical settings, signal acquisition in consumer devices inevitably suffers from degraded signal quality due to poor sensor interfaces, motion artifacts, and environmental interferences, such as ambient light and power-line coupling, which can be >60dB stronger than the wanted signals. These challenges result in demanding AFE specifications, even under tight constraints on sensor area, chip size, and battery capacity, which are all necessary for wearable devices. This paper presents a multimodal biosensing SoC that addresses these issues to enable reliable Photoplethysmography (PPG), Electrocardiography (ECG), and Bio-impedance (BIOZ) acquisition under highly variable conditions in wearable devices. It contains a 130dB-DR PPG readout, which is 11dB higher than the state of the art [1], to tolerate strong ambient light; an ECG AFE with a pseudo Right-Leg-Drive (RLD) tolerant of >130V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PP</sub> common-mode interference (CMI) from power-line coupling, which is 3× higher than the technique presented in [2]; and a BIOZ AFE for body-fat measurement using 1cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> electrodes rather than the typically >10cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> electrodes in conventional body-fat meters. The area for the entire PPG/ECG/BIOZ AFE is ~3mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is around 50% smaller than prior AFEs with similar features [3], [4].