Earbud-Embedded Micro-Power mm-Sized Optical Sensor for Accurate Heart Beat Monitoring

This work demonstrates that accurate hear beat monitoring, via photoplethysmography (PPG), can be performed at micro-power consumption level with an ear worn device. This is achieved thanks to the combination of a highly sensitive low power PPG chip, an optimal optical module, and an ergonomic design of the wearable device. The monolithic optical sensor is fabricated in a 180nm CMOS image sensor (CIS) process and features an area of 1.5mm by 1.5mm only. It implements an array of pixels, newly designed for PPG, and reaching a quantum efficiency of 85% up to 800nm. The presented sensor consumes 60 μA at 122Hz sampling frequency, including digital communication. The optical design optimization enables high signal-to-noise ratio (SNR) PPG signals with less than 10 μA average LED current. Heart beat monitoring, using the presented wearable device, is performed on 7 subjects and compared with a medical electrocardiogram (ECG). The earbud device correctly detects 98.47% of the beats on a total of 72.21 hours of recordings. The inter-beat interval (IBI) estimation from these recordings features a mean absolute error (MAE) as low as 7.52ms.