Earbud-Embedded Micro-Power mm-Sized Optical Sensor for Accurate Heart Beat Monitoring
Assim Boukhayma, Anthony Barison, Serj Haddad, Antonino Caizzone
Abstract
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.