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

Abstract

This paper presents a wearable optical health monitoring device embedding a monolithic sensor chip with newly designed pixels, and an optical module optimized for photoplethysmography (PPG). The monolithic optical sensor at the heart of this device implements an array of novel pixels designed specifically for PPG and featuring a quantum efficiency (QE) as high as 85%. The sensor readout chain, as well as the logic circuits, implements low power design techniques leading to only 60 $\mu \text{A}$ current consumption at 122 Hz operation frequency, including digital communication. Thanks to the high QE and optical module optimization, medical grade PPG recordings are enabled with less than 10 $\mu \text{A}$ emitter current. The optical sensor is embedded in an ergonomic ring device together with system electronics in order to enable in-field test of beat detection accuracy on 7 subjects. The ring device detects correctly 97.87% of the beats on a total of 72.21 hours of recording. The inter-beat interval (IBI) estimation from these recordings features a mean absolute error (MAE) as low as 8.10 ms. This performance is achieved with less than 70 $\mu \text{A}$ current consumption at the level of the PPG module.

Keywords

• CMOS image sensor
• heart beat detection
• low power integrated circuit
• monolithic optical detector
• photoplethismography sensor
• wearable electronics