A CMOS Imager for Reflective Pulse Oximeter with Motion Artifact and Ambient Interference Rejections

This paper presents a dual-mode CMOS imager for reflective pulse oximeter and fingerprint capturing capability. The same sensing array with 3-transistor active pixel sensor (3T-APS) is used in both modes for area and cost efficiency. In the oximeter mode, a coarse-fine ADC is applied to extend the system resolution. A periodical tracking and subtracting (PTS) technique is proposed to prevent the physiological signal (photoplethysmogram, PPG) from saturation caused by the motion artifact. The 60Hz time-varying ambient light from power line noise is suppressed by the post processing of the moving averaging filter with a sampling frequency (fs) of 120Hz. A prototype with 64×64-pixel array and 20×20um
²
pixel pitch was fabricated in TSMC 0.18um CMOS technology. The measured result shows a motion rejection capability of ±199× AC swing, a motion tracking speed of 61.4kHz, and an ambient interference rejection capability of -36dB. The achieved SpO
₂
accuracy is ±0.51% (@Sp02 = 97%) at a power consumption of 380uW (without LED driver).