An Embedded Auto-Calibrated Offset Current Compensation Technique for PPG/fNIRS System

TL;DR

This paper introduces an auto-calibrated offset current compensation method for PPG and fNIRS systems, enhancing sensitivity by effectively reducing offset current effects through a mixed-signal approach and dual loop technique.

Contribution

It presents a novel mixed-signal auto-calibration and dual loop compensation system for offset current in PPG/fNIRS devices, improving signal sensitivity.

Findings

Effective offset current compensation demonstrated in prototype

Enhanced signal sensitivity in PPG/fNIRS recordings

System operates successfully on a 3.3 V supply

Abstract

Usually, the current generated by the photodiode proportional to the oxygenated blood in the photoplethysmography (PPG) and functional infrared spectroscopy (fNIRS) based recording systems is small as compared to the offset-current. The offset current is the combination of the dark current of the photodiode, the current due to ambient light, and the current due to the reflected light from fat and skull . The relatively large value of the offset current limits the amplification of the signal current and affects the overall performance of the PPG/fNIRS recording systems. In this paper, we present a mixed-signal auto-calibrated offset current compensation technique for PPG and fNIRS recording systems. The system auto-calibrates the offset current, compensates using a dual discrete loop technique, and amplifies the signal current. Thanks to the amplification, the system provides better sensitivity. A prototype of the system is built and tested for PPG signal recording. The prototype is developed for a 3.3 V single supply. The results show that the proposed system is able to effectively compensate for the offset current.