Towards maximizing the sensing accuracy of an cuffless, optical blood pressure sensor using a high-order front-end filter

Abstract

A high-order filter as part of an analog front-end circuit for an optical, cuffless photoplethysmography (PPG) sensor is developed herein to maximize the sensing accuracy of measured blood pressures (BPs). The BP device consists physically of light emitting diodes and photo-diodes (PDs) to sense the dynamic change of intravascular blood volume based on the known principle of PPG and then calculate the BP based on reflective pule transient time in a PPG signal. The photoplethysmography (PPG) signal acquired by the PDs are expected to be processed by an excellent front-end circuitry to reduce its noise and DC offset but without much distortion to obtain accurate BP prediction. This front-end is accomplished herein by a transimpedance amplifier, a critical high-order band-pass filter and a programmable gain amplifier, which is followed by microprocessor and a wireless module. The band-pass filter is optimized with a passband from 0.2 to 7.2Hz, where the low-pass is in a 4th order while the high-pass is in 2nd order. The low-pass is designed for reducing noise including those due to ambient lighting, while the high-pass is for reducing DC drifting caused mainly by breathing and/or subject slow motion. 46 subjects were tested with the designed high-order filters in comparison with reference device. The advantages of employing high-order low-pass filter versus first-order is clearly seen in experimental data with an accuracy on predicting BP reaching +/- 3mmHg, as stopped to +/- 7mmHg using commonly used first-order filters.