An Integrated High-Gain Wide–Dynamic Range Photoplethysmography Sensor for Cardiac Health Monitoring

Abstract

This article presents a low-power, high-gain, and wide–dynamic range (DR) photoplethysmography (PPG) sensory system with a novel direct current (dc) compensation loop and light-emitting diode (LED) dimming control. The proposed PPG system exhibits a signal-aware performance, where, at strong PPG signal levels, the dc compensation loop’s capability of removing dc photocurrent is increased and the LED’s driving current is reduced. This ability allows the entire DR to be utilized by the useful alternating current (ac) component of the signal and reduces power dissipation. Fabricated in 0.35- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text {m}$ </tex-math></inline-formula> standard CMOS technology, the proposed PPG sensor occupies an area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.744~{\text {mm}}^{{2}}$ </tex-math></inline-formula> . The proposed system exhibits a high gain and a low input-referred noise current of 19.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{M}\Omega $ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$11.23~{\text {pA}}_{\text {rms}}$ </tex-math></inline-formula> , respectively, while dissipating only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9.9~\mu \text {W}$ </tex-math></inline-formula> of power at the receiver’s analog frontend. The proposed dc compensation loop can remove up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$80~\mu \text {A}$ </tex-math></inline-formula> of dc photocurrent for 2% total harmonic distortion (THD), achieving a DR of 137 dB. An external microcontroller unit (MCU) uses a machine learning (ML) algorithm to extract blood pressure (BP) from the analog output of the implemented PPG sensor chip. Measurements from eight human subjects show a mean absolute error (MAE) and a standard deviation of 2.25 ± 2.82 mmHg for systolic BP (SBP) and 5.01 ± 2.10 mmHg for diastolic BP (DBP).