Abstract
In capturing high-quality photoplethysmographic signals, it is crucial to ensure that appropriate illumination intensities are used. The purpose of the study was to deliver controlled illumination intensities for a multi-wavelength opto-electronic patch sensor that has four separate arrays each consisting of four light-emitting diodes (LEDs), the wavelength of the light generated by each array being different. The study achieved the following: (1) a linear constant current source LED driver incorporating series negative feedback using an integrated operational amplifier circuit; (2) the fitting of a linear regression equation to provide rapid determination of the LEDs driver voltage; and (3) an algorithm for the automatic adjustment of the output voltage to ensure suitable LED illumination. The data from a single centrally-located photo detector, which is capable of capturing all four channels of back-light in a time-multiplexed manner, were used to monitor heart rate and blood oxygen saturation. This paper provides circuitry for driving the LEDs and describes an adaptive algorithm implemented on a microcontroller unit that monitors the quality of the photo detector signals received in order to control each of the individual currents being supplied to the LED arrays. The study demonstrated that the operation of the new circuitry in its ability to adapt LED illumination to the strength of the signal received and the performance of the adaptive system was compared with that of a non-adaptive approach.
Highlights
Reflectance photoplethysmography (PPG) is a low-cost non-invasive technique for measuring dynamic changes in the optical properties of living tissue and is becoming increasingly popular in commercial real-time health monitoring products
This paper presents new work that includes the design of a PPG system that is able to adapt the illumination of the acquisition site according to the characteristics of the received signals
The light-emitting diodes (LEDs) driver circuit was implemented to ensure a linear steady current source which is composed of the series negative feedback of an integrated operational amplifier
Summary
Reflectance photoplethysmography (PPG) is a low-cost non-invasive technique for measuring dynamic changes in the optical properties of living tissue and is becoming increasingly popular in commercial real-time health monitoring products. It has been used to measure oxygen saturation (SpO2), heart rate (HR), blood pressure (BP) and cardiac output (CO) as well as provide an assessment of autonomic function and for detecting peripheral vascular disease [1]. The ZenPPG device, configured with two red LEDs and two infrared LEDs, was developed to monitor blood perfusion in tissue [2]. A second configuration with two red LEDs and four infrared LEDs was able to acquire PPG signals and provide a measurement of SpO2 [3]. Ruiz et al [4] developed a PPG prototype for the study of heart rate variability (HRV) using reflected green light. Asada et al [5] designed a ring sensor which
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