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Analysis of radio frequency reflectance type PPG sensor based on systolic amplitude and source wavelength

Nivedita Daimiwal, Betty Martin, M Sundararajan, Revati Shriram

PPG (photoplethysmography) sensors are capable of measuring blood volumetric changes in the subcutaneous vessels. The transmission type of measurement is limited to only a few specific positions whereas the reflectance type of PPG sensor can be easily applied on various parts of the body surface. To determine the quality of reflectance type photoplethysmography signal, various parameters are analysed like the source-detector distance and the source wavelengths (660 nm and 860 nm). The experimental set up for light reflectance measurement is surface mount light-emitting diodes (LEDs) with two different wavelengths (660 nm and 860 nm) of light were tested in the experiment and by varying the distance between the source and the detector. The excitation signal used for LED is AC varying from range of 300 kHz and above and analysing the advantages as compared to DC excitation. Detector used is OPT101. The work in this paper analyses systolic amplitude (SA) using MATLAB for two wavelengths 660 nm and 860 nm. The sensors are designed for two distances, 1 cm and 1.5 cm respectively. Observations were taken for two different wavelengths with two different distances for 10 females in the age group 20 to 50 years. The results show that SA amplitude decreases as the source and detector distance increases. In Paired t-test using Minitab 17, the mean of SA of PPG signal acquired with a distance of 1 cm between the source and detector was observed to be greater than the mean of SA of PPG signal which was acquired with a distance of 1.5 cm (p<0.1). Systolic amplitude of 860 nm for oxy-Hb is greater than 660 nm for deoxy-Hb, so we concluded that the system can be used for measurement of concentration change in deoxy-Hb and oxy-Hb due to increase in blood during brain activity.