Cover story t sponsored FeATUre the
Figure 5. Descriptive statistical properties of the magnitude squared wavelet coherence values are shown in four relevant frequency ranges from 0 Hz to 20 Hz.1
major frequency bands around the peaks of the PPG signals measured by the gold standard ADPD107 and the new integrated ADPD188GG optical module. Major components of the signals were not modified, except the very slow baseline fluctuations (<0.25 Hz) and the high frequency components (>40 Hz), which were filtered. Wavelet coherence and correlated
comparisons were computed to compare the stability of the two signals within the most dominant frequency range. Figure 3 shows almost identical patterns of the results of the two PPG systems at the level of individual waveforms and their averages. To continue the comparison at the deep data
level, two different correlation-based methods were applied. Correlation coefficients and P values (R, P) were computed between every upcoming PPG wave, following each other. A different kind of variability of the signals was also tested by comparing every individual PPG waveform to the average. Based on the comprehensive correlation tests,
we can conclude that it was not possible to observe any significant difference between the
Instrumentation Monthly April 2021
two compared PPG systems, neither on the level of the individual waveforms nor on the level of the individual waveforms vs. averages. Wavelet methodologies are very sensitive to
differences in specific frequency bands. For that reason, wavelet coherence functions were calculated to compare the two PPG signals. Based on the results analysed in the case of all 11 subjects, no significant difference was observed in the frequency domain or in the phase domain between the two signals (see Figure 4). While developing a new product, it might also be helpful to look at the specific frequency bands, which can be extracted from a given signal, to be able to optimise the specifications. In this test, basic statistical properties of the magnitude squared coherence between the two compared PPG systems were analysed within all the relevant frequency ranges, as shown in Figure 5. The entire spectrum was divided into six specific frequency ranges to analyse the variability of similarities between the signals. For all 11 subjects the coherence value was
higher than 0.95 within all the frequency bands around the peak of the PPG signals, which tells us that there is a very high similarity between
gold standard and the new integrated ADPD188GG.
ConClusion
The ADPD188GG is a fully integrated optical module targeted for measuring heart rate, heart rate variability and oxygen saturation, and for monitoring continuous blood pressure estimation. Because the module includes optics and electronics in a tiny package, it helps the designer and companies without optical knowledge to shorten their overall design cycle. The module is optimised for applications where reflective measurement methodology is used with wavelengths at 525 nm; however, external LEDs can also be used to measure at different wavelengths or to measure based on a transmissive principle. We have proven that the integrated system is not limiting us to meet the required specification for the various use cases in both out-of-hospital or clinical-based systems. For more information, visit
analog.com/healthcare.
Analog Devices
analog.com/healthcare 11
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