System Overview

The heart rate/respiratory rate measurement device, we had designed operates by making a PPG measurement and extracts heart rate and respiratory rate signal from the raw PPG sensor output. The sensor we are using is the Nellcor DS-100A which interfaces through a D-Sub 9 connector. The Sensor comprises of an Infrared and Red LED and a photodiode sensitive to both the above wavelengths. PPG measurements require only one LED operational, (Although SpO2 measurements require both.)

PPG Principle
FIG 1. The Principle of transmitive and reflective PPG.

The PPG sensor (FIG 1) works by shining light on a finger and measuring the amount of light transmitted through it. This signal gives an indication of pulsating motion of the blood vessels, which is synchronous to the heart rate. The respiratory rate information manifests itself as modulation of the frequency, amplitude and baseline of the heart rate signal (FIG 2 and 3).

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FIG 2. The Scope waveform of the PPG data processes by the heart-rate filter. The frequency modulation and amplitude modulation (green trace) can be clearly seen.
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FIG 3. The scope waveform of the PPG Data processed by the respiratory rate filter. The baseline modulation is filtered out and amplified.


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