ADPD4000 signal time reversal

The trend of PPG signal we got is going up slowly and going down quickly. The real life's signal should be going up quickly and going down slowly. Anyone know why the signal is different from the normal blood wave form?

 the normal one should be like the following:

 

  • 0
    •  Analog Employees 
    on Apr 27, 2021 3:31 PM

    Hi Paul,

    Nice observation!

    PPG is a photometric measurement of cardiac cycle associated blood volume changes in the biological tissues of interest. The blood volume is high during systolic phase and low during diastolic phase due to the pressure difference. Photons from the light source(s), typically LEDs, are either absorbed or scattered in the biological tissue, and a fraction of the photons are detected by the photodiode. The photocurrent varies, depending on the detection mode (reflective vs transmissive) and the wavelength of the light. For example, in a reflective PPG measurement, less photons are scattered and reach the photodiode when the blood volume is high, resulting in a lower photocurrent. The output of photometric analog front end (AFE)  such as ADPD4000/ADPD4100 is directly proportional to the photocurrent. Hence, the PPG signal falls rapidly during systolic phase and rises slowly during diastolic phase in a reflective PPG measurement. To interpret PPG signal, you can flip the data in your post processing.

    Hope the explanation above helps.

    Regards,

    Glen B.   

  • Hi Glen,

    Thanks for the response. This explains why the plots we got is kind of time reversed. Is IR do better than red?

  • 0
    •  Analog Employees 
    on Apr 28, 2021 1:52 PM in reply to pchen

    Hi Paul,

    You are welcome! You would need to flip the data since the reflective PPG is inverted.

    IR typically can travel much longer distance in the tissue than red does. Hence the chance that photons are modulated by arterial blood become higher and the waveform with IR has more high frequency features during the diastolic phase. 

    Regards,

    Glen B.