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AFE4400: PPG signal is very bad by reflection using afe4400 with DCM03 sensor

Qingyun He
Prodigy 30 points
Part Number: AFE4400


Hi, I am doing a project using afe4400 to measure the PPG signal of finger with DCM03 sensor, but the signal is very bad in most cases.

So I do a test. I increased the LED currents both for IR and RED LED by 10 every 30 seconds from 5 to max. and I found the DC signal doesn't increases with the increasing current. It seems the photodiode is  saturated. But the LED cucrrent is very little at the beginning(lesser than 50mA * 15/256)  and I cann't select  lesser current for afe4400(full-scale 50mA only) . 

Could you help me to solve this problem? Thank you.

  • 0
    TI__Genius 12575 points
    Hi Qingyun,

    When you say the signal is bad, how does it look like? Is it all ambient signal or it has some PPG signal. It would be of great help if you could share the raw data.
    Following controls will help you reduce the saturation of signal chain.
    1) Reduce the gain (i.e. RF_LED) and also second stage gain.
    2) Reduce the LED current
    3) AMBDAC to cancel out the DC

    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    Prodigy 30 points
    Thank you for your reply, my question had been solved.
  • 0 in reply to Prabin Yadav
    Prodigy 30 points
    Prabin,

    I have another question to ask you. I want to use the AMBDAC function to reduce the ambient light component. How often should I change the cancellation current? Change it every sample time or a period of time for averaging ambient DC?
    Thank you.
  • 0 in reply to Qingyun He
    TI__Genius 12575 points
    Hi Qingyun,

    If the ambient light is of low frequency then taking the average of few samples and correcting them will be enough. This also reduces the overhead on the MCU.
    Whereas for fast ambient you have to correct very often and it comes with overhead for MCU.
    I suggest you can try both and pick the one that works best for you.

    Regards,
    Prabin