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AFE4404 EVM: Problems with PPG measurements at wrist with RED and IR LED

dida
Prodigy 30 points
Other Parts Discussed in Thread: AFE4404, AFE4404EVM

I'm developing a SpO2 measurement device at wrist using AFE4404 EVM and its sensor board. Unfortunately, I can not get useful PPG signal with enough SNR. I have increased the driving current of RED and IR LED, and decreased the gain of the TIA at the same time. The duty cycle was increased to 2%, the sampling rate was 100Hz and the average number was 6. The power of the AFE4404 EVM is from a laptop which is powered with its battery, the sensor board was surrounded by black tape with LED chip exposed.

I changed the position at the wrist many times, and at only one time I got some useful PPG signal. But with the same configuration, I can not repeat the results. Only I got were DC components around 0.6V to 0.8V with tiny AC components and noise.

I want to know if the LED chips (SFH7050 and NJL5513R) within the AFE4404EVM is suitable to the wrist SpO2 application? If so, are there any parameters I can tune to realize it? Many thanks!

  • 0
    TI__Mastermind 49906 points
    Hi Dida,

    Red and IR LEDs are particularly challenging in getting consistent PPG signals from the wrist.
    There are multiple factors that are critical to get PPG signals from the wrist.

    1. User physiology
    a. Skin tones
    b. tissue thickness
    c. location of arterial bed on the bed
    2. Optics
    a. wavelength
    3. System Mechanics
    a. Placement of the sensor on the wrist

    Based on your inputs, it seems more like the user physiology and mechanical problems.
    You can probably check for the following:
    1. How are you placing the sensor on the wrist? Is it held in place using a watch strap or are you holding it? You may want to use a watch strap in case you are holding it.
    2. How are the PPG signals with Green LED?
    3. Have you used the offset cancellation DAC settings if the signal chain is saturating with higher LED currents and TIA gains?
  • 0 in reply to Praveen Aroul
    Prodigy 30 points

    Hi Praveen,
    Thank you for your reply. My situation are as follows:
    1. I use a black wrist band to fix the sensor board to my wrist.
    2. The PPG signals with green LED are usually good, but not always good. At some positions or pressure, I can't get good PPG signal with green LED too.
    3. I did use the DC offset cancellation setting. It worked for PPG signals with Green LED, but it had no  observable effect on the DC components of PPG signals with Red and IR LED.

    Can you give me some advices? Thanks !