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AFE4490: IR signal very noisy

simon_corv
Prodigy 20 points
Part Number: AFE4490

Hi,

We designed our own customized board including the AFE4490 photometric front end combined with two external led. Now we are trying to receive an accurate signal out of the front end, and thus some questions regarding the settings of ADPD144 arose because the signal from our IR leed is very noisy compared to the RED led signal.

The signal itself is very accurate (clear pulse waveform after filtering) but if we increase the led current, the IR signal gets very noisy and the RED signal not. Currently we are working with 6.15 mA, which is much less than the maximum forward current of the LED. Moreover we are using the same Rf and Cf value to compare the output. So now we are just wondering if it's only a design problem or if there is a way figuring out how to decrease the noise part by setting some register appropriately. Generally it's not a big problem applying different settings for both led, but we just wanted to know if you have any experience in this issue.

Regards,

Simon

  • 0
    TI__Genius 12575 points
    Hi Simon,

    Higher noise in IR channel could be because of LED, input ( reflected light from body) or combination of both. To find the root cause can you do following tests:
    1)Use a reflective surface instead of body : Vary the TX Current and observe the noise. Please make sure that you do not saturate the ADC. If this test gives good result, it concludes that LED is working fine. Bad results suggests that LED is behaving differently across the TX current.

    Can you also look at IR-Ambient channel instead of IR alone?

    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    Prodigy 20 points

    Ahhh, we totally forgot to check the other channels. Up to now we were only reading out the LED-Ambient channel. Thanks for the advice.

    So after we did the recommended procedure, we found out that it has most likely nothing to do with the LED. There's no great difference in the noise level between the two LED when reading out only the LED digital sample. But, however, there's some difference in the ambient channel and so in the LED-Ambient channel. We could see this behavior in both cases, using a reflective surface and using the body. Somehow the ambient light level is higher for the IR LED. But we can definitely work with the data from the LED channels, although we don't have a good explanation why the ambient channels are so different.

    Thanks again,

    Simon