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AFE4490: Ambient Noise Phase Offset in Ambient versus LED_ON

Jason Naramore
Prodigy 20 points
Part Number: AFE4490

We have observed a phase offset in the AMBIENT versus LED_ON signal that seems to be due the ambient noise caused by overhead fluorescent lights. The image below shows the measured noise signals without any current driving the LEDs. The frequency of this noise is centered at 5Hz, but seems to depend on sampling rate. When the ambient is subtracted from LED_on, the noise is amplified due to the 180 degree phase offset. The amplitude of this noise below is a few mV, but we've seen the same noise with amplitudes much higher, around 100mV.

The circuit schematics in the AFE4490 spec sheet show the INP and INN channels connected to opposite ends of the amplifiers, basically on different circuit paths. Is there some sort of filter being applied to these channels separately that are causing this phase difference in ambient noise?

Settings:

Gain: 1M stage 1 gain, 4x stage 2

Sampling rate: 62.5 Hz, 25% duty cycle

Appreciate any help on this issue...

Jason

  • 0
    TI__Genius 12575 points

    Hi Jason,

    One reason for this kind of behavior is high frequency ambient light. This high frequency signal aliases back as low frequency and that is why its location changes with the sampling frequency.

    LED and AMB phases are sampled at two different time instances , so if the ambient light changes significantly within these instances there are chances that you might get 180 out of phase data  in LED and AMB phase.

    One  way to fix/verify this problem is to change the separation between the sampling phase of LED and AMB. You may have to do couple of hit and trails to find what separation works best for the given ambient light.

    Also  can you share image showing "The circuit schematics in the AFE4490 spec sheet show the INP and INN channels connected to opposite ends of the amplifiers, basically on different circuit paths"

    Regards,

    Prabin

  • 0 in reply to Prabin Yadav
    TI__Genius 12575 points

    Hi Jason,

    I am closing the thread assuming your query was resolved.

    Regards,
    Prabin