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AFE4404: ambient values

Konan Kim
Prodigy 210 points
Part Number: AFE4404

Hi,

I am working on calibration code for AFE4404 + SFH7072. I've encountered the following problem.

1) Even though OFFSET values were 0(ambient offset or DC offset), minus ambient values were measured during calibration. The values were not big values - like -690 ~ -1000.

    Is this normal ? Rf=250K, ILED was about 10(red or IR). Measured at finger tip.

2) Another case: Ambient values were very big like 230000 while red/ir values were 1450000 or so. This is weird because I pressed the sensor tight enough with my thumb so that no ambient light can come in.

   Strangely if the red/ir values were subtracted by the ambient values, the values were near the target threshold values, 0.6V. In this case, OFFSET values were 0. Can somebody explain it ?

   PD module was out of order or AFE4404 was not set properly ?

Thanks for your advice.

Konan

  • 0
    TI__Genius 12575 points
    Hi Konan,

    If I understand your question correctly, you are worried that the ambient values are very less in first case. This is true, you can leave the PD uncovered so that all the phases have the ambient light. Now if you wave your hand on top of PD, you can modulate the ambient light, which confirms that you are indeed sampling ambient light for the given register settings.

    ADC code of 1450000 roughly translate to ~830mV which is towrds saturation. The behavior that you see in case 2 could of following reasons.
    1) If any of the phase is saturated then the others phases ( including the ambient) phase will have the corrupt values. So try observing all the 4 phases and avoid any saturation.
    2) Failure to meet the requirements as stated in Table 7 of the datasheet will result in corrupted values.

    Regards,
    Prabin
  • 0 in reply to Prabin Yadav
    Prodigy 210 points

    Hi Prabin,

    Thanks for quick reply.

    Regarding question 1), my question was asking if minus ambient values are normal under the condition of OFFSET register value = 0. (Low ambient values are ok, of course.)

    As for question 2) I will observe the 4 phases and will check the requirements of the datasheet. But this issue is always happening for 1 PD module, for the other 2 PD modules, no problem. (with the same AFE chip)

    Thanks,

    Konan

  • 0 in reply to Konan Kim
    TI__Genius 12575 points
    Hi Konan,

    You will get the channel offset ( can be +ve or -ve) when OFFSET is set to 0 for the ambient phase. Also as observed by you this will be pretty small.

    Since the AFE is same for all the PD module, I feel the problem lies in PD module .

    Regards,
    Prabin