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AFE4490 Second stage dark current offset difference

Other Parts Discussed in Thread: AFE4490

Hi,

I am currently testing a prototype board with an AFE4490 module.

When I look at the ambient light signal of the RED and the ambient light signal of the IR LED than I see a DC offset difference.

This difference is not there when I disable the second stage amplifier. It only appears when I enable the second stage.

When I detach the photo diode from the board than the DC offset still exists, but the difference is a little less.

The TIA_GAIN and the TIA_AMB_GAIN settings for both LEDS are equal.

Does someone have a similar experience with this issue?

Regards,

Martin

  • Hello Martin,

    How much DC offset difference do you notice? Could you provide more details?

    Also could you provide your AFE4490 register configuration?

    Regards

    Praveen.

  • Hi Praveen,

    The DC difference varies between 1000 ADC units up to 10000+ ADC units depending on the Stage2 Gain value.

    AFE register map:

    AFE4490_CONTROL0    000000
    AFE4490_LED2STC        003A98
    AFE4490_LED2ENDC    003B5E
    AFE4490_LED2LEDSTC    003A98
    AFE4490_LED2LEDENDC    003B5F
    AFE4490_ALED2STC    000000
    AFE4490_ALED2ENDC    0000C6
    AFE4490_LED1STC        001388
    AFE4490_LED1ENDC    00144E
    AFE4490_LED1LEDSTC    001388
    AFE4490_LED1LEDENDC    00144F
    AFE4490_ALED1STC    002710
    AFE4490_ALED1ENDC    0027D6
    AFE4490_LED2CONVST    000002
    AFE4490_LED2CONVEND    001387
    AFE4490_ALED2CONVST    00138A
    AFE4490_ALED2CONVEND    00270F
    AFE4490_LED1CONVST    002712
    AFE4490_LED1CONVEND    003A97
    AFE4490_ALED1CONVST    003A9A
    AFE4490_ALED1CONVEND    004E1F
    AFE4490_ADCRSTSTCT0    000000
    AFE4490_ADCRSTENDCT0    000000
    AFE4490_ADCRSTSTCT1    001388
    AFE4490_ADCRSTENDCT1    001388
    AFE4490_ADCRSTSTCT2    002710
    AFE4490_ADCRSTENDCT2    002710
    AFE4490_ADCRSTSTCT3    003A98
    AFE4490_ADCRSTENDCT3    003A98
    AFE4490_PRPCOUNT    004E1F
    AFE4490_CONTROL1    000114
    AFE4490_SPARE1        000000
    AFE4490_TIAGAIN        008000
    AFE4490_TIA_AMB_GAIN    004400
    AFE4490_LEDCNTRL    011E1E
    AFE4490_CONTROL2    020000
    AFE4490_SPARE2        000000
    AFE4490_SPARE3        000000
    AFE4490_SPARE4        000000
    AFE4490_RESERVED1    000000
    AFE4490_RESERVED2    000000
    AFE4490_ALARM        000000
    AFE4490_LED2VAL        000000
    AFE4490_ALED2VAL    000000
    AFE4490_LED1VAL        000000
    AFE4490_ALED1VAL    000000
    AFE4490_LED2-ALED2VAL    000000
    AFE4490_LED1-ALED1VAL    000000
    AFE4490_DIAG        000000

    Regards,

    Martin

  • Hello Martin,

    When ENSEPGAIN (D15 of reg TIAGAIN) is set, then Stage2 enable and gain for LED1 and LED2 phases are separately controlled.

    STAGE2EN1 (D14 of reg TIAGAIN) enables stage 2 for LED1, LED1_AMB and STG2GAIN1 (D10:D8 of reg TIAGAIN) sets the stage 2 gain for LED1, LED1_AMB.

    Similarly STAGE2EN2 (D14 of reg TIA_AMB_GAIN) enables stage 2 for LED2, LED2_AMB and STG2GAIN2 (D10:D8 of reg TIA_AMB_GAIN) sets the stage 2 gain for LED2, LED2_AMB.

    Based on your register settings,

    ENSEPGAIN is set,
    STAGE2EN1 = 0 and STG2GAIN1 = 3'b000
    STAGE2EN2 = 1 and STG2GAIN2 = 3'b100.

    i.e., LED1 stage 2 is disabled and LED2 stage 2 is enabled and LED2 stage 2 gain = 4

    This is the reason why you notice the DC offset difference only when Stage 2 is enabled.

  • Hi Praveen,

    That explains it.

    I am using datasheet revision SBAS602F from october 2013 and this STAGE2EN1 and STAGE2EN2 feature is not documented.

    I downloaded the lastest datasheet revision SBAS602G from june 2014 and this feature is documented.

    Thanks for the help.

    Regards,

    Martin