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Is the AIC3204 ADC Gain before or after the ADC?

MikeH
Guru 15580 points

I am using the ADC data overflow event (via INT1) to trigger an "Audio Clip" LED on my custom board. I want the ADC to overflow when my Line1 input exceeds 0dBv (1.4v peak = 1VRMS), To accomplish this I have routed IN1_L/R to the PGA_L/R via a 40k resistor, and set CM1 to LPGA_P/M via a 40k resistor. I have also set the PGA gain to 0dB and the  ADC gain to +6db (since the full scale input voltage required to produce a full scale ADC output is .5VRMS and I want to clip at 1.0VRMS, which is 6dbv higher than .5VRMS).

However, the ADC never overflows with the above settings. If I increase the ADC gain to +6.5db, the ADC will clip at 1.04VRMS. I suspect that the PGA is saturating before the ADC so that the ADC never overflows. Is this correct?

My main question is this:

Is the gain device for the ADC ("ADC Channel Volume" Registers 0, 83 &84) physically located BEFORE or AFTER the ADC?? The reason I ask is, if it is AFTER the ADC (per Fig 1-1 of the datasheet - shown below), its setting should have no effect on the ADC overflow. However, my experiments indicate that it definitely DOES affect the overflow signal level.

Below is Fig. 1-1 from the datasheet. I assume the box labeled "Gain Adj." is the "ADC Channel Volume". Notice that it is AFTER the ADC, which means its setting should not affect the level at which the ADC overflows...

Also, what is the function of the boxes labeled tpl and tpr?

Any guidance on this issue would be much appreciated.

  • 0
    TI__Mastermind 46195 points

    Mike,

    Yes, you are probably seeing analog saturation first. The ADC gain is applied in the processing engine after the ADC, so it correlates with your observations.

    Tpl and Tpr are phase adjustments.

    Regards,

    J-

  • 0 in reply to J-
    Guru 15580 points

    J,

    Thanks for the feedback. However, I think you missed my point.

    If the ADC gain is AFTER the ADC, then increasing the ADC gain would not affect the level of the signal ENTERING the ADC, and would therefore NOT have any affect on when the ADC overflows. However, increasing the ADC gain definitely DOES have such an effect, which leads me to believe that the ADC gain is BEFORE the ADC.

    This question is now actually moot since I have decide not to use the ADC overflow as a clip indicator. There are too many other potential saturation points before the ADC for this to a reliable way to detect audio clipping. But thanks anyway for the response.

  • 0 in reply to MikeH
    TI__Mastermind 46195 points

    Mike,

    The ADC gain is after the ADC modulator.

    The ADC overflow logic is after the ADC and after the gain block. Think about it as ADC 'channel' overflow, which would encompass the entire ADC channel from analog in to I2S Out.

    Regards,

    J-

  • 0 in reply to J-
    Guru 15580 points

    J.

    Thanks for the clarification.