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TLV320AIC3254 - micPGA gain before or after signal combination

Robert Wolfe
Mastermind 7860 points
Other Parts Discussed in Thread: TLV320AIC3254

Hello,

Does the gain of the mic PGAs preceding the ADCS on the TLV320AIC3254 codec occur before or after differential signals are combined?, i.e. each component of the differential combination is first gained and then they are combined, or the resulting combination/addition is gained.

Regards,

Robert

  • 0
    TI__Guru* 84915 points
    Hi, Robert,

    The PGA gain is applied after the differential signals are combined. Each analog input will have an individual gain determined by the input resistor (10k, 20k, 40k). Then, differential signals will be combined. Finally, the PGA gain will be applied.

    Best regards,
    Luis Fernando Rodríguez S.
  • 0 in reply to Luis Fernando Rodriguez S.
    Mastermind 7860 points

    Thanks for the quick response. Follow-up question please: I see on the application note for this part that the differential inputs to the mic PGA, for the left ADC, are either IN2_L and IN2_R or IN3_L and IN3_R. What happens if a different pair are combined?, i.e. IN3_L to positive terminal and IN2_R to the negative terminal.

  • 0 in reply to Robert Wolfe
    TI__Guru* 84915 points
    Hi, Robert,

    The TLV320AIC3254 only supports three fully-differential inputs. They are located in pairs as the datasheet explains: IN1_L/R, IN2_L/R and IN3_L/R. If a different pair is combined, it will be taken as single-ended input.

    Best regards,
    Luis Fernando Rodríguez S.
  • 0 in reply to Luis Fernando Rodriguez S.
    Mastermind 7860 points

    Thanks again for the reply Luis.  It's a bit confusing to me, though, based on our experience.  We have routed IN3_L to the positive terminal of the Mic PGA for the left ADC.  And IN2_R has been routed to its negative terminal.  And when looking at the resulting ADC data, the two signals are being mixed together ... IN2_R is being subtracted from IN3_L.  But this is not the fully-differential inputs the datasheet explains, i.e.  IN1_L/R, IN2_L/R and IN3_L/R.  But it appears, from our experience, the Mic PGA will still mix the signals together, even though they are not the fully-differential pairs.  Is that expected?  What have we lost by doing this, and not using the fully-differential pairs for mixing the signals?

    Regards,

    Robert

  • 0 in reply to Robert Wolfe
    TI__Guru* 84915 points
    Hi, Robert,

    We would expect these results as you mentioned. However, we cannot guarantee that this would be the best mode to subtract the signals since the datasheet doesn't mention this. We would recommend to use the suggested differential pairs to ensure the correct behavior of the codec.

    Best regards,
    Luis Fernando Rodríguez S.
  • 0 in reply to Luis Fernando Rodriguez S.
    Mastermind 7860 points
    Ok, we will make the correction. I am wondering if it is the reason I am seeing unexpected behavior when adjusting the Mic PGA gain on-the-fly, i.e. after some period of time, I write a new gain to Page 1, Register 59, and the mixed signal at the ADC does not respond as would be expected for the updated gain (I confirm the gain has actually been written, by reading the value back after writing it).

    Thanks,
    Robert