ADAU1372 ADC references

I'm considering using the ADAU1372.  What, precisely, are the AINxREF pins?  In the datasheet, it says to tie them to CM, but on the eval board, they're only AC-coupled to GND.  Which is recommended? (Standard line-input audio).

Also, can these references be used for proper balanced audio inputs?

Thanks!

  • 0
    •  Analog Employees 
    on Apr 6, 2016 10:33 PM

    Hello Joe42,

    They are the reference for a pseudo differential input stage. Originally we intended for these to be tied to ground through a cap but with further investigation determined that they could be tied to CM to save costs. They are internally tied to CM through a high impedance so you actually do not need to tie them to anything. But, it is a good idea to not leave it floating so we recommend to tie them to the CM pin.

    No, they will not work well for proper balanced inputs.

    Thanks,

    Dave T

  • 0
    •  Analog Employees 
    on Apr 13, 2016 12:39 AM

    Hello Joe42,

    I would like to update my answer due to recent research and experimentation. We found that the best way to do this is the way it is done on the evaluation board. That is to terminate each reference pin through a cap to ground and not to tie the common mode pins to the reference inputs. If you do you will find the CM voltage slightly changes when turning PGAs on and off and that will change the gain slightly on the DACs. If your setup will be static (not changing the PGA bypass) then you can tie it to CM but you will probably see a larger gain error difference from unit to unit.

    So I am changing my recommendation to using four caps to ground.

    Dave T

  • Dave, does your new recommendation apply to the ADAU1761 and ADAU1961 as well?

    Is there any noise-related benefit to terminating through a cap to ground, or is it perfectly okay terminating to CM (which also has a cap to ground) if not turning PGAs on and off?

    Thanks!

    Ed

  • 0
    •  Analog Employees 
    on Apr 14, 2016 4:20 PM

    Hello eingber,

    We are still working on understanding the issue fully but at the moment we have noticed this anomaly. It is a problem only if the pop-suppression is turned on. We will be doing some further work on it. The CM node is the most quiet place to terminate it to which is why we had suggested you do that in the datasheet. On the evaluation board we terminate it to ground and it is quiet on that board but it is a small board with little else on the board that would cause ground noise.

    So yes, if you are not turning the PGAs on and off then it will stay stable and it will be a quiet place to terminate to. So you can save the costs and use the CM pin. I just wanted you to be aware of this so you could make an informed design decision and not have a surprise late in your design cycle. We are going to continue to work on understanding this issue but it will be a little while before we have more answers.

    Dave T

  • Hello Dave,

    I have some problem with the ADC reference tied to ground through a cap (like eval board).

    Conditions: - PGA enable

                       - Input : Sinus -30dBu 1kHz

                       -Output of ADC routing to DAC ( just for debug)

    It's working until PGA gain is less than 15dB after this value, the signal is satured at 2V.

    It's working if we tied the reference to CM, but why?

    Thanks!