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AD1955 gain error in External Digital Filter mode

Hello,

I want to use the AD1955 DAC in External Filter mode. But when I use this mode I have gain factor of about 1.12 higher compared to the DAC used in PCM mode.
In PCM mode, I have a full scale of  8.64mA as shown in the datasheet.
In External Filter mode, I measure a full scale of 9.66mA.
Is that what is expected in this external filter mode?

It might be a coincidence since I don't know the inner workings of the AD1955, but in PCM mode 8.64mA FS is 9xIref (Iref=0.960), and in External Filter 9.66mA is around 10xIref...

Thanks,
Nicolas

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  • Hello Nicolas,

    I will apologize in advance but my answer is somewhat speculative not a definite answer. This part was designed over 30 years ago and all the designers are either retired, moved on or passed away. Normally a part this old would have been obsoleted a long time ago but it is such a good part that it is still a competitive part!  It is a tribute to those original designers!

    In our digital filters we use to decimate the sigma-delta data to PCM format we often add in a margin to prevent clipping of the analog stages. Also the THD also can rise slightly as it gets close to clipping. Since we have to write the specifications over all temperatures and silicon skews we will build in a margin so that X input will never be clipped and will meet specs over all temperatures and skews. So this is probably what you are seeing. The 9.66mA level may clip in some parts or at extreme temperatures so we would have backed that down a little. So I do not think you have done anything wrong and I would say this is expected. 

    Dave T

  • Hello Dave,

    Thanks for your answer.

    If I want to have the same output ranges in External Filter mode and in PCM mode, I see two ways:
    - apply a digital factor on the data sent do the DAC in extarnal filter mode
    - use the "volume" register of AD1955 do reduce the output in external filter mode

    Is one method better than the other?

    Nicolas

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  • Hello Dave,

    Thanks for your answer.

    If I want to have the same output ranges in External Filter mode and in PCM mode, I see two ways:
    - apply a digital factor on the data sent do the DAC in extarnal filter mode
    - use the "volume" register of AD1955 do reduce the output in external filter mode

    Is one method better than the other?

    Nicolas

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