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problem with 24 dB headroom available inside Sigma DSPs

Question asked by dimitrisTymph on Aug 26, 2015
Latest reply on Sep 2, 2015 by mmmike

Hi forum,

 

I am facing this problem when using an EVAL-ADAU1X61EBZ.

 

The manual of the ADAU1761 clearly states that

  • there is 4 bit available internally that provide 24 dB of extra headroom before saturating the DSP core internally.
  • the typical available input in the ADC is 1 Vrms

 

Please correct me if I am wrong but I am assuming that with 1 Vrms input I will have +24 dB inside the DSP before I clip the signal.

So I would expect to be able to boost a 1 Vrms input signal by 24 dB before I clip it.

But this is not what I experience, nor what I see inside SigmaStudio. I only seem to have 4 dB of headroom before the signal clips internally.

 

here is the experiment I am doing that does not give me the expected results:

 

  • I am feeding 1 Vrms input in IN1 of the EVAL-ADAU1X61EBZ
  • In the "Record Input Signal Path" of SigmaStudio I am using PIN10 of IN1 to pass the signal through the DSP2.png

 

  • Inside the Schematic my design looks like this:

     3.png

  • in the "Playback Output Signal Path" my comfiguration looks like this:

      1.png

 

  • When I am measuring the DIFF OUT L output of the EVAL-ADAU1X61EBZ I am measuring 400 mV rms.

 

My questions are:

1) where is the 24 dB of headroom?

2) Why is there only 4dB of headroom appearing in the Schematic?

3) I would expect that my output would would also be 1 Vrms. Instead, it is 400 mV. Where is this attenuation coming from?

 

thank you for your time

Dimitris

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