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I am using an TI PCM1804 to convert my audio signal.

The PCM1804 is configured as follows: Master Mode, with 12.288MHz input clock Sample rate: 48kHz

The testsignal is a 440Hz sine with an amplitude of 4.5V_pp. In order to get a balanced signal from the signal generator, I use a circuit which is identical to the one in the PCM1804 evaluation kit: http://www.ti.com/lit/ug/sleu015/sleu015.pdf

The circuit looks as follows: Circuit

After analyzing the ouput, unfortunately there is no 440 Hz sine but a distorted signal. I can post a picture of it in the next post.

Can anybody help with this issue? I have no idea where this comes from. Thanks in advance! audio_horrible

Output picture: enter image description here

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  • \$\begingroup\$ Screenshot of the output can be found here: i.stack.imgur.com/us1iy.png \$\endgroup\$
    – le_audio
    Aug 7, 2017 at 20:04
  • \$\begingroup\$ Reduce a lower input voltage and what do you get? Also what audio analyzer are you using? \$\endgroup\$
    – Jason Han
    Aug 7, 2017 at 23:05
  • \$\begingroup\$ Analyzer is Saleae Logic 8, Audio analyzation via Matlab \$\endgroup\$
    – le_audio
    Aug 8, 2017 at 16:39

1 Answer 1

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The ADC is returning the data as signed (two's complement). But you're treating it as if it was unsigned.

https://en.wikipedia.org/wiki/Two%27s_complement

Here's a table (taken directly from Wikipedia) that explains what happens:

Bits   Signed    Unsigned

011    3         3
010    2         2
001    1         1
000    0         0
111    -1        7
110    -2        6
101    -3        5
100    -4        4

The "distortion" you see is caused by that transformation.

There should be some option in the software you use where you can select if the data is signed or unsigned. Anyone who knows more about that is free to edit my answer.

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