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TAC5212EVM-K: Audio output (75Hz) is causing clipping to a square wave in single ended output mode, headers should be in single-ended mode, very confused.

Brian Neltner
Prodigy 20 points
Part Number: TAC5212EVM-K

Tool/software:

I am having difficulty using the TAC5212EVM-K configured for mono output

I am configuring it via PPC, and set it to use the OUT1M using the USB audio interface to let me do some basic stability testing with Windows.

I generated a 32-bit 48000Hz sample rate 75Hz sine wave audio file being played back. The signal is not distorted in the software (Audacity) and it is properly configured for 32-bit 48000Hz playback. The TI driver on the lower right icons is there, and was also set to 32-bit output.

When I configure the output to have a gain of -8dB (dropdown, not slider) I see a bad signal that certainly looks like half of a differential output (below).

 

With lower output gain selected (dropdown not slider), it instead produces a higher amplitude fully clipped square wave. Yes, when I *decrease* the output amplitude in the program, it *increases* the output amplitude and causes it to turn into a square wave (below).

 

I see it constantly rail the output regardless of the volume control setting in Audacity, Windows, the TI icon on the tray, or the PPC software.

I can only guess that some jumper must be wrong, but the eval board documentation doesn't show the full audio path and is transparently wrong -- at a bare minimum J59 and J60 are not even four pin header. We don't know how to tell what's incorrect, but it all seems fine when we slowly read the datasheet. I attached photos showing the configuration of the output for reference.

Please let us know if you have any hints, thanks. I'd love to be able to consider using this part but this is rough for a basic performance check. All I was looking to do was measure the AC voltage stability... which at present is not even measurable.

  • 0
    Prodigy 20 points

    To head off the obvious, decreasing the audio bit depth to 16-bit did not help.

  • 0 in reply to Brian Neltner
    TI__Genius 12965 points

    Hi Brian,

    Just to be clear, what happens when you select mono single-ended at OUT1P as the output? Also, check the blue switches AKA SW1 in the user guide, make sure you have the output drive you desire selected. I will look more into your jumpers tomorrow. 

    Best,
    Mir

  • 0 in reply to Mir Jeffres
    Prodigy 20 points

    In that mode it simply rails, with the attenuation seeming to slightly change the duty cycle as if it is clipping a different section of a sine wave.

    All of the SW1 switches are set to off, I understand them to just be test loads or bias voltages.

    I connected J36 as per page 10 to make the shield a return for single ended mode, left it AC coupled, and J49 is not connected.

    Just from observation it seems like it's operating in current mode and hitting an open circuit, or maybe there's some disconnected feedback, should I expect to be able to run it into an oscilloscope? Maybe I am missing the need for a test load during operation.

    I am assuming I don't need to connect a return cable from OUT1P to OUT1M (or the reverse), if I do then that would explain a lot. I see the connection requirement for the ground shield but i don't see any notes about it not using the shield as the return.

  • 0 in reply to Brian Neltner
    TI__Genius 12965 points

    How are you sending the audio through? Are you using audio over USB, or externally generating I2S? If you are using audio over USB, make sure that the switches on the AC-MB are both set to ON.

    Can you check the Audio Serial Bus tab and send what it looks like here? Also, what does the Rx/Inputs (DAC) section look like?

  • 0 in reply to Mir Jeffres
    Prodigy 20 points

    Audio is over USB, with no signal playing via the Windows driver the output is flat and the frequency of the output signal is correct -- it is just railing.

    I'm out of the lab now and cannot capture a screenshot, but I'm using the AC-MB. I didn't change any settings on the audio serial bus or gpio/interrupt tabs, but maybe the defaults are no good for my configuration? I also did not mess around with the advanced tabs. The SW2 is set to on/on.

    From reading more I think the OUT1P, OUT1M, OUT2P, and OUT2M give four single ended outputs as long as the shield is connected to ground, I had hoped that might be the reason but I think that part is okay.

  • 0 in reply to Brian Neltner
    Prodigy 20 points

    Dear Mir,

    I tried loading up the preset for single ended output and it behaves as I expected. The configuration looks the same to me, the device config page and playback config page are identical but I must have missed something somewhere.

    Thanks

  • +1 in reply to Brian Neltner
    TI__Genius 12965 points

    Hi Brian,

    Sounds good, I'm glad it works for you. You can record what the I2C commands are for the presets with the I2C monitor page so you can run those to initialize the device in a working state for you. Let me know if you have more questions.

    Best,
    Mir

  • 0 in reply to Mir Jeffres
    Prodigy 20 points

    Oh that's a relief, I was dreading having to figure out what I2C commands it worked out to.