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I'm using a TAS5754MDCAR DAC in a Bluetooth receiver. DAC_OUTA and DAC_OUTB are connected to the internal amplifier inputs per the Datasheet (as below): reference circuit design

When I play an audio file and view DAC_OUT on my scope, I see a completely normal audio waveform and everything works as expected. However, I'd like to have line outputs, so I have connected the DAC outputs to RCA jacks. When I connect the DAC outputs to an external amplifier (I've tried several), the audio waveform looks fat, like this:

A normal audio waveform and a thick audio waveform

EDIT: I'm still monitoring the same DAC_OUT point (which is connected to the external amp's input). I'm not looking at the output of the amp.

I tried disconnecting the on-chip amplifier, but the results are exactly the same. The end result is fine--it sounds normal and works as expected. But it's bothering me that I don't know why the waveform changes like this when connected to the input of an external amplifier, and I'd like to learn about what I'm seeing and why.

Out of curiosity, I connected several other sources to the same external amp and didn't see anything like this--the original audio waveform was still intact.

Closer examination of the "thick" waveform shows that it's actually a high frequency sine wave (in this case 1.20 MHz).

1.20 MHz waveform

What am I seeing here? The scope I'm using is a Siglent SDS 1204X-E. Thanks for your help!

DavidB
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  • Can you explain how you power your circuit (Can you be specific on how you ground it) and can you elaborate on what amplifier you used. It looks like some grounding issue. Also, do you hear the noise on the other side of the amplifier? – Julien Jul 26 '23 at 21:25
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    The circuit is powered from an on-board buck converter that supplies 12V. There is virtually no noise on the output--it's very quiet. Per the data sheet, all of the grounds are connected to system ground. I would have expected to have analog ground separated out but the data sheet says to connect it to ground. In the end, it sounds and works "fine" and I wouldn't have noticed anything if not for connecting the scope and poking around. – DavidB Jul 26 '23 at 21:31
  • I think you have an issue of ground loop when you connect the oscilloscope. These can be tricky to confirm since the instrument prevent the measurement. The simplest way is to use a floating oscilloscope (but you probably don't have one of those!) I'd have to think a bit to offer you a test setup that would allow to confirm or infirm this issue – Julien Jul 26 '23 at 21:35
  • Thanks, that would definitely explain it--especially since there are no other symptoms. Unfortunately, you're correct--I don't have a floating oscilloscope. – DavidB Jul 26 '23 at 21:58

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