This is not a (harmonic) distortion.

This is a modulation between sample rate and the actual sine wave frequency and happens with every DAC.

The sample rate represents a non-infinite time resolution.
Constructing a certain sine-wave frequency out of equal-distant steps (the sample rate) that have no common denominator leads to this amplitude modulation effect if both frequencies narrow each other.

Imagine what e.g. a 16 kHz sine-wave at 44.1 kHz sample rate looks loke in the digital domain.

This page has a good explanation of the effect:
https://dsp.stackexchange.com/questions … -frequency

If you do a single shot on your scope you can get something like this:

I‘m not quite sure (can‘t test, don‘t have my measurement equipment at hand), but I think the reconstruction filter even has an influence on that, as the effect is related to aliasing.

Try the “Sharp” filter, it should make this go away.

It has the “Sharp”-filter on all the time.

If this happens on ADI-2 with Sharp filter try to revert to an earlier firmware - you might have found a bug.

It’s already there.
If you’re interested in digital technology’s theory, manual pages 84-86 have a very detailed description of the filter options.

Aliasing and it’s related issues is the single most important phenomenon of digital, even the most prominent parameter for sound, more than bit depth - if you’re starting with 16 bit.