And how can it completely avoided? you will always have some stray capacitance even with battery powered devices.
Whats why we don’t test with "real world" conditions but with synthetic tests.
DACs don’t get tested with music but with special test tones.
The same way you can measure CM impedance or rejection ration.
And that's why with proper testing you would isolate how much the DAC alone is contributing or rejecting.
Whats your problem with this? what would you uses to express a ratio? is said "compared to listening level."
Comparison is you shuld not need an external device for this. filtering shuld be build in.
It doesn't matter if stray capacitance is present; what matters is whether or not ground-related hum or noise is present at audible levels. Whether you measure that distance from the listening position, a foot or two from the speakers, or with your ear pressed against the speaker driver, is a question reasonable people can disagree on, precisely because we're talking about an engineering and personal-preference question of practical sufficiency, rather than literal 100% perfection.
As for your "that's why DACs don't get tested with music but with special test tones," no, in fact, you're incorrect about that. With a musical signal it's difficult to impossible to measure noise and distortion with anything close to the precision necessary to ensure (a) repeatable results with the same gear, and (b) comparable results between different pieces of gear tested at different times by different people.
That is why we don't test DACs with music. And as far as a DAC is concerned, a 32-tone test tone and actual music are essentially the same, with the only difference being that the 32-tone signal is more regularized and controlled so that results can be repeatable and comparable.
As for isolating the DAC's role in ground-loop noise, I think that's more difficult than you're making it out to be. A reviewer like Amir would have to create a setup with a known ground loop in it, and he'd have to more or less randomly pick one DAC to serve as his reference for the "baseline" ground-loop noise. And - not a trivial thing - he'd have to determine what components of the overall measured noise were coming from the ground loop, because a ground loop doesn't always generate just 50 or 60Hz hum, and it doesn't even just generate obvious low-order harmonics - a ground loop can also increase the amplitude of higher-level noise that sounds identical to the hiss we hear from virtually all gear's self-noise (or self-noise being passed through or amplified from upstream in the chain).
And forever into the future, Amir would have to test every single DAC with that exact setup. And we would have to assume that the amplitude and frequency content of that ground loop never changes over time - which in my understanding is far from a safe assumption.
And after all that, we'd still end up with measurements that were of extremely limited value, if any value at all, because from several of Amir's reviews and in the widely varied experience of countless members of this forum (including me), we know that it's almost always possible to significantly reduce or for practical purposes eliminate audible ground-loop noise - and that it's foolish to think you can depend on something like a DAC to make your ground loop problem go away.
I noticed in your prior comment that you claimed you were citing facts that "many at ASR don't want to believe." I almost commented on that in my prior post; I will do so now: I see this tired "ASR members are hypocrites because they don't want to believe scientific facts that are inconvenient" trope come up periodically here on the forums. And I've noticed that every time someone trots it out, the "hard facts" they'e banging on about are not in fact things that ASR members don't want to believe. Instead, they are facts that ASR members acknowledge but don't think
matter. And if a ground loop or stray capacitance is always present, but you can set up your signal chain so that it does not produce audible noise from your speakers, it doesn't matter that it's present. You could say the exact same thing about distortion: always present, can't deny that. But if it's at -120dB, who cares? (And if it rises to -80dB at 20kHz, who cares either - our hearing acuity isn't good enough to detect that.) We know that you can run an unfiltered 20kHz square wave into a Class D amp and cause its measured performance to degrade significantly. But that's not "a fact many at ASR don't want to believe." It's just a fact that doesn't matter.
So if you want tests that say, "this DAC has noise at -140dB but what happens if we put it in a system with a terrible ground loop problem?" then you're free to run those tests yourself. Until you do, and until you can produce some evidence or reasoning for why such tests would matter, I don't see the point.
