Yes, agree, edechamps, the N+D is the correct way to look at this. I've attached the N+D plots for both the AK5397 and AK4490. On the QA401, both ADC and DAC are 0 dBFS @ = +6 dBV, so if you connect in loopback, and then drive to 0 dBFS, then both are at the same point of operation, which would be at the very far right on the AK plots. In this region, according to AKM, there could be as much as a 29 dB rise in N+D. Some from harmonics, and some from noise. You'd see about 23 dB of that from the ADC, and 6 dB of that from the DAC.
But notice as you drop to -5 dBFS, the combined N+D degradation drops to just a few dB.
Blumlein 88, this behavior is in all sigma delta converters to some degree...the way you "hide" this in an audio DAC is you scale all incoming digital data by, say, -5 dB in the driver before you feed that to the hardware. In other words, if you send it a true 0 dBFS signal, the DAC driver would give it -5 dB of gain, and then it would output a -5 dBFS signal when you told it to output a 0 dBFS signal. And then you call that new lower signal 0 dBFS. And just like that, the DAC (or ADC) exhibits no growth to N+D at 0 dBFS. But in doing so, you throw away dynamic range/noise floor/etc....it's all a tradeoff. The QA401 does some of this (and thus avoid the full 29 dB penalty shown in the graph), but preserving the -115 dBV noise floor is important.
AmirM, I'm always happy to drop a unit in the mail on short notice when you get a time slot opened. Just let me know