@amirm sorry if from time to time I go back with the same question, but I don't understand the linearity test results in general... For instance here the linearity test is superb, while in DX3-PRO is no so good (dominated by noise floor starting at 90 db), though DX3 has a lower noise level than m900 as shown in this review in the intermodulation test. Can you help me in understanding?
Thanks
Sure.
Linearity is a measure of single frequency voltage accuracy. A very sharp narrowband filter gets rid of all but the tone generated in digital domain. Instability in this voltage due to noise, or power supply ripple, etc. will cause linearity to suffer differently than normal measurements. In other metrics, entire bandwidth of audio or more is used so what ails them does not correlate with linearity and vice versa.
Put simply, linearity is a vertical measurement of accuracy (with the dimension being volts). THD, DNR, etc. are an area of measurement of volt by frequency spectrum.
Going back to your answer, as a broadband measurement, DX3 Pro does better than many other DACs. But with respect to measuring just voltage at this one frequency, its value seems to fluctuate as we get lower and lower resulting in that exponential rise.