Introduction:
The THD+N vs Amplitude measurements are good for testing a devices linearity and noise performance and see whether there is any dependence of noise with signal level. In this test a stimulus in the form of a 1kHz sinewave is fed to the EUT (equipment under test) to see how it responds. In an ideal test what is fed into the EUT should also be present at its output in proportion and without any other frequency components or artifacts present if the EUT is perfectly linear. However in the real world some non linearity is always present and we expect to see some frequency artifacts in the form harmonics of the original fundamental test frequency. The random uncorrelated noise is also added to this measurement.
We also use this test to show up a flaw in some Esstech Sabre DAC implementations in the form of a hump in THD measurements at a certain band of signal amplitudes.
No evidence of ESSTech IMD/THD hump !!
DAC - THD+N vs A @ 1kHz.png