I see in the specs you mentioned 'no measurable noise floor modulation' - how did you do the measurement and with what stimulus?
Good question. Noise floor modulation is very difficult to measure, as ADC's (even the very best) have very significant noise floor modulation. Firstly, what is noise floor modulation? When a sine wave signal is used in a DAC, you get different types of distortion - harmonic distortion (distortion of integer multiples from the sine wave fundamental) enharmonic distortion (distortion products that are non integer) and changes to the noise level. So for example you may have a DAC that produces noise at -120dB with -60dB sine wave (traditional dynamic range test) but the noise with a 0dB sine wave maybe -115dB - thus the noise has increased by reproducing a higher level sine wave - in this case the noise floor (seen by doing an FFT measurement) would increase by 5dB.
Now noise floor modulation is highly audible - it interferes with the brain's processing of data from the ear - and immeasurably small levels of noise floor modulation is audible. I know this as I have listened to noise floor modulation at around -200dB - these numbers are derived from simulation - and heard the effect when the noise floor modulation mechanism was switched on and off.
The problem with noise floor modulation is measuring it, as ADC performance is far worse than Hugo's, and certainly worse than Dave's. With my old Audio Precision (AP) I used to use a fixed frequency passive notch filter to remove the fundamental, then fed the residual into the AP. From this one could determine noise floor modulation, but the AP was not good at resolving small noise floors. But around October 2014 AP launched the APX555, and this had a clever system to enable more accurate measurements of distortion and noise floor. What this instrument does is the employ two ADC's per channel, and an automatic notch filter, so one ADC uses notched out fundamental, and another ADC for the fundamental. The instrument then stitches the two plots together in the digital domain.
It also had a very high purity analogue oscillator - the system has residual THD at 2.5v of -150dB. Since I need a high purity analogue source to test the pro ADC project, and since Dave at that time exceeded the old AP measurement capacity, once AP launched the APX555 I went out and purchased one.
So we can now see the performance of Dave using the APX555:
Here we have a 1kHz signal at 2.5v RMS and distortion is below -150dB (blue trace). The no signal is the red trace, and you can see that the noise floor is identical in both cases, close to -180dB. 0dB is at 6v RMS, the maximum output of Dave. Clearly, there is no measurable noise floor modulation at all, even with the noise floor close to -180dB.
Rob