The measurement is done by measure in DGS with 50 Ohm inserted into the channel.

The unit of measurement is counts, so we need to do calibration. The calbration factor is 0.61V for 1000 counts (see entry #1315). So we have

Noise_ADC = ~ 3e-6  [V/sqrt(Hz)] 

or alternatively

Noise_ADC  = 20*log(3e-6) = -110 dBVrms

This is almost 40dB higher than the noise of Network analizer we always use. So if we use DGS to record data, we will use at least 100 amplification given by pre-amplifier (SR560).