@IVX a few mention Cosmos ADC has a lot of DC, which makes it not great for recording (like for null difference analysis)
Are you working on a newer Cosmos ADC? Maybe which has DC filtering?
Not sure if you are using it, but DeltaWave has the option to remove DC in computing the null. It's also not going to affect the null RMS value by much if not removed, since Cosmos ADC is already around -90dBFS DC, IIRC. By the way, Multitone Analyzer now has a DC filter option, as well.