Hi Ivan,
Your plot is showing 192Ksps it looks like.
I took the QA403 on my desk and ran 3 sweeps, starting with 64K FFT for 48Ksps sample rate. And 96K I used a 128K FFT, and at 192K I used a 256K. For the sweeps, you can see the 48K has a bit of peak (0.01 dB at 20 kHz) and then drops off sharply. That is purely the digital filter inside the ADC.
For the 96 and 192K sample rates, you can see a bit of what looks like a first order filter rolloff before the sharp drop. That is an RC in the front-end of the analyzer. At 96K, you should be down about 0.1 dB at 37 kHz. And at 192k, the 0.1 dB point comes a bit sooner, but the 0.5 dB point comes a bit later.
The -3 dB point on my QA403 shows:
At 48K, ~22 kHz
At 96K, ~45 kHz
At 192K, ~88 kHz
That looks about the same as yours.
