Low-pass filter on ADC?

Before using Hifiberry, I have successfully recorded ultrasonic sounds with microcontrollers and specialized microphones. Using the same mic hardware, I tried recording ultrasonic sounds with the DAC+ ADC and the DAC+ ADC PRO and am unable to see the high frequency signals in most spectrograms. After analysis, we get a very clean spectrogram below 20 kHz and then we quickly lose all signal above. We've tried using Linux's arecord, audacity, python, you-name-it to try and obtain this high frequency data, but have had little success (and yes, we are setting the sample rate to 192 kHz). Rarely, we do get a recording that seems to have no low-pass filter, but it is random and very infrequent. Is this expected behavior and is there a way to permanently disable this filter? Thanks!


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