has anyone tried to measure actual analog bandwidth of AVR ADC? I'm using it for sampling of kind of funky waveform with fundamental frequency ~42kHz so I'm using equivalent time sampling 2.5MSPS, S&H circuit is evidently fast enough for this. This way I get 59 samples per period which seems to be enough for true-rms calculation. Question is what is the limitation? Datasheet of ATmega48 says 38.5kHz. That seems to me a bit pessimistic because I'm getting almost identical waveform as on the ADC input measured by DSO including much higher harmonics.
With known transfer function I can theoretically apply correction in frequency domain (2.5kB of code memory is plenty to implement some FFT/IFFT for the filter).