If you are looking at the switched waveforms (high frequency) directly your sampling rate will need to be much higher than the switching frequency to meet the Nyquist criterion. If the sampling does not satisfy Nyquist, no amount of filtering will give you valid results. In principle every sampled data system needs a physical filter ahead of the sampling point for accurate results. In some cases the source of the signal is bandwidth limited sufficiently to avoid the need for an explicit filter.
What is the amplifier switching frequency? How many harmonics do you need to get good waveform reproduction? What is the sampling rate? What is the amplifier bandwidth?
You should use the special low pass filter to remove the switching noise (about 100kHz) in order to carry out the correct and repeative measurement. However, this switching noise will not affect the quality of listening as the speajer is the natural low pass filter. Mots of the engineers use the Audio Precision AUX-0025 for this application.
Digital filtering can be used provided that the digitized signal is alias free (anti-aliasing filters are not effective if the signal has already been aliased). NI has developed
some VIs below implement and test the 'standard' low-pass and high-pass filters specified in AES 17. The tests pass for sample rates between 44.1k Hz and 204.8k Hz.
The switching frequency is above 500kHz and the energy level is much higher than the over-sampling converter as AES17. So you better to use the external low pass filter such as AUX-0025 or built as some IC manufacturer suggested. The switching frequncy make the input section of the instruments work in abnormal situation and create wrogn result.....
