What's the frequency of the input? 800kS/s is aggregate, so you can only resolve frequencies of 400kHz/(number of channels your fpga calls) so really, for the 9221 you shouldn't try and capture a signal above 50kHz unless you aren't planning on using all of your channels. Also, based on your title I think you are already doing this, but you need to use FPGA mode, the scan interface can only do ~1kS/s.
Also, can you post an image of the waveform you are getting?
Jon
Message Edited by jaraalmonte on 02-22-2010 05:14 PM