If your signal has frequency components that are higher than the Nyquest frequency (half your sampling rate), they will show up as alias frequencies that are lower. This can cause false results.
Your analog signal must be filtered such that only frequencyies suitable for your sampling rate are digitized. Look at some of the information here at the NI site, e.g.: http://zone.ni.com/devzone/conceptd.nsf/webmain/2EAC3B85027D8EE5862568C0005554AE?opendocument or http://sine.ni.com/apps/we/niepd_web_display.DISPLAY_EPD4?p_guid=B45EACE3DBE356A4E034080020E74861&p_node=DZ52008&p_submitted=N&p_rank=&p_answer=&p_source=External
