Hi Watersky,
Welcome to the NI forums! This is behavior that usually occurs when the input is unwired, as mentioned in this KnowledgeBase article. Which pins are you using to perform your measurement, and what is your input configuration (eg. Differential, RSE, NRSE)? Are you measuring a floating or grounded source? Table 1 in the Field Wiring and Noise Considerations Devzone article is a handy reference to see how to hook up your signal in different configurations.
-John
John Passiak