If you are measuring a floating signal, then you should connect bias resistors to prevent saturation. Check out the board's user manual for a wiring diagram that shows where to connect the bias resistors. http://www.ni.com/manuals.
Also check out this KnowledgeBase:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/7b99932232e8c10b86256297005ea780?OpenDocument