Here is the link to our online accuracy calculator: http://www.ni.com/advisor/accuracy/
For more information about calculating the accuracy of a system check out this KB: http://digital.ni.com/public.nsf/websearch/8BA2242D4BCC41B286256D1D00815B90?OpenDocument
You can also look at particular user manuals from http://digital.ni.com/manuals.nsf/MainPage?ReadForm&node=132020_US
Regards, Jyoti F National Instruments Application Engineer