Are you seeing this offset when acquiring data from an analog input channel with your PXI-6281, or when generating data with an analog output channel? I can't tell from your initial post. Assuming you are performing analog input, devchander may be right about the measurement mode being the cause of the problem. If the sine wave source you are reading is referenced to a different ground than the DAQ board, you will want to use a Differential measurement mode to avoid a ground loop. Take a look at the Measurement System Types and Signal Sources section in the NI-DAQmx Help (Start > All Programs > National Instruments > NI-DAQ) for more information about the different measurement modes.
Travis G. Applications Engineering National Instruments www.ni.com/support