Hi Dan,
Are you using DAQmx tools (
look link)? I am not sure how to do it internally but you can try one of the subset of names taken from DAQmx C reference help installed with the driver:
Internal Channels for M Series
_ao0_vs_aognd
_ao1_vs_aognd
_calref_vs_aignd
_aignd_vs_aisense
Have you tried to use LabVIEW with MathScript to run the code (read more)?
Good luck,
Gerardo O.
RF SW Engineering R&D
National Instruments