Someone here may know exactly how to do this (I don't). If you read the AD7708 Datasheet, it tells you the protocol for digital communication with the chip; registers, timing, etc. This, along with a review of the serial functions in LabView, and a good bit of messing around will get you there.
However, if you call Labview , they've probably got techs who can help you right away.
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"Reason is not automatic. Those who deny it cannot be conquered by it. Do not count on them. Leave them alone." - Ayn Rand
This is not a very specific question, what hardware do you have? What interface and how far did you get talking to the chip? What version of LabView? Here is a NI SPI digital reference library if the SPI interface is giving you trouble.