The AD7768 manual says that the start_n pin needs to be tied high if not used.
The AD7768EVAL schematic shows that start_n is not tied high by default.
The ad7768 HDL reference design brings this pin into the Zynq's GPIO interface
The ad7768 linux driver, and the DTS file, don't reference GPIO anywhere. Is the this reference design pulling this pin high somewhere else that I don't see?