Hi Sandeep,
Thanks for the reply. Moving the data bus over as you suggested solved the problem. Thanks for pointing that out, I appreciate it.
In hindsight I probably should have caught on to this earlier based the behaviour, however I would like to point out that (unless I'm missing something) the documentation for the ads7881/7891EVM is patently incorrect. Table 5 on page 5 shows the pinout of the data bus, claiming that the *LSBs* are aligned between the two chips, not the MSBs.
If one trusts the EVM documentation above, the host processor will read the data incorrectly. It would be pretty obvious how things are actually connected from seeing the wiring on the board, except that the schematic wires the data bus using a bus line, so you don't see the physical connections to P3. You have to cross reference the ads7881 datasheet pinout and note the pin number mismatch in the evm schematic to catch this error. That's debugging time needlessly wasted.