Q: Are there special grounding considerations for the AD7656/AD7656-1?
A: At least one ground plane should be used. It can be common or split between the digital and analog sections. In the case of the split plane, join the digital and analog ground in only one place, preferably underneath the AD7656/AD7656-1, or at least as close as possible to the part. Multiple connections are acceptable as long as the multiple connection mimics a solid GND plane.