The AD7609 datasheet says: "When the serial interface is selected, the DB[15:9] and DB[6:0] pins should be tied to AGND."
But, I'm a bit nervous about shorting so many outputs, in case the part accidentally starts in parallel mode (say, bad solder joint on the PAR/SER pin could be read as random logic state - incorrect operation is bad too, but not as bad as damage from shorted outputs).
In an earlier prototype board design with the AD7608, I accidentally left these DB pins unconnected, and it seems to work.
Only DB[15:14] are documented as type DO/DI but not specified what they do as inputs. The other pins are specified as type DO only.
So, I'm considering to use two pull-down resistors for DB[15:14] and leave DB[13:9] and DB[6:0] floating. Or is this wrong?