I am planning to have independent AGND and DGND ground planes in my system, and plan to short the two GNDs together at or under the ADC.
If I use the AD7915, would the bypass capacitors for VDD (2.5V power rail for ADC core) be connected to AGND or DGND?
Initially I would have thought it would attach to DGND, since it drives the ADC SAR structures and likely has some high-frequency digital content.
But I noticed in one of the packages that the VDD pin is placed right next to the analog IN+/- pins, and I know the pin layouts on these ADCs is derived to allow a AGND/DGND split across the part body center. This seems to imply that it would be better to bypass VDD to AGND…
It's not discussed in the layout section of the datasheet on how the GND is implemented but it was outlined in the user guide UG340 http://www.analog.com/media/en/technical-documentation/user-guides/UG-340.pdf .Single ground plane is used to minimize the effect of high frequency noise interference where VDD bypass caps is connected to AGND.