"Understanding that the primary real estate consumed on the SAR converter chip is analog, it makes sense to connect the power and ground pins on the same planes. While implementing the layout you should connect AGND and DGND to the analog ground plane. With all of the converters, the power supply stratgey should be to connect all grounds, positive supply and negative supply pins to the analog planes."
"As with SAR A/D converter, the Sigma-Delta converters can have multiple analog and digital ground and power pins. Once again, the common tendency of a digital or analog design engineer is to try separating these pins into separate planes. Unfortunately, this is a misguided tendency, particularly if you intend to solve critical noise problems with the 16-bit and 24-bit accuracy devices."
"In both cases, with high-resolutions DAR A/D converters and Sigma-delta converters you need to connect them directly to the lower-noise, analog ground and power planes"