I am approaching layout stage for a board using an AD9858. My understanding, for all mixed signal ICs, was to tie both AGND and DGND together as close to the package as possible and partition logic and analog components so that noisy digital currents would not interfere with the device operation. The connection of the AGND and DGND pins would be as close to the device as possible (usually underneath the device).
The AD9858 datasheet gives no layout instruction. However on checking the AD9859 datasheet I was confused to find the following layout advice.
"Likewise, the ground connections (AGND,DGND) should be kept sperate as far back to the source as possible(i.e. seperate the ground planes on a localized boards, even if the grounds connect to a common point in the system)."
This seems contradictory to the information in many App notes for example AN-280 and in other DDS data sheets. Does anyone have experience of using the AD9858 or AD9859? Are these devices different in some respect?
I'd appreciate any advice anyone has on this matter.