Q: In most of DDSs’ datasheet, analog ground (AGND) and digital ground (DGND) pins are separate, but in most of the DDS evaluation boards AGND and DGND pins are not separated. Why?
A: To start with, the pin names AGND and DGND refer to what’s happening inside the component, and not the way in which they must be connected externally. How to deal with them externally is the main concern.
There are a lot of ways in dealing with systems having AGND and DGND. We can see in the books that having separate ground planes and connect them at a single point of lowest impedance (star ground) means better board performance.
However, in reality it is not always like that. Having separate grounds optimized means multiple spins on PCB, and hence multiple revision on the board.
The following are some reasons why most of our DDS evaluation boards do not have separate ground planes:
- we do not have the luxury to do multiple revisions on the boards for optimized separate ground planes.
- the DDS group has tested the boards with single ground plane and found the board performance is just as good.
- it can be very difficult to properly split up GND planes on a densely populated PCB.
At the customers’ side, they have the freedom to do the layout they think is much better than the evaluation board has. So long as the layout conforms to the specifications of the DDS, they are good to go.
In addition, here are helpful links that talks about AGND and DGND.