altiumground
I have a PCB design that requires two ground pours on the bottom layer, one is AGND and the other is DGND. Most of the board is AGND ('L' shape) and the remaining should be DGND.
I have tried to pour polygon with AGND over the entire board, draw another polygon for cutting a square out of the AGND pour and tried drawing yet another polygon for DGND. I think this is a naive approach and it did not work.
If anyone can enlighten me it would be super!
One could write a book on this topic (and few did). I can only give you a few hints as this topic is too big.
Rule 1: Current loops must be minimized. (But even this is a book for itself)
Rule 2: Minimize common signal paths (e.g. apply star grounding).
Rule 3: Don’t care about top planes, this is not really important. But if you use top planes, then contact them with a hell lot of visa to the bottom plane.
I will give you some additional hints to these rules:
1) Identify each current source in your design and provide a minimized loop for the return current. Even IC supply pins are high frequency current sources, that’s why you need a bypass cap as near as possible to short this current source to the other IC supply pin (Caps are a short for high frequency).
2) If you have identified a current loop of a signal trace, try to keep this current loop away from ALL other current loops, because they will harm your signal. Properly done, this will result in a star grounding concept, where each of the current loops is isolated and your GND current return paths are connected in a single point (not necesseraly a GND plane!).
3) If you follow these rules, you might not need GND plane separation. I think GND plane separation is generally a bad thing, because it may contradict rule 2.
I did this: Draw a circle on the top layer & select it.
Tools->Convert->Create Polygon from Selected Primitives
Double click on polygon and select Fill Mode -> Solid Copper
Tools->Polygon Pours-> Repour All
Best Answer
Option 1: Just bite the bullet and make the AGND pour in an L shape.
Option 2: In the polygon manager, adjust the pour order so the DGND pour happens before the AGND pour.
I'm not clear what you mean about "draw another polygon for cutting a square out of the AGND". If DGND is poured first, and your design rules are set correctly, Altium will respect the different nets clearance rule and produce a clearance between the DGND polygon and the AGND being poured around it.
However I actually prefer the first option because it is less likely to cause trouble if you edit the design later, shelve some of the pours, need to add a new polygon that overlaps the first two, etc.