I recently had a discussion with a guy who believes that by putting a big slot in his PCB ground plane he will protect his analog domain from the noise originating in his digital domain. I don't agree with this view (hence the discussion), for various reasons, but to keep this post on point I want to focus on the guy's analogy---or way of thinking about the ground plane. His reasoning went something like this: When a digital IC switches from one state to another, the device will produce a large di/dt that depletes the immediate area (and decoupling cap) around the IC for charges. This, in turn, disturbs neighboring charges ("pulling" them in / "pushing" them out, gah) and creates waves from the point in the ground plane and outward just like a stone dropped in a pond. Even with a decoupling cap, the current needed at the instance the IC changes states will be almost infinite so it will cause these "waves" in the ground plane during the first few nano-moments, etc.. I'm not even sure if that's an analogy or what it is. I tend to think in terms of currents and where/how they return to their source, so this picture of a "pond of charges" or what the hell it is, bothers me. I tried to play the game of analogies and suggested it was more like removing a drop from the Atlantic, and that the pond analogy was more accurate if you added Hurricane Katrina to the picture. The discussion didn't go anywhere from there... (I'm just an engineer, not a particle physicist) So, any thoughts on IC injecting and removing charges from the ground plane and creating ripples across the plane?