Electronic – Trace Inductance when routing power nets for PCB

Combining digital and analog grounds is quite a contentious issue, and is might well fire up a debate/argument. A lot of it depends on whether your background is analog, digital, RF etc. Here is some comments based on my experience and knowledge, which is likely to differ from other peoples (I am mostly digital/mixed signal)

It really depends on what kind of frequencies you are running at (digital I/O and analog signals). Any work on combining/separate grounds will be a work in compromise - the higher the frequencies you are operating at, the less you can tolerate inductance in your ground return paths, and the more relevant ringing will be (a PCB that oscillates at 5GHz is irrelevant if it measures signals at 100Khz). Your main aim by separating grounds is to keep noisy return current loops away from sensitive ones. You can do these one of several ways:

Star Ground

A fairly common, but quite drastic approach is to keep all digital/analog grounds separate for as long as possible and connect them together at one point only. On your example PCB, you would track in digital ground separately and join them at the power feed most likely (power connector or regulator). The problem with this is when your digital needs to interact with your analog, the return path for that current is half across the board and back again. If it's noisy, you undo a lot of the work in separating loops and you make a loop area to broadcast EMI across the board. You also add inductance to the ground return path which can cause board ringing.

Fencing

A more cautious and balanced approach to the first one, you have a solid ground plane, but try to fence in noisy return paths with cut outs (make U shapes with no copper) to coax (but not force) return currents to take a specific path (away from sensitive ground loops). You are still increasing ground path inductance, but much less than with a star ground.

Solid Plane

You accept that any sacrifice of the ground plane adds inductance, which is unacceptable. One solid ground plane serves all ground connections, with minimal inductance. If you're doing anything RF, this is pretty much the route you have to take. Physical separation by distance is the only thing you can use to reduce noise coupling.

A word about filtering

Sometimes people like to put a ferrite bead in connect to different ground planes together. Unless you're designing DC circuits, this is rarely effective - you're more likely to add massive inductance and a DC offset to your ground plane, and probably ringing.

A/D Bridges

Sometimes, you have nice circuits where analog and digital is separated very easily except at an A/D or D/A. In this case, you can have two planes with a line of separation that runs underneath the A/D IC. This is an ideal case, where you have good separation and no return currents crossing the ground planes (except inside the IC where it is very controlled).

NOTE: This post could do with some pictures, I'll have a look around and add them a bit later.

I will start with a shift of terminology here -- instead of "grounds", we have "returns" -- namely a power return and a signal return. The power return is the easy case -- it should be a star topology with the star point at the power supply output, tying to the signal returns of each board at a single point in turn, and also to the chassis ground at a single point.

Now, we have the RF and processing boards to deal with. Each board has a signal return, which is handled through a ground grid at a minimum, if not a full ground plane. This return is contiguous, even on the RF board, and also is interconnected between the two boards alongside the digital signals passing between them.

The digital board can be laid out freely, provided the loops are kept small. This is not true for the RF board, though, which needs to have a layout that is strictly partitioned between analog nets and digital nets -- there should be an "analog area" on the board that has only analog signals, and a "digital area" on the board that has only digital signals. The return currents will then proceed to stay with the signals they are paired with, thus keeping them separated without the need to do any ground plane splitting.

Finally, connector shields need to be returned to the chassis ground by the shortest path possible, and there also needs to be a "bridge" between the chassis (I/O) ground and the signal return located with the I/O signals going to the outside world. (ESD/EMI protection devices, as well, return to the chassis aka I/O ground.)

For more information on this topic, I recommend reading Henry Ott's fabulous book, Electromagnetic Compatibility Engineering, and in particular, Chapters 16 and 17 on circuit board layout.