Re: vias between parallel planes and traces?

Michael Noone wrote:
John Popelish <jpopelish@xxxxxxxx> wrote:>

Before I try to answer your ground plane questions, I would like you to describe why you have a ground layer. What (and be as specific as possible) is it supposed to do for your circuit? Does it do different
things for different parts of the circuit? You may find that you are answering your own questions.

OK well the top ground plane is for a very specific purpose: it is directly below a compass module that is very sensitive to interference and whatnot. Unfortunately I had to run some signal lines directly beneath it so I moved them to the bottom layer and then put the ground plane above them on the top layer to shield the module from the signal lines (they're SPI lines that will be running at about 5Mhz as I recall). The data*** for the compass module very specifically stated not to run any traces directly beneath the module, so this was the best I could do.

Excellent information. I suspect a compass module is not only sensitive to electric fields but also to magnetic fields. This means that you want as little current passing through that section (you can section the ground plane and tie the pieces together at points of your choosing that guide currents) of the ground plane as possible, so that section should be isolated from supply current fed through the ground plane. And any traces that pass under it, if they carry any significant current, should be in the form of transmission lines with the return current passing along side, going the other way, to cancel the external magnetic field from that current. In this case, such cancellation should be used for all large currents in the general vicinity of the compass.

The other ground plane is there, to me, for three main reasons:

-simplify layout by making a ground signal always readily available
-give lots of copper to ground signal to decrease resistance
-provide a sort of shield to the circuit board. (this is something I've always been told)

Well, the shield can be useful to keep outside electric fields away from your traces and components, or from communicating between them, but also to keep the electric fields of your traces and components from getting out into the surroundings.

The ground plane can also be a voltage reference for either logic or analog circuits, or both. It is especially important to keep supply current out of the parts that are analog references.

And, of course, the plane is a path for supply current, if that doesn't conflict with the above use. Noisy and high current supply grounds may be better run as traces, to keep the I*R drop caused by these currents out of a clean ground plane.

So with all that said I'm still not entirely sure about how to answer my questions... Any suggestions?

Just to think about all the possible uses of the ground plane and how one use may be in conflict with another. Every pour should have specific purpose(s) in mind, not done just because you can.
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