Ground Planes

[grapefruit]

Howdy,

I'm thinking about using ground planes on my PCB's as I get all my PCB's done professionally through work. I was just wondering, when using a ground plane on a double layer PCB, can there be a problem with capacitance between tracks and the ground plane in high impedance areas like the input preamp? I was thinking it may be best not to use the ground plane in this part of the circuit.

Thanks,
Stew.

[Processaurus]

Hi, I would say go with the ground plane for your manufactured circuit board, it would be hard to imagine how it could be a bad idea.  The capacitance of a ground plane .050" or 1.5mm to an inch long trace on the other side would be pretty insignificant to audio frequencies, think of your instrument cables, they have their shield about that far away from the signal wire in the middle for many feet.  I see ground planes on many modern effects, as well as other analog stuff, like soundcards (alright, those are part analog).  They can make laying out a board easier, and have a very low resistance from each point on it.  It may not make a day or night kind of difference in your circuit's s/n ratio though, since you're probably putting it in a metal enclosure anyway.  If theres digital stuff in the circuit, or opamps being used as an lfo generator, I understand you'd want to keep the digital ground seperate from your analog ground plane except at one point near where the power supply ground comes in, to keep little ticks and stuff out of your sound.

This is just stuff I've learned at my work, if there's any EE's with an educated perspective on this, I'd be interested to learn more about what ground planes are good (and not good) for.

[niftydog]

hell, use one if you want, it shouldn't be any different. (except for maybe the cost!)

I tend not to bother, but I do use the polygon tool in protel to fill up any spare space with a ground plane, even on a single sided board.

Basically, it just takes all the empty space on the board and fills it with copper that's connected to ground.

I don't think it has much of an effect at audio frequencys though.

[Paul Perry (Frostwave)]

Ground planes (or not) shouldn't make much (if any) difference at the frequencies stomp boxes are working at.
One disadvantage, if you are just whacking in big areas to save wasting etchant, is that soldering a component lead into a hole in a ground plane can be difficult because so much het is conducted away! and worse, if you have to do a repair. (in layout progs you can make a kind of target pattern around the hole to help).
A ground plane is not an automatic solution to all problems at high frequencies either: with a plane, you can't really tell exactly where currents are flowing, while with traces, you are certain, and might be able to take obvious steps.

[niftydog]

paul is right, as usuall. But there are tricks to getting around the heat sinking during soldering. Protel can automatically put in heat releif connections which allow the pad to be somewhat separated from the big chunks of copper in the ground plane. Instead of being connected 360 degrees around the pad, it's connected by four "tracks" going off at right angles to each other. Less heat conduction = easier to solder.

Also, ground planes don't have to be solid copper. Again, Protel can automatically create a grid pattern ground plane with a user variable gap and track width setting resulting in less copper covering the same area.

[Satch12879]

Ground loops (R.G. pointed this one out); watch your trace routing.