Does it matter where ground traces are routed?

[mth5044]

When making a layout, does the ground trace need to always go around the outer edge? I mean, in general, are there traces best kept separate? I'd imagine so, as they are like bare wires, but I'm not sure if it matters in projects such as effects pedals. Perhaps only when gain stages are involved?

[esnabez]

I think that this article by R.G. Keen is a good overview and starting point:

http://www.premierguitar.com/Magazine/Issue/2008/Apr/The_Ins_and_Outs_of_Layout.aspx

[PRR]

> Perhaps only when gain stages are involved?

Many effects pedals have a LOT of gain. (Even if that gain is knocked-down before the output, the internal gain can be treacherous.)

> I'm not sure if it matters in projects such as effects pedals.

I think that is the case. Most pedal-size stuff, much guitar-level stuff, isn't likely to make trouble.

R.G. may have more to say, but as a general thing: you run the common ground around the ouside of the board because everything connects to it. Like the beltway loops around large cities: you "can" go straight through downtown, but it is often easier to go around.

You can make a case that a PCB ground should not go ALL the way around. A closed loop makes a better antenna. Like a beltway might attract hotrodders who drive around and around and around; breaking the loop foils that. There are audio systems where you must NOT loop all the way around. But again I think this will not be a problem in small effects.

In a few situations, a beltway ground (loop or near-loop) does a little to suck-out ambient buzz. The effect is small, if you have a real problem you need shielding. And for the specific case of "stomp" pedals, mechanical stompage kinda demands metal cases, which happen to be effective shields.

[R.G.]

Bottom line - yes it matters sometimes. The lower the frequencies and the lower the gain, the less it matters. The higher the frequencies and the higher the gain, the more it matters. The higher the impedance of the circuits, the more it matters.

For routing individual ground traces, you have to know and think about what currents flow in that ground conductor. This is because although copper is a very good conductor, it's not a perfect one. It's best to think of copper as just a low-valued resistor when you're thinking about grounding. For instance, a classical mistake in wiring power amplifiers is to return the speaker ground back to the amplifier board. Those amperes of current in the speakers flow across the board on their way back to the power supply from whence they came. In doing so, they force the "ground" on the board to wiggle up and down by a surprising amount. This board ground is the reference for the input circuits, and with their ground wiggling around, they take it in as signal. If at any frequency it reinforces the input, you can have an oscillator which promptly overheats the output devices. The first symptom in these cases is often death.

This is the whole point behind star grounding - the wire that takes MY current back to the power supply may not be the fattest, but only MY current is polluting it.

Good grounding is so mysterious because you have to know and think about currents, not voltages like we normally do.

[Paul Marossy]

For instance, a classical mistake in wiring power amplifiers is to return the speaker ground back to the amplifier board. Those amperes of current in the speakers flow across the board on their way back to the power supply from whence they came. In doing so, they force the "ground" on the board to wiggle up and down by a surprising amount. This board ground is the reference for the input circuits, and with their ground wiggling around, they take it in as signal.

That's a really good point. Speakers do move a lot of current, comparatively speaking.

[Sic]

...
This is the whole point behind star grounding - the wire that takes MY current back to the power supply may not be the fattest, but only MY current is polluting it.

Good grounding is so mysterious because you have to know and think about currents, not voltages like we normally do.

Great info R.G.

[Processaurus]

It's easy to get anal about grounding, once you start to understand the cases where routing is important, but with low current effects circuits, most of the time willy nilly grounding will work just fine (which is nice for people just getting into electronics).  Think about the common practice of using a stereo jack to connect the battery (-) to ground when a mono plug is inserted, that means that all the current from the circuit is going through the same wire that a hi impedance input (guitar) is referenced to.  That is a fairly horrific, worst case scenario from a grounding perspective, but most pedals work just fine, and with manageable noise.

From my pedal specific making/breadboarding experiences, conscientious ground routing has only been a night and day improvement when using logic (including opamps used as LFO's and comparators) chips, and charge pumps in the same circuit as hi impedance, hi gain analog parts.  The rest of the time it has worked fine to have a fat ground bus along the bottom or edge of the board, or a ground plane (there's some strip board I use with a bare ground plane on top, which is really convenient to solder right onto when you need a ground).

Definitely a good subject to be aware of, though, going from analog pedals on to bigger projects.

[cloudscapes]

I like to imagine my traces as water pipes. helps me decide where to branch ground traces (traces in general), to star ground or not, to keep noisy (high gain, high frequency) traces away from audio, etc

[Paul Marossy]

I like to imagine my traces as water pipes. helps me decide where to branch ground traces (traces in general), to star ground or not, to keep noisy (high gain, high frequency) traces away from audio, etc

Not a bad analogy. I used to design plumbing years ago, and so GPM flows for a certain pipe size is a very intuitive sort of thing for me. Not hard to apply that concept to wire sizing and such.

[mth5044]

Very well explanations, thanks a lot!