Layout tip: bipolar power supply planes?

[mth5044]

I'm attempting make a bi-layer PCB for the FSH-1 which uses +/-9v, +/-8v, and 0v.

Schematic: http://www.tonepad.com/getFile.asp?id=138

I like to make one side of the PCB a ground pour, then route +9v and most traces on the other side. In this case, there are multiple reference voltages. My thought was to make 'vref' (gnd) still a pour on the back, but also route the power through the pour to clear things up on the signal side. Any pros or cons to this?

Thanks

[R.G.]

Ground conductors are best left wires at DC, planes at RF, and the shift between wires, to signal/ground trace pairs, and finally planes depends on the highest frequency being carried. Ground planes are of marginal use at DC and audio as far as power distribution is concerned.

However "ground" is many things. I split up the concept of ground into reference ground, shield ground, and sewer ground, the power supply return for current that's already been "used". They're different. What makes them different is what currents flow through them. You do NOT want sewer ground currents flowing through your reference ground conductors. That makes for oscillation, crosstalk, and noise. Reference grounds with shield currents flowing through them lead to RF pickup. And one ground, with all of that mess in it could do anything. It could work right - but that's only one of the possible outcomes.

Ground planes are useful for three kinds of things. One is where you just can't get enough copper in the ground return any other way. Another is where you use circuits which are sensitive enough that you need shield ground everywhere to cut down on parasitic capacitances. The other is where the frequencies are so high that electromagnetic field effects forces ground currents to flow selectively right under the signal trace carrying the signal, a kind of automagic transmission line pair that goes anywhere the signal trace goes.

For pedal PCB layouts, ground planes are solidly optional. They don't necessarily hurt and I admit to running ground planes so that I can be lazy and not go route ground traces; I let the program fill in everywhere and put vias wherever I need ground. But it's a convenience, not a circuit necessity.

So, down to cases. Unless you have some HIGH gains and HIGH impedance inputs to shield, putting ground in a plane is not much help. Audio is too low a frequency for the electromagnetic field stuff to be a problem. Pedals are (usually) too low a power level to just need more copper. If you have digital logic on the board, planes for the digital sections may save your... er, may save you  from unbearable digital grunge. If not, a plane can let you save some routing time.

Splitting a plane up in to pieces and sections with traces in the plane layer is problematic. To the extent that a plane is needed, running breaks across it to hold isolated traces can defeat much of what you want a plane for, and force return currents and signal currents to mix in places where they're forced to go by the splits in the plane. If you really, really need a plane, don't do this. However, I suspect you don't really need one, so do whatever, and if there's a problem, do a new board.

Ah. That. Planes are a bi**h to modify if you have to cut traces and otherwise hack them in the first iteration board. Likewise, you have to use thermal breaks in pads to the plane or they will be really hard to solder to. And even with thermal breaks around pads in the plane, they can be really hard to unsolder if you have a problem.

In many ways, planes are like whether to buy a 50 cal Desert Eagle. If you need that much handgun, you already know it. 

[amptramp]

Ground planes also act as thermal conductors and can help get a lot of heat off the board, which can be a good idea for a power supply.  Copper has 220 times the heat conductivity of epoxy-glass board material.  This is a slightly different consideration from circuits that need more copper for electrical conductivity reasons.  In many instances, you can prove that convection and radiation are unnecessary if you have enough heat conduction - we used to do that for all out spacecraft work where there was no atmosphere (so no convection) and the only radiation path was often to surfaces that were at the same or higher temperature.  Where possible, we used all four forms of heat transfer (conduction, convection, radiation, telepathy) but conduction was the most reliable (and we never got telepathy to work all that effectively).

[liquids]

I started doing PCBs, on 1-sided FR-4 copper clad...

My main reason for running ground everywhere I had empty board space is that....I could retain more copper, and keep the copper out of the etchant.

I ended up linking some 'blocks' in various areas either via traces, or jumpering, so that it's as one big connected ground, and connect all off-board wiring to one centralized area/row/region on the board.   I figured it also wasn't bad to have ground running physically close to all other voltages, signals, via parallel traces etc.

But I avoided making a ground that is as one circularly connected loop, if that makes sense  - think of circle around the outside of the board - it seems to me that's a big no-no.

So in short, this probably does nothing for me except...well...maybe save some etchant from being saturated with copper?  

[mth5044]

Thanks guys!