two circuits on one pcb using eagle

[Beo]

I'm putting together some multi-effect 2 in 1 and 3 in 1 pedals. To optimize space and to use pc-mount pots, I'm designing layouts in Eagle. Each circuit will still have its own input and output, each going to a dedicated 3PDT bypass stompswitch. All circuits are negative ground, and can use a common 9v or higher dc source.

Are there any rules or suggestions on what to-do and not to-do with the circuit design? Should I use two separate ground planes or can I use just one? Should I be careful to keep input/output traces from circuit 1 away from input/output traces of circuit 2? Any other proximity concerns between two circuits?

As a working example, my first design is for a Muff - Rat combo.

Thanks,
Travis

[Hides-His-Eyes]

As long as you follow the full procedure for seperating your 'dirty' and 'audio' grounds it shouldn't be a big deal, but I might keep them seperate anyway depending on the circuit.

[Beo]

What exactly constitutes a non-audio "dirty" ground? Power filter caps and diodes I presume. Should the LFO side of an LDR arrangement have a separate ground? What else is "dirty"?

[Taylor]

LFOs and digital signals (including clocks in BBD circuits, any kind of logic) are "dirty". On my tap tempo tremolo, for example, the PIC and LED side of the optocoupler, the audio section, and the power section each have their own ground pours, which only connect at the point that the ground wire enters the board. Then of course a lot of folks use spaghetti wiring, so all that work was negated   but at least I tried.

For 2 fuzzes, not sure it really matters too much. You could potentially get squealing, but avoiding this requires deep knowledge of where all the currents are flowing at various points of a circuit, so it's not easy to come up with rules for that. I would go ahead and use 2 separate ground planes for the 2 circuits, and connect them inside the PCB right at the ground wire.

[Beo]

Thanks Taylor. Good info.

[caress]

taylor, how do you get multiple ground pours to connect at a single point in eagle?  i've tried a few different things but could never figure it out...

[Taylor]

Oh, I don't use Eagle, so I have no idea, sorry. I didn't notice before that this was an Eagle-specific question.

But my general approach is to make the separate pours, then run a trace (fat) from a pad connected to one pour, to a pad connected to the other. Seems like no spcial software trickery should be necessary for that.

[Hides-His-Eyes]

taylor, how do you get multiple ground pours to connect at a single point in eagle?  i've tried a few different things but could never figure it out...

You give each ground its own pad, and after you run the DRC, you connect them with a single trace at the minimum width your fab can do. (6mm for dorkbot, 10mm for futurlec, wouldn't go below 20mm if you're self-etching). This will then fail a DRC if you recheck obviously!

[caress]

But my general approach is to make the separate pours, then run a trace (fat) from a pad connected to one pour, to a pad connected to the other. Seems like no spcial software trickery should be necessary for that.

you'd be surprised by eagle and the trickery involved to get it to do the things you want sometimes...   

@hides-his-eyes, thanks for that!  yeah DRC check was always messing with me and why i got confused...  i was also under the impression that this connecting trace should be as small as possible?  though i'm sure it doesn't really matter that much for our purposes.

[Hides-His-Eyes]

I've always read to make it small, but all my PCBs come from fabs, not from home etching.

[Beo]

What about the ground for a voltage divider that feeds a tranny or FET? Should that be separated from Audio Ground?

[Hides-His-Eyes]

What about the ground for a voltage divider that feeds a tranny or FET? Should that be separated from Audio Ground?

No, not unless it's doing something digital. couple it with a cap though.