isolating digital and analog supplies?

[tempus]

Hey all;

I've built a PIC controlled switching matrix, which essentially switches my various stompboxes in and out of the signal chain. It uses FETs to accomplish the switching, and is very quiet, except for a little thump sound when I hit a switch. Without going into a lot of detail, I suspect the noise is not coming from the switching of the stompboxes, but from the actual PIC switching itself. I'm powering this thing with a 12v wall wart, which feeds a 9v regulator (for the stompboxes). The output of the 9v regulator goes to a 5v regulator (for the PIC). Is there some way to isolate the 2 supplies, in the hopes that my switching configuration will finally be dead silent?

Thanks

[defaced]

5v DC-DC converter.  Mouser. 

[cloudscapes]

You can probably power both the 5v and 9v regulators from 12v. Check the datasheet.

Are you star-grounding?

Where are your decoupling caps, and how many?

[tempus]

Thanks for the replies.

The DC-DC converter solution is pretty expensive, but if it works may be worth checking out.

You can probably power both the 5v and 9v regulators from 12v. Check the datasheet.

Are you star-grounding?

Where are your decoupling caps, and how many?

I am powering both regulators from the 12V wall wart. Do you mean try powering them in parallel?

Yes I am star grounding.

I've got 2 decoupling caps at the PICs supply pins.

Thanks

[amptramp]

You could power the regulators from two separate wall warts, which would enable you to have single-point grounding between digital and analog supplies.  Added bonus: you can power the 5 volt regulator from a wall wart which is lower than 12 volts, such as 9 volts and reduce dissipation on the 5 volt regulator.

[cloudscapes]

I am powering both regulators from the 12V wall wart. Do you mean try powering them in parallel?

Yes I am star grounding.

I've got 2 decoupling caps at the PICs supply pins.

Thanks

I assume you're using 0.1uf (or thereabouts) caps right next to the PIC's pins.

But do you have filtering caps elsewhere? I like to put 10uf-47uf caps right on the main power rails, then again on the output of any 9v or 5v regulator. An extra 0.1uf there doesn't hurt either.

[tempus]

You could power the regulators from two separate wall warts, which would enable you to have single-point grounding between digital and analog supplies.  Added bonus: you can power the 5 volt regulator from a wall wart which is lower than 12 volts, such as 9 volts and reduce dissipation on the 5 volt regulator.

I was kind of hoping there was some way other than using 2 separate transformers/wall warts to save space, especially since the one I'm using is 500ma which should be enough for my PIC and pedals. Or does a uC use a lot more current (esp when switching) than I surmise?

I assume you're using 0.1uf (or thereabouts) caps right next to the PIC's pins.

But do you have filtering caps elsewhere? I like to put 10uf-47uf caps right on the main power rails, then again on the output of any 9v or 5v regulator. An extra 0.1uf there doesn't hurt either.

Yes, I have filtering caps at the regulators in and out as well.

[Gurner]

I do a fair amount of switching with PICs...they're very quiet in my opinion.

getting to the bottom of these problems can be very circular, so if you're just using a pic pin to switch a gate voltage to a fet, then to eliminate the pic as the troublesome component, why not - as a test - temporarily manually switch a control voltage to your switching fet's gate (i.e. remove the pic IO pin from the gate and use a jumper wire with the right voltage on it  ....you could even use a separate power source for the jumper wire ....proving they share the gnd of course))....at least then it'll start narrowing down where the noise is actually being generated