Re: Multilayer PCB - Shielding - Which supply on each layer?

On Thu, 30 Jul 2009 01:44:38 -0700, Son of a Sea Cook
<NotaBrewster@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 29 Jul 2009 23:24:34 -0700, Robert Baer <robertbaer@xxxxxxxxxxxx>
wrote:

John Larkin wrote:
On Wed, 29 Jul 2009 10:51:30 +0200, Bill <a@xxx> wrote:

Hi,

I need to design a 4-layer PCB for an analog circuitry that measures
EEG (10 uV to 1000 uV peak to peak, 0.5 Hz to 50 Hz).

My supplies are:
+5 V (for analog)
+3.3 V (for digital)
0 V (for analog and digital)
-5 V (for analog)

I know about (and have experience with) star topologies, to minimize
contamination from one section to others. That doesn't worry me.

I'm concerned about SHIELDING. I don't have much experience working
with uV signals, so I need to pay attention to what supplies/signals I
place on each layer.

I'm also not concerned about the +3.3 V supply, since it doesn't need
to power any IC close to the analog input.

I'm concerned mainly about +5 V, 0 V and -5 V. Where would you place
them, in the 4-layer PCB?

I was thinking of something like this:



I'd suggest...

Layer1 : signals + soldering pads for SMD parts.
Layer2 : ground plane
Layer3 : power pours
Layer4 : signals

which is much easier to kluge, should you need to.

John

Wouldn't having shielding planes surrounding sensitive signals be better?

Why would one ever want sensitive signals taking long paths? Nowadays
converting a signal to digital is cheap, even when 8 or 16 channels are
involved.

No one wants sensitive signals taking long paths, but sometimes you
need to amplify before A-to-D conversion is possible. And sometimes,
you need to amplify so much, that you need to break that gain into
several stages, not to have output-to-input feedback. That's how you
end up needing shielding.

Hell, a simple (or complex) data logger would be an excellent PCB layout
to examine.

It depends on the voltage levels and frequency bands it deals with. My
application is not RF, but it deals with uV, so I'd rather learn from
RF PCBs than from PCBs of data loggers.
.

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