Elroy A. Tesch, Jr. wrote:
> We design high speed line scan cameras and use internal and
> external ground planes (external for shielding) on our PCB's.
> My question is this-
> Should the internal and external planes be tied throughout the board (at
> vias, etc.) or should the external planes be tied back only at a single
> soure (power connector) and leave the internal plane(s) for all component
> connections?  I am wondering if there are advantages of one over the other.

I apoligize if this post repeats itself. Something to do with my server.

If multiple "signal" ground planes are used, then
sprinkle connections between them throughout the board.
This increases the conductivity of the ground plane,
thus improving your ground.

As far as using ground planes externally for "shielding",
depending upon alot of factors (frequency and edge rates
to name only two) you can inadvertently construct a
fairly good 'two dimensional' transmission line with your
signals sandwiched between the two planes.

I assume you're intent is what's referred to as a "passive"
ground plane for sheilding.  Therefore, sprinkling
ground interconnections amongst all the ground planes
to include your top and bottom passive ground shield planes
would be defeating this purpose.

Personally, I would:

1. Start with as many ground connections among ALL the ground planes
   to CHASSIS around the outside perimeter of the board as possible.

2. Leave the center of your top and bottom shield planes free of
   connections to the other signal ground planes.

3. Be careful how components are connected/isolated from the top layer.

4. 'Possibly' put a chassis connection (ground screw) as close to the
   center of the board as possible connecting all planes to chassis.

This specific technique does not guarentee good technique.
You will have to analyze your specific circuit to guard against
ground loops back through the ground planes, common mode noise, etc...

Have you noticed a significant difference between
this shieldingtechnique and others?

Without this specific technique, -15dB under Class A
limit has been achieved with video transport boards.

*******************************************************
Doug McKean
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The comments and opinions stated herein are mine alone,
and do not reflect those of my employer.
-------------------------------------------------------
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*******************************************************
Doug McKean
[log in to unmask]
-------------------------------------------------------
The comments and opinions stated herein are mine alone,
and do not reflect those of my employer.
-------------------------------------------------------
*******************************************************

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