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.

	The short answer is to keep an eye on where you want differing
	types of current to flow.  The "signal" ground is really the
	return currents for your signals.  The "shield" gnd probably
	doesn't want any of this current flow and therefore tying back
	to the common source is proper.

	You will need to be sure that all signals have an appropriate
	return path, other than the shield; this is very much integrated
	with your circuitry and your overall stackup.

	Good luck,
-- 
 
      Jeff Seeger                             Applied CAD Knowledge Inc
      Chief Technical Officer                      Tyngsboro, MA  01879
      [log in to unmask]                               508 649 9800

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