[SI-LIST] Re: Split planes and ground return wires
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 20 Feb 2003 13:54:19 -0500
Marc,
I think it's only fair to present an opposing view, that everything
might work OK even with a lousy return path.
Your boards are not terribly large. If the signals crossing between
boards go right into a low capacitance receiver pin with very little
trace length, then the return currents might be small enough (in time)
to be almost ignored.
Signal current, and therefore return current, are among other things a
function of the impedance of the load being driven. Usually this is the
trace or wire impedance that comes after the point in question, for the
first nanosecond or so whenever the signal switches.
In the limit as the trace/load after the "split" goes to zero, the area
under the current curve also falls to zero. When the length of trace
being driven is much smaller than the signal's risetime, the fact that
it crosses a split becomes much less important.
Think of it like a bunch of short (~1 inch? long) wires that stick up
from one board and go nowhere. Unless the risetimes are really short,
say 100ps or less, we don't normally think about return currents in that
case, nor the fact that these wires don't have a controlled impedance.
On the other hand, long multiconductor cables with insufficient returns,
or traces that cross splits and go some distance, can cause major
headaches.
If your signals drive moderately long traces on the second board, or
touch a number of IC pins there, or have parallel terminations, then you
really do need to worry about keeping the return currents in non-signal
wires where they belong.
Regards,
Andy
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