This may be a niave question,
but if Kirchoff's law states that for every current there is a return
current, then it seems that everything that happens on a power plane would
have a corresponding effect on the ground plane.

So why is a ground plane any better reference plane than a power plane?
(they are both "power planes" in effect?)

Jack

        -----Original Message-----
        From:   Douglas McKean [SMTP:[log in to unmask]]

        Kirchoffs Laws dictate that for every current going
        out, there has to be a current coming back. Notice
        the "return current" arrow in the Gnd Plane just
        below the current coming out of IC#1.  The current
        has to seek a complete circuit.  Energy doesn't
        come out of nowhere.  Notice also, that the current
        going into IC#2 has also a return current in the
        Gnd Plane.  All's well and good since the return
        current is indeed in the ground plane where it
        should be.

        But wait.  Where is the return current for the
        signal in S2???   It's in the Pwr Plane!

        Bad.  Very bad.  How does that return current
        get back to the source of the signal in IC#1?
        Through the power plane.  Since the return
        current is now in the power plane, it can affect
        anything else in the power plane.



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