Hi there,
Good question, I knew that AC follows the path of least impedance, but
had know idea at what frequency this effect would become apparent. Bonus
question, same deal, as long as it's got the path of lowest impedance.
But I have an extension to the bonus question. Assuming you have
PWR-GND plane pairs in a multilayer stackup, is one inherently noisier
than the other? ie is it preferable to route next to the GND plane....or
does that depend? I guess a factor would be the coupling between the
planes, but who is to say the GND will be less noisy than a PWR rail?
Also when we have 3 or 4 plane pairs, but with each PWR plane being a
different voltage, with different noise levels, how does this effect our
"clean" GND rail? In the end we just do what the engineers ask and put
sensitive signals next to GND planes, it sounds obvious and safe to
route next to GND planes, but without further investigation I feel there
is a lot of "ASS U ME ing" going on.
Regards,
Steve
Steve Kingdon
PCB Design Engineer
Allied Telesyn Research
>>> [log in to unmask] 03/12/04 07:54:54 >>>
Return Current Path vs. Frequency
-=-=-=-
Easy question:
Imagine a "U"-shaped trace above a solid ground plane,
with a connection at each end.
Inject a signal through the "U", and vary the frequency.
At what frequency will you see the return current
follow the shape of the U rather than the shortest path?
-=-=-=-
Bonus question:
What if I replace the ground plane with a power plane?
(I say it makes no difference. )
thanks,
Jack (the "arguing with a new engineer" guy)
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