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yungman
yungman is offline
#1
Dec16-12, 03:32 PM
P: 3,842
I have been thinking about Ohm's Law in one particular situation. say I have a trace on top of a ground plane on a pcb. A signal is driven from point A and terminated by a load at point B. Looking at the return current path using Ohm's law V=IR. This implies most of the current should be running in a path of least resistance on the ground plane, which is the straight line from point B back to point A. This is because any path deviated from the straight line from B to A is a longer path, whereby, have higher resistance.

But we all know signal is travel as EM wave, the trace and the ground plane form a microstrip guided structure. If the signal from A is of high frequency ( over say 50KHz give and take), the ground return path pretty much follow the trace and most current concentrated right under the trace. So if I snake the trace around while going from point A to B, the current is not following the path of least resistance anymore. In fact the current follow the path of least impedance as the guided structure will have the lowest characteristic impedance when the return path is closest to the forward path.

How does Ohm's Law explain this. I can see Ohm's Law will work in "point form" at any point, but in macro situation, electromagnetic theory take over.

Please correct me if my assumption is wrong.
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