Impedance between Power Planes

A high speed signal propagated down a trace and returns to the nearest power plane. If the nearest power plane is not the ground plane it first hops into the nearest metallic plane and then goes into the ground plane next to it. If the metallic plane and the ground plane are nearby, they can be assumed to be electrically AC short . They are not DC short. No DC current will flow through the plane unless we put a jumper between them. But they are electrically ac short. They are electrically short because they act as a huge capacitor. And we can assume a capacitor to be electrically short for AC signals. In other words the ac impedance between the adjacent power planes is very small.

The characteristic impedance between two long wide planes can be estimated when the separation between the planes is very small as compared to the width of the planes. The characteristic impedance in is given by





We have assumed that h is very small as compared to w.

Example – Find the characteristic impedance between two long power planes, separated by 5 mils and having widths of 4 inches. Assume the Dielectric constant of the material between the planes to be 4.0

Solution – The characteristic impedance is given by




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