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## Here’s a simple first attempt at making the connections.

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**Here’s a partial schematic we’ll use to illustrate the**advantage of a ground plane. The idea is that an output pin on the microprocessor is driving an external gate. The logic gate, U2, will have some input capacitance, such that a transient return current must flow in the ground connection. (ignore the pin numbers - it’s just an illustration)**This layout provides a return-current path with a large loop**area. Inductance is a complicated function of geometry, but in general it increases with increasing loop area.**Our schematic effectively looks like this. A step change in**voltage on the processor pin will resulting in ringing of the parasitic LC circuit, possibly resulting in unintended transisitions of the inverter output.**Using a ground plane on the back side (solder side) of the**board reduces the loop area. But why? Where will the return current flow? .**If the return current takes the shortest path - the path of**least resistance – the loop area is roughly the same as before.**But the transient current will take the path of least**impedance, which is to say, least inductance, and therefore the one with the minimum loop area. The ground plane provides a return path directly underneath the trace, no matter how the trace is routed.**Now let’s suppose we need to route a signal which must**cross the previous signal. The crossover trace on the back side only removes a small amount of copper, but completely disrupts the return current path. If crossovers like this can’t be avoided, then it’s time to go to a 4-layer board where an entire inner layer can be committed to the ground plane.**4-layer PCBs from ExpressPCB**1.75 mils, 1.25oz Cu 12 mils, k=4.6 1.4 mils, 1oz Cu 28 mils, k=4.7 1.4 mils, 1oz Cu 12mils, k=4.6 1.75 mils, 1.25oz Cu (Via to inner layer)