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Printed Circuit Board Power Bus Decoupling Questions

4. What is the best method for connecting decoupling capacitors if my PCB has power and ground planes, but they are not closely spaced?

If the power and ground planes of the board you are decoupling are more than ~0.5 mm apart, then the inductance of the loop area between the planes can no longer be neglected. This applies to most 4-layer boards built with a 1-mm core and multi-layer boards that have signal layers in between the power and ground layers. For these boards, it is important to locate the decoupling capacitors near the device being decoupled. If the decoupling capacitor is properly mounted very close to the correct power pin, it will effectively reduce the noise on the power bus up to frequencies well above 1 GHz.

More information can be found in the LearnEMC Tutorial: Power Bus Decoupling Guidelines for Printed Circuit Boards with Widely Spaced Power Distribution Planes.