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Issue #210 January 2008
Problem 2—Which connection(s) to a decoupling capacitor should be shortest? Why?
Answer 2: The most important connection is between the supply side of the capacitor and the supply pin of the chip.
The biggest transient currents in most integrated circuits are related to the switching of the output pins. When a pin switches from low to high, a pulse of current is drawn from the chip’s external supply to charge the output node, which includes the capacitance of the PCB trace and the input gate of the next chip. This current is primarily supplied by the decoupling capacitor.
Note that this current “returns” to the ground end of the capacitor NOT by way of the same chip’s ground pin, but rather by way of the PCB ground plane and the ground pins of the chip(s) that it is driving. For this reason, it is not particularly important that there be a short ground connection between the driving chip and its capacitor, but it is important that the capacitor be bonded as directly as possible to the PCB ground plane.
When an output switches from high to low, the capacitance of the output node must be discharged, which causes another pulse of current. However, this pulse does not involve the decoupling capacitor at all. Instead it flows from the capacitance of the output node, through the pulldown transistor and into the ground plane, where it is “returned” to the other end of the distributed node capacitance. This is why the ground pins of all chips need to be bonded as directly as possible to the PCB ground plane.
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