EMC Question of the Week: August 7, 2017
On boards with closely spaced (<0.3 mm) power and ground planes,
- decoupling capacitors should share vias with the power and ground pins of active devices
- decoupling capacitors should be located between the power and ground pins of active devices
- the location of decoupling capacitors is not critical
- decoupling capacitors are not required
Answer
The best answer is "c". Generally, on boards with closely spaced power and ground planes, all of the decoupling capacitors are "global" (i.e. they respond to changes in the power plane voltage and do not serve a specific device). In this case, it is good to distribute them around the board with a sufficient number in the general vicinity of the devices that draw the most current from the planes. The exact location of each capacitor is not critical and there is no advantage to sharing vias with the active devices. In fact, attempting to share vias generally increases the connection inductance of the capacitor and/or the active device with undesirable consequences.
Of course, it is dangerous to rely on simple rules for decoupling without understanding the physics behind them. There are certain situations where the location of a decoupling capacitor could be important on a board with closely spaced planes. Also, when the power and ground planes are not closely spaced, it is best to rely on "local" decoupling capacitors. In this case the location is extremely important. For more information check out the Circuit Board Decoupling Information on this website.
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