EMC Question of the Week: August 29, 2022

decoupling capacitor mounted to a circuit board

A power bus decoupling capacitor that meets the high-frequency current needs of one pin of one component is generally referred to as a _________ decoupling capacitor. 

  1. local
  2. high-frequency
  3. single-point
  4. X


The best answer is “a.” By definition, local decoupling capacitors are connected in a manner that favors one active component or one pin of an active component. Global decoupling capacitors provide current for many components sharing the same power bus.

On circuit boards with widely spaced power and ground planes (or no power plane at all), local decoupling capacitors are typically the primary source of high-frequency current for fast-switching active devices. The precise location and method of connecting local decoupling capacitors is usually very important. 

On boards with closely spaced power and ground planes, it is a standard practice (and usually a good idea) to connect all of the decoupling capacitors directly to the planes. All of the capacitors respond to voltage changes in the planes, and their relative location has almost no effect on the amount of charge they provide to the active component. In this case, all of the decoupling capacitors (high-frequency and low-frequency) are global. 

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