EMC Question of the Week: November 11, 2024

decoupling capacitor placed between IC power pins and the vias to the VDD and GND planes

When is the decoupling capacitor, trace, and via placement indicated in this figure appropriate?  

  1. almost always
  2. when the VDD and GND planes are closely spaced
  3. when the VDD and GND planes are widely spaced
  4. almost never

Answer

The best answer is “d.” If a printed circuit board has a GND plane, the GND pins of the active devices should generally connect directly to the plane with no intervening trace. High-speed active devices should not share their GND plane connections with any other component, including decoupling capacitors.

The GND pin of an active device is usually part of the return-current path for single-ended signals. It should have a low-inductance connection to the current return plane (on this board labeled GND plane). Attempts to force the current in this connection to pass under a decoupling capacitor increase the connection inductance and do nothing to improve the power bus decoupling.

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