EMC Question of the Week: April 5, 2021

An example of bad printed circuit board decoupling

Two 0603 MLCC capacitors are used to decouple a single power input of an IC. One of them has a nominal value of 0.10 μF. For the best high-frequency performance, the other one should have a value of 

  1. 0.10 μF
  2. 10 μF
  3. 0.001 μF
  4. 10 pF


The best answer is “a.” Two capacitors connected to the same power input through a trace effectively form a pi-filter due to the inductance of the connection between them. If the two capacitors have values that differ by an order of magnitude or more, the larger-valued capacitor will go self-resonant at a lower frequency than the smaller-valued capacitor. At frequencies between the two self-resonant frequencies, one capacitor looks like an inductor and the other like a capacitor. They will resonate at a frequency between the two self-resonant frequencies and may introduce ringing on the power bus. 

For this reason, decoupling capacitors (or filter capacitors) connected to the same trace in parallel should generally have the same nominal value unless measures are taken to ensure that any ringing is damped (e.g. using at least one capacitor with a sufficiently high ESR).

Note that the decoupling shown in the figure (the two capacitors in the upper right) is particularly poor. Not only do the capacitors have significantly different values, but they share the same connection to the ground plane.

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