EMC Question of the Week: September 30, 2024
An active device is mounted on an 8-layer board with a GND plane on Layer 4 and a VCC plane on Layer 5. Which of the four decoupling capacitors depicted in the figure provides the best high-frequency decoupling?
- (a.)
- (b.)
- (c.)
- (d.)
Answer
The best answer is “b.” When power is distributed on closely spaced planes beneath the active component, the decoupling capacitors with the lowest-inductance connection to the planes are best able to provide charge to the active device quickly. Option (b.) minimizes the connection inductance by connecting directly to the planes through vias that are near each other. Other capacitors on the board connected in the same manner as (b.) will also contribute significantly to the high-frequency decoupling.
Options (a.), (c.) and (d.) all have traces between a capacitor pad and at least one of the via connections. In fact, option (c.) [which was recommended in a recently published application note] is the worst option. On boards with a solid GND plane, the GND pad of a decoupling capacitor should always connect directly to the plane.
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