EMC Question of the Week: December 23, 2024
Which parameter of the connection between a decoupling capacitor and the power planes is most important to consider when attempting to optimize the high-frequency performance of the decoupling capacitor?
- connection inductance
- connection capacitance
- connection resistance
- characteristic impedance
Answer
The best answer is “a.” At the highest frequencies where the capacitor is effective, its ability to supply the current necessary to stabilize the voltage between the power planes is limited by its connection inductance. In typical circuit board geometries, none of the other parameters listed are even remotely important.
The connection capacitance (capacitance between the vias) is too small compared to the capacitor and plane capacitances to have any effect. The via resistance is also small relative to the inductive reactance at the frequencies where the capacitor is designed to operate.
There is no benefit to controlling the characteristic impedance of the connection, because there is no source or load resistance to try to match. Also, at the frequencies where the decoupling capacitor is effective, the length of the vias is much too short to warrant transmission line modeling.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at