EMC Question of the Week: June 4, 2018

Which statement is generally true about the "equivalent series inductance" values published in the datasheets of SMT ceramic capacitors?

1. They are more important than the nominal capacitance values.
2. They characterize the high-frequency behavior of the capacitor.
3. both a. and b.
4. They are not meaningful, and should generally be ignored.

Answer

The best answer is "d." It is true that the high-frequency behavior of SMT ceramic capacitors is largely determined by their equivalent series inductance (ESL); however this inductance strongly depends on how the capacitor is connected to the rest of the circuit. The ESL numbers published in capacitor datasheets  are generally much lower than the connection inductances that can be achieved in practice. Also, since there is no standard method for determining these numbers, they cannot be used to compare capacitors made by different manufacturers.

Inductance is a property of current loops, and therefore it is not possible to assign an inductance to a capacitor that represents a small fraction of the current loop in any given application. ESL numbers are generally obtained by making a measurement of the inductance of a capacitor in a given circuit and comparing that value to the inductance of the same circuit with the capacitor shorted. This method is seriously flawed, since the inductance of a current loop is not the sum of its parts. As a result, ESL numbers published in datasheets are strongly dependent on the method and apparatus used to obtain them. Furthermore, there is virtually no correlation between the ESL numbers published in a datasheet and the connection inductance that determines the high frequency impedance of the capacitors in real applications.

Nevertheless, the connection inductance remains the single most important parameter affecting the high-frequency impedance of SMT ceramic capacitors. Capacitors that employ techniques such as reverse aspect ratios or interdigitated mounting pads can exhibit significantly less connection inductance than standard capacitors. Applications where the equivalent series inductance of capacitors is critical should be evaluated individually, without regard to any ESL values published in a datasheet. 

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