EMC Question of the Week: February 16, 2026

In a conducted emissions test setup, the DUT is 40 cm from a vertical metal plane. Changing the separation distance from 40 cm to 20 cm, we would expect the value of the parasitic capacitance to the plane (C_PAR) to increase by approximately  

1. 0 dB
2. 3 dB
3. 6 dB
4. -6 dB

Answer

The best answer is “a.” The parasitic capacitance between a typical device under test and a large nearby metal plane is virtually independent of the distance between them unless that distance is on the order of a few centimeters or less. And, even if the DUT is up against the plane, the capacitance is only weakly dependent on the separation distance.

C_PAR is an important parameter because it is responsible for the common-mode current on the cables attached to the DUT. Fortunately, this parameter (also sometimes called a self-capacitance or capacitance to earth) is primarily determined by the design of the DUT. It has relatively little to do with the test setup.

C_PAR can often be estimated with enough accuracy to ensure compliance with conducted or radiated emissions specifications. However, if necessary, it can be accurately determined using simple 3D numerical models. 

In a conducted emissions test, moving the DUT closer to the vertical plane typically has no impact on the common-mode cable current. In most cases, this plane is eliminated allowing the same setup to be used for both conducted and radiated emissions measurements.

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