EMC Question of the Week: July 14, 2025
The sum of the currents flowing into a circuit node (e.g., a circuit board signal trace) is essentially zero as long as we can neglect the
- parasitic inductances
- parasitic capacitances
- parasitic resistances
- laws of physics
Answer
The best answer is “b.” All conductive objects with any physical size can hold a certain amount of charge when they have a voltage relative to other objects or infinity. The ratio of that charge to the voltage responsible for that charge is a mutual capacitance if the voltage is relative to another object. It is a self-capacitance if the voltage is relative to infinity.
At high frequencies, the parasitic self and mutual capacitances represent current paths that do not necessarily appear in the schematic. It's usually not practical (or necessary) to model these parasitic capacitances. However, EMC and signal integrity engineers need to be aware of them. They also need to be able to quantify these capacitances well enough to know when they might make a difference in an EMC test (or when they might affect the signal integrity).
Note: We can make virtually any model work if we neglect the laws of physics. So, while "d" may also be technically correct, it is not the "best" answer.
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