EMC Question of the Week: July 3, 2017
Ultimately, the current that flows out of an integrated circuit pin and on to a circuit board trace has to find its way to,
- whatever is connected to the other end of the trace
- the signal ground
- the product's earth ground
- one or more of the integrated circuit's other pins
Answer
The correct answer is "d". Kirchhoff's current law requires that the current flowing out of an integrated circuit (IC) has to equal the current flowing back in to it. All of the current leaving an IC through a signal pin has to find its way back to the IC.
By design, we expect most of the current to reach whatever is at the other end of the trace, but it can't just stop there. It must return to the source. Also, due to trace's mutual capacitance with other conductors, the current reaching the load end of the trace may not equal the current at the source end.
The conductor that carries most of the current back to the source may or may not be labeled "ground," but currents will take the lowest impedance paths back to the source regardless of how these conductors are labeled.
None of the currents flowing out of an IC are trying to reach earth ground.
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