EMC Question of the Week: June 17, 2024
An IC drives a circuit board trace connected to pin 2 with a 20-MHz clock signal. The 100-MHz component of the current driven onto the trace is 1 mA. The 100-MHz current returning to the IC through one or more of the other pins is also 1 mA provided the
- load is linear
- load is nearby
- trace radiation is negligible
- none of the above
Answer
The best answer is “d.” The steady-state current flowing out of the IC will always equal the steady state current flowing back in. This is Kirchoff's Current Law. It holds whether we describe the current in the time domain or the frequency domain. It holds whether the load is nearby, far away, or missing altogether.
Of course, variations in the load can have a major impact on the amount of current drawn from the IC, including changes in the time-domain and frequency-domain representation of the signal current. Nevertheless, the steady-state current leaving the IC must always equal the current returning to it.
Note: Hypothetically, some of the current entering or leaving an IC could be a displacement current. For example, if the IC package was driven like a monopole antenna, conduction current drawn from the ground pins would be offset by the time-varying electric field lines (displacement current) emanating from the IC and terminating back on the circuit board ground plane. Nevertheless, this current would be negligible for any reasonably-sized IC package at 100 MHz, and the current exiting the IC on one pin would have to equal the sum of the currents returning to the IC on the other pins.
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