EMC Question of the Week: September 16, 2024
The yellow components on the circuit board in the figure are in series with some of the low-speed I/O traces. These components are probably
- resistors
- capacitors
- inductors
- TVS diodes
Answer
The best answer is “a.” Series resistors are often placed in connections between a microcontroller and an external cable. Low-speed signal terminations are often capacitances, so the resistor filters high-frequency noise. The resistor also helps to protect the processor from voltage transients coming in on the cable.
If the signal termination is capacitive, series capacitors would not be helpful. And even with a resistive termination, series capacitors would block the low-frequency signal, while passing the high-frequency noise.
Series inductors would be inappropriate because they could resonate with capacitive inputs or with the cable capacitance.
Finally, these components are not TVS diodes, because a TVS diode would always connect to ground at the connector.
Of course, inputs that require better filtering may have both a series resistor and a shunt capacitor. Inputs that require better transient protection may have both a series resistor and a shunt TVS diode.
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