EMC Question of the Week: May 2, 2022
A digital signal source drives a 20-cm microstrip trace terminated with a CMOS input at 10 Mbps. The sum of the trace capacitance and the input capacitance is 18 pF. Which component below would be a good choice to slow the transition time?
- series resistor
- series inductor
- shunt capacitor
- ferrite bead
Answer
The best answer is “a.” Series resistors are a reliable, inexpensive means for controlling the transition times of signals terminated in capacitive loads. They also have a relatively small footprint and work over a wide range of frequencies. In this example, a 250-Ω resistor could be added to achieve a transition time of approximately 10 nsec.
Series inductors should not be used to filter digital signals with capacitive terminations. A key reason for this is that LC circuits can resonate. Also, inductors are larger, more expensive and have a bigger footprint than resistors that would provide similar control over the transition time.
Shunt capacitors can slow the transition time of the voltage waveform, but they don't reduce the high-frequency current drawn from the source. Depending on their placement and connection inductance, they can also introduce unwanted parasitic oscillations.
Ferrite beads should never be used to filter digital signals with capacitive terminations. They are generally larger, more expensive, and work over a narrower frequency range than resistors. They also tend to be inductive at some frequencies raising the possibility of setting up an unwanted parasitic oscillation.
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