EMC Question of the Week: February 8, 2021

microcontroller on a circuit board

In order to control the transition times of CMOS digital logic, it's generally best to use a 

  1. series resistor
  2. series ferrite
  3. series inductor
  4. shunt capacitor


The best answer is “a.” The transition times of CMOS digital logic are generally determined by the source resistance and the load capacitance. By default, many CMOS devices will transition in less than 1 nanosecond even when bit widths are hundreds of nanoseconds or longer. Adding a series resistor allows the circuit designer to control the transition time, which can significantly reduce unwanted crosstalk and improve the emissions and immunity of digital signals.

Series ferrites are usually a poor choice due to their higher cost, limited bandwidth and potentially non-linear behavior. Series inductors are also a poor choice because they can resonate with capacitive CMOS loads. A shunt capacitor slows the transition time of the voltage waveform, but does not reduce the high-frequency current in the circuit.

Another effective way of controlling the transition times in CMOS circuits is to use logic devices that internally control their own slew rates. These devices have the advantage of working with matched terminations as well as unmatched CMOS loads.

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