Common mode chokes are one of the most used components used to filter noise at PCBs. There are plenty of types and sizes, as well as manufacturers. The structure is always similar: two or more windings, so the signal or power lines go through the choke and go out from it with its common-mode noise attenuated. The schematic is as follows:

The schematic is quite straightforward to follow. However, problems come when designing a PCB and when choke facing different part numbers, manufacturers, and, footprints. The reasons are:

• The number order presented in the choke schematic is not always the same. Sometimes 1-2 is used for the signal #1 input-output path, as well as 3-4 for signal #2
• The pad numbering of the PCB footprint can change from one model to another, as well as from one manufacturer to another
• Choke footprints tend to be rectangular, but sometimes there are almost squared, leading to confusion about how to place correctly the choke

To avoid mistakes when placing a new choke in a PCB, it is recommendable to do the following:

• Number the schematic pins in a coherent way for all the chokes in the component libraries. For example, always number the pins clockwise.
• Verify each footprint against the schematic so:
  • The inputs and outputs are not short-circuited
  • The inputs are not swapped