How to Select and Use Ferrite Cores
Ferrite Placement Depending on Current Mode
Depending on the mode of the current being targeted, different attachment methods of ferrites are recommended.
For Common Mode Only
For example, in the diagram on the left, an efficient method of targeting common mode is to attach a ferrite to the signal and ground wires together. The currents within these wires are travelling in the same direction, therefore they do not cancel out. The magnetic flux is generating a field within the ferrite, which helps to filter common mode noise.
Normal (differential) current travels in opposite directions; it is transmitted through the signal and returns along the ground. In this case, the current of the two cables are opposing, meaning they cancel out.
For Normal (Differential) and Common Mode
magnetic saturation and/or wave shape distortion could occur when suppressing normal mode noise
In the right diagram, the attachment of ferrites targets both common mode and normal (differential) mode. In this scenario, signal and ground lines are individually filtered with separate ferrites. In regards to normal (differential) mode, this attachment of ferrites is effective because the currents do not cancel out. In the case of common mode, both signal and ground lines are still being filtered.
A concern when attaching ferrites in this manner is magnetic saturation. If a large current flows through the power line in normal mode, the magnetic flux causes saturation of the ferrite. Magnetic saturation can cause the ferrites to overheat. This makes the ferrite less effective at filtering noise. To counteract this, applying more ferrites is recommended to alleviate strain.