Most of us get this wrong when we first hear about EMI.
They tell us that filters and shields are the key to solving EMI issues and that if we have EMI problems, these are the go-to solutions.
But what they don’t tell us is that the way you design your layout has a bigger impact on the EMI performance of your circuit than the filter does.
If you are failing Radiated Emissions (or Conducted Emissions) EMC tests, the first thing to look at in your design is most likely not the filter but the stackup.
A well-planned filter will do nothing if your stackup is not set up correctly, without consideration for return currents and electromagnetic fields.
The same goes for PCB layout. If your layout is not designed with common-mode and differential-mode currents in mind, the likelihood that the signal will couple to other parts of your circuit and bypass your filters is extremely high.
This is why having a filter when your board is not designed properly will not do much to solve the issue.
Parasitics play a much larger role than your schematic diagram and SPICE simulations suggest.
This is not to say that a good filter design should not be included from the start—of course, it should be.
The filter must be designed properly and be an integral part of the project.
But relying too much on your filter to compensate for poor PCB design is definitely not going to save your project from failing EMI.
Not to mention the fact that adding extra components and filters after the project has already gone live will only increase costs and create complexities during production.
EMI is not black magic.
Planning ahead is what will give you predictable success.
I hope this helps,
Dario
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