Looks like an innocent mistake, but this can cause a great deal of EMI troubles.
To put it simply, think about your chassis as acting like a Faraday cage for your product.
This should allow you to contain high levels of emissions from your product and provide a high level of protection from external emissions into your product.
The problem happens when we use pigtail connections.
First of all, we break this Faraday cage, so now the emissions can escape, and external emissions can access our device.
The second important part is that we are basically setting free common mode currents, which are much more effective in radiating emissions compared to differential mode currents.
This means that even a tiny amount of common mode current is going to create the same level of radiation as a larger differential mode current.
So when we use shielded cables, we have to remember that this will only be effective if we make a 360-degree low impedance connection between the shield of the cable and the chassis of the product.
In this way, the shield of the cable will act simply as an extension of the Faraday cage.
I hope this helps,
Dario
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