FEDEVEL
Platform forum

Arda , 07-09-2024, 02:00 PM

Hello everyone,

I am looking for some guidance on how to properly handle chassis ground and signal ground connections in my designs. Specifically, I am dealing with chassis ground from Ethernet PHY, USB PHY, and similar components.

I have come across various suggestions, including:

1. Tying chassis ground directly to signal ground.
2. Connecting chassis ground to signal ground via a 0.01uF/2kV capacitor near the power supply
3. Short them together in the mounting holes
More method approaches can also be named.

I would appreciate any insights or recommendations on the following points:

• What are the pros and cons of each method?
• Are there any industry best practices or standards that should be followed?
• How do these methods impact EMI/EMC performance?
• Are there any specific scenarios where one method is preferred over the other?

Thank you in advance for your help!

QDrives , 07-09-2024, 09:18 PM

Most of my designs have a single ground (plane). This is the easiest.
Biggest downside to it, is when the power ground, shield and protected earth are all connected, some leakage current can flow. On ships this can be disastrous.
So there are reasons the do not have a direct connection between your main signal ground and chassis (shield).

For that you want a capacitor with enough capacitance to bridge the two for higher frequency noise.
2kV? Do you want that much voltage difference between them?
I stated "with enough capacitance" for this. Take an ESD event of 15kV. Human body model is 100pF. Simple capacitive divider: a 10nF this would then be 1.5kV. However, if you use a 100nF, you have 150V.
I would also add a high value resistor (like 1meg) in parallel so that it is not floating.

The connection point with the capacitors is there where the two meet. This could be mounting holes, but often near the connectors.

The impact on EMC/EMI is how you design it specifically. This is mostly the layout. Parasitic inductance is often much higher.