How to properly ground an EMI shield ?

[kender]

Colleagues,

I’m designing a signal conditioning board for industrial environment. The board will reside inside a plastic enclosure on a DIN rail. I need EMI shielding. The enclosure will not provide shielding, because it’ plastic. I’m going to make a small sheet metal “can” that will solder onto the board. The question is: should I ground it to signal ground or earth ground (i.e. the DIN rail)?

Please let me know if I forgot to mention anything important.

Cheers,
Nick

 

[kevpat]

grounding emi shields

I don't really think you should use either. If the board that you are designing has I/O connectors, and I am assuming it does, then you should have a seperate I/O ground and ground the connector shells to this then ground connectors to the can. If you are going to solder the can to the board you can extend the I/O ground all the way around the peripheral of the PCB then have several exposed ground pads for the can to solder to. The spacing between the ground pads depends on the highest radiating frequency on your board, no greater than 1/20th the wavelength of the highest frequency.

Hope This Helps.

 

[kender]

Re: EMI shield grounding

[...] If the board that you are designing has I/O connectors, and I am assuming it does [...]

The board has one block of screw terminals for 4-20mA interface: signal, return and shield. Probably, I should not assume that the return terminal on my side will be at the earth ground potential.

 

[ricksidd]

Re: EMI shield grounding

I think EMI shielding requires earth ground.......
-Sidd

 

[kender]

Re: EMI shield grounding

… PCB then have several exposed ground pads for the can [shield] to solder to.

All EMI shield cans that I saw had throughole pads. I’ve never seen a purely SMT EMI shield. I wonder why?

 

[vaibhav.k]

EMI shield grounding

EMI shield can be connected to earth .If u dont have earth u have to connect it ground through capacitor

 

[kevpat]

Re: EMI shield grounding

...All EMI shield cans that I saw had throughole pads. I’ve never seen a purely SMT EMI shield. I wonder why?....

I don't think there is any advantage to either having SMT or thru-hole cans. I just said ground pads because the cans that I have used all have tabs to solder to the PCB or attached to the PCB at the screw holes that attach the PCB to the chassis.

By the way after your additional info I would ground the can to your DIN rail.