PCB grounding

Hi to all,

I am designing a PCB with a digital circuit based on a STM32F103 MCU with
USB,LAN and RS232 ports. The power comes from an external wall type
12V/2.5A switching type regulator. There are 3 grounds in my PCB : a
digital ground (GND), a connector frame ground (FGND) which is connected to
GND through a 3.9nF/1KV capacitor and the input ground (the common which
comes from the external power supply) which is connected to GND through a
differential line filter. 
The pcb according to local regulations has to be enclosed into a metal box
and has to be compatible with all CE regulations accordind EMI and ESDs.
In your opinion where the metal frame should be connected ? to GND, to
connector frame ground or at the common power supply ground ? 

Thanks in advance,

Angelo
	   
					
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Posted through http://www.EmbeddedRelated.com

On Nov 15, 4:31=A0am, "abrous" <abrous3d@n_o_s_p_a_m.hotmail.com> wrote:
> Hi to all,
>
> I am designing a PCB with a digital circuit based on a STM32F103 MCU with
> USB,LAN and RS232 ports. The power comes from an external wall type
> 12V/2.5A switching type regulator. There are 3 grounds in my PCB : a
> digital ground (GND), a connector frame ground (FGND) which is connected =
to
> GND through a 3.9nF/1KV capacitor and the input ground (the common which
> comes from the external power supply) which is connected to GND through a
> differential line filter.
> The pcb according to local regulations has to be enclosed into a metal bo=
x
> and has to be compatible with all CE regulations according EMI and ESDs.
> In your opinion where the metal frame should be connected ? to GND, to
> connector frame ground or at the common power supply ground ?
>
> Thanks in advance,
>
> Angelo
>
> --------------------------------------- =A0 =A0 =A0 =A0
> Posted throughhttp://www.EmbeddedRelated.com

Your "local regulations" should specify this.  Tests can be done to
make sure that you got it right.

Most of the time the metal box is connected to frame ground.  It makes
sense to have the device's "frame" connected there.

Most of the time the USB host will have frame ground shorted to
digital ground.  That introduces additional problems.

Have fun,
RK

RK