[SI-LIST] Re: EMI Filters on I/O Lines & Impedance Matching
- From: Hithesh <hitheshn@xxxxxxxxx>
- To: HUBING@xxxxxxxxxxx
- Date: Thu, 3 Jul 2014 10:06:34 +0530
Todd,
Can you please explain why connecting Ferrite between digital and chassis
ground is a bad idea.
The usual assumption is that the Ferrite will attenuate the noise from
coupling to the digital ground.
-Hithesh
On Thu, Jul 3, 2014 at 7:43 AM, Todd Hubing <HUBING@xxxxxxxxxxx> wrote:
> Carson,
>
>
> You are correct and your link is an excellent example of why designers
> should never rely on application notes. The PI-filter you're referring to
> uses two different grounds that are essentially isolated at 100 MHz. The
> effect on the RGB signal will depend on the routing, but in a 150-ohm
> system, a 100R@100MHz ferrite bead will definitely affect the signal
> amplitude at 100 MHz, which may or may not be what was intended.
>
>
>
> Ferrite beads between a digital and a chassis ground are usually not a
> good idea. Routing all the RGB signal returns through the same 0-ohm
> resistor to the digital ground is nearly always a very bad idea. App note
> designs like this tend to generate EMI through poor signal routing while
> simultaneously attempting to reduce EMI by attenuating the signal.
>
>
>
> Todd
>
>
>
> Todd Hubing
>
> LearnEMC.com
>
>
>
> -----Original Message-----
>
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Carson Au
>
> Sent: Wednesday, July 02, 2014 8:04 PM
>
> To: si-list@xxxxxxxxxxxxx
>
> Subject: [SI-LIST] EMI Filters on I/O Lines & Impedance Matching
>
>
>
> Hi,
>
> It is often usual to see something like a PI-filter (C-L-C) filter on I/O
> signal lines in an attempt to filter out frequencies that will negatively
> affect EMI. For example, many VGA front-ends will include such filters on
> its signal lines (see page 72 of the COM Express Design Guide -
>
> http://www.picmg.org/pdf/picmg_comdg_100.pdf)
>
>
>
> What are the effects of such filters on the transmission line termination?
>
>
>
> Even for the simplified case of a 100R@100MHz ferrite bead in series with
> a
>
> 50 ohm source-terminated transmission line, even at such low frequencies
> of 100MHz, your series termination of the transmission line will be greatly
> affected! Would it not?
>
>
>
> Regards,
>
> Carson
>
>
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