[SI-LIST] Re: EMI Filters on I/O Lines & Impedance Matching
- From: "Orin Laney" <olaney@xxxxxxxxx>
- To: <hitheshn@xxxxxxxxx>, <HUBING@xxxxxxxxxxx>
- Date: Wed, 2 Jul 2014 23:10:58 -0700
He DID explain. A properly designed ground is a low impedance common
reference. Why on earth (pun intended) would you want to decouple from your
reference? If you really want to "attenuate the noise", why not sever your
ground connection altogether? For the same reason you don't do that, you
don't rationally put beads in series with ground connections either. The
low impedance of ground is precisely the place where noise currents can flow
with least adverse effect, which results in the cleanest possible signals
referenced to it. Isn't it the cleanliness of the signal that counts, not
ground per se? Decoupling from ground gives only a slightly cleaner ground,
but a far noisier signal. That ranks right up there with protecting a fuse
by ensuring that critical components blow first.
Orin
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Hithesh
Sent: Wednesday, July 02, 2014 9:37 PM
To: HUBING@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: EMI Filters on I/O Lines & Impedance Matching
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
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum is accessible at:
> http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
