[SI-LIST] Re: EMI Filters on I/O Lines & Impedance Matching
- From: Todd Hubing <HUBING@xxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 3 Jul 2014 14:27:37 +0000
Perhaps the simplest way to view this is to remember that the metal labeled
digital “ground” is the signal current return path. Above a MHz, signal
currents are easily controlled by proper routing of the return adjacent to the
signal traces. With good routing, it is not necessary to block high-frequency
currents from flowing to the chassis with a ferrite. Low frequency (e.g. kHz)
currents are not so easily controlled and it is sometimes necessary to isolate
low-frequency current returns from the chassis ground. However ferrites do not
provide good isolation at low frequencies, so there is rarely (if ever) a good
reason to have a ferrite between chassis ground and the digital current return.
Todd
From: Hithesh [mailto:hitheshn@xxxxxxxxx]
Sent: Thursday, July 03, 2014 12:37 AM
To: Todd Hubing
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [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
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