[SI-LIST] Re: Ferrite Beads on Differential Signal Lines?

  • From: Joel Brown <joel@xxxxxxxxxx>
  • To: craigleefrancis@xxxxxxxxx
  • Date: Fri, 12 Sep 2014 14:25:46 -0700

It depends on what your load impedance is.
If it high compared to the ferrite bead (i.e. 100 ohms) then the ferrite
won't do anything.
Putting a capacitor after the ferrite forms a filter that is somewhat R
LC and RC but may distort the signal.
Even then it only helps differential mode noise, most of the time common
mode noise is the problem.

Joel


On Fri, Sep 12, 2014 at 2:11 PM, Craig Francis <craigleefrancis@xxxxxxxxx>
wrote:

> SI-LIST,
> Thanks for the insight.
>
> My application is like Randy described: low data rates (think 1Mbps) driven
> by microprocessors and cables between PCBs within a chassis.
> Some people put ferrites on single-ended signals before the connector to
> reduce the high frequency content of the signal and prevent unnecessary
> high-frequency radiation.
>
> Murata sells ferrite arrays for this purpose:
> http://www.murata.com/en-us/products/emc/emifil/bl but I understand that
> they are in the business of selling ferrite beads.
>
> I was wondering why I never see the ferrites applied to differential pairs,
> only common-mode chokes. But it sounds like there are better matched
> filters for differential pairs to reduce common mode conversion.
>
> However, there appears to be a large disagreement on using ferrites on
> signal lines at all. Application notes and books offer contradictory
> information and now I am more confused than I started.
>
> Thanks,
> Craig Francis
>
> On Fri, Sep 12, 2014 at 2:04 PM, steve weir <weirsi@xxxxxxxxxx> wrote:
>
> > Some generalizations are more general than others.
> >
> > Steve
> > On 9/12/2014 8:47 AM, Orin Laney wrote:
> > > All generalizations are false.
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx [mailto:
> si-list-bounce@xxxxxxxxxxxxx]
> > On
> > > Behalf Of Lee
> > > Sent: Friday, September 12, 2014 8:15 AM
> > > To: clfranci@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: Ferrite Beads on Differential Signal Lines?
> > >
> > > It makes no sense to put ferrite beads in any signal line.  How did
> this
> > get
> > > started?
> > >
> > > -----Original Message-----
> > > From: Craig Francis
> > > Sent: Thursday, September 11, 2014 4:11 PM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Ferrite Beads on Differential Signal Lines?
> > >
> > > SI-List,
> > > I haven't seen any literature concerning using ferrite beads on each
> > signal
> > > line of a differential pair.
> > > Usually only a common-mode choke is used to reduce common-mode EMI.
> > >
> > > Can ferrite beads be used on a differential pair to increase the the
> rise
> > > time of the signal and reduce unnecessary high-frequency content that
> may
> > > radiate?
> > >
> > > The two ferrite beads would be sized to allow the bandwidth necessary
> for
> > > the differential signal's highest data rate.
> > >
> > >
> > > Thanks,
> > > Craig Francis
> > >
> > >
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