[SI-LIST] Re: Ferrite bead question
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Zhangkun" <zhang_kun@xxxxxxxxxx>, joel@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 10 Apr 2006 09:29:54 -0700
Joel,
I agree with all the respondents who have cautioned you on the use of
ferrite beads in the power leads of devices. In all of the cases, I have
investigated, ferrite beads have been used to treat a symptom rather than
the problem. The problem being that the power delivery system had too much
noinse or ripple on it.
Lately, I have seen many cases where placing a ferrite bead in the power
lead of a device, very often the power lead of very high speed serdes, has
made the part perform poorly or below spec.
Yes, thousands of applications notes insist that ferrite beads be placed in
the power leads and the applications engineers will demad you use them or
they won't guarantee the circuit will work properly. The other side of
that same coin is that when you ask if they will guarantee that the circuit
will work properly if you follow the applications note exactly, the answer
is still no! The reason is that there has been no analysis to prove that
the use of a ferrite bead is a good idea.
I have measured wavforms of a 3.12 GB/S serial link where the eye just
barely makes the minimum signal amplitude with the ferrite bead in place
and has large amounts of margin when the ferrite bead is shorted out.
This, even with the manufacturer's recommended capacitors attached. This
data was taken from a demo board supplied by the manufacturer who was not
aware of the degradation caused by the ferrite bead.
The right solution is to design a power delivery system with power supply
noise within the limits of the circuits being supplied and this is not very
difficult to do.
I have done the SI rule generation for three terabit routers with
thousands of 3.125 GB/S and 4.8 GB/S serial links in them and used no
ferrite beads in them any where and they worked to specification the first
time wit hproper margins. So maybe people who want you to use ferrite
beads should be challenged with why they want you to add these parts.
My fellow SI engineers call ferrite beads "voodoo" beads and "get lucky"
beads for good reason.
This is not likely to make the ferrite bead salesmen happy, I'm sure.
> [Original Message]
> From: Zhangkun <zhang_kun@xxxxxxxxxx>
> To: <joel@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 4/10/2006 1:14:21 PM
> Subject: [SI-LIST] Re: Ferrite bead question
>
> Dear Joel
>
> As to my experience, it is not enough to select bead based on the P/N.
The decoupling circuit should be considered very carefully.
>
> In several cases, we use beads and the noise become larger. The following
element should be considered:
> 1.The resonance between bead and capacitor.
> 2.The frequency span of the noise source.
>
> A simulation is suggested. It is very simple.
>
> Hope this is helpful
>
> Best Regards
>
> Zhangkun
> 2006.4.10
> ----- Original Message -----
> From: "Joel Brown" <joel@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Saturday, April 08, 2006 2:33 AM
> Subject: [SI-LIST] Ferrite bead question
>
>
> > I often see recommendations in application notes for ferrite beads on
DC
> > power lines. Sometimes a particular P/N is specified and sometimes just
> > an impedance is given.
> > Wouldn't I want to choose a ferrite bead that has the highest impedance
> > possible at the frequencies of interest, and that the current rating
and
> > DC resistance are appropriate? Would the frequencies of interest be the
> > fundamental switching frequency of the part and possibly the 3rd and
5th
> > harmonics?
> > Thanks - Joel
> >
> >
> >
> > -- Binary/unsupported file stripped by Ecartis --
> > -- Type: text/x-vcard
> > -- File: joel.vcf
> >
> >
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