[SI-LIST] Re: Ferrite bead question
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: <leeritchey@xxxxxxxxxxxxx>, "Hassan O. Ali" <hassan@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 11 Apr 2006 13:34:38 -0700
Lee
I thought the purpose of all power supply filtering was to provide a low
pass filter between some noise source and the load of interest. The R in a
RC network, the L in a LC network or the Ferrite in a Ferrite based system
when working with the capacitance on the load side is a low pass filter.
The job of the power supply distribution network downstream of the R, L or
Ferrite is supposed to provide a low impedance source to the load. Thus
when the load looks back towards the filter or upstream it sees a low
impedance voltage source.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey
Sent: Tuesday, April 11, 2006 1:15 PM
To: Hassan O. Ali; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ferrite bead question
Hassan,
You have the right idea. The problem is, the loads downstream from the
ferrite bead need a low impedance source of current over a wider range of
frequencies and ferrite beads prevent this.
It is true that the usual reason given for using a ferrite bead is there is
noise on the power rails due to less than optimum design of the power
distribution system, which is thedesign engineer's job, not the IC
manufacturer. IC manufacturers need to have the power supply design skill
the same as the rest of us, else how can the demonstrate and test their own
parts?
> [Original Message]
> From: Hassan O. Ali <hassan@xxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 4/11/2006 11:08:30 AM
> Subject: [SI-LIST] Re: Ferrite bead question
>
>
> I always thought that ferrite beads were primarily used to surpress noise
FROM (rather
> than INTO) high-speed and clock-like devices. I thought ferrite-beads
were used as a
> measure against power-supply corruption and also as an EMI prevention. I
thought ferrite-
> beads were harmless at lower (switching) frequencies over which voltage
regulator
> modules are supposed to operate as long as their impedances were low
enough not to cause
> excessive voltage ripples. If the above is correct, is it then fair to
blame
> semiconductor folks for something that board designers have to make a
decision on?
> Please someone tell me if this understanding is wrong.
>
>
> Best regards.
>
> Hassan.
>
>
>
> On Apr 11, <art_porter@xxxxxxxxxxx> wrote:
> >
> > Back in the good old days when I was actually designing stuff instead
of =
> > trying to sell stuff, use of beads was considered a serious sign of =
> > weakness.=20
> > But like anything else, if it works, fine, but you need to understand =
> > why it works. If you understand why a bead works, often that will lead =
> > to an insight as to how to fix the problem without using a bead.=20
> >
> > Art Porter
> > Agilent =20
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] =
> > On Behalf Of Joel Brown
> > Sent: Tuesday, April 11, 2006 10:46 AM
> > To: leeritchey@xxxxxxxxxxxxx
> > Cc: Zhangkun; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Ferrite bead question
> >
> > Lee,
> > I can't say that I disagree with any of the points you made.
> > The problem is these days semiconductor companies have changed their=20
> > support model.
> > Most have done way with FAEs and factory applications engineers
unless=20
> > maybe if you are a large customer.
> > More often than not support is provided by email and the responses
are=20
> > slow in coming and usually don't adequately address the question.
> > Now when I encounter an application note with a ferrite bead=20
> > recommended, the chances of the IC manufacturer providing information=20
> > that meets all your criteria is slim and none. The suggestion that
the=20
> > IC manufacturers provided a current spectrum is a good one, but it
seems =
> >
> > they are more preoccupied with the inner workings of their chips than=20
> > supplying information about how their parts effect the power system.
My=20
> > inclination at this point is to include a footprint for a ferrite
where=20
> > I think it might help, then make measurements with various ferrite
beads =
> >
> > and a 0 ohm resistor and see which part results in lowest noise=20
> > measurements. The only problem is that a 0 ohm resistor would have
more=20
> > inductance than vias connecting the bypass caps directly to the power=20
> > plane. This has definitely been a learning experience, thanks to all=20
> > that posted responses.
> >
> > Joel
> >
> >
> > Lee Ritchey wrote:
> > > Joel,
> > >
> > >
> > > As others have observed, when the solution using a ferrite bead has =
> > been
> > > engineered to satisfy the following set of conditions it is warranted
=
> > and
> > > helpful.
> > >
> > > 1. There is a problem that needs fixing.
> > > 2. The proposed solution fixes the problem.
> > > 3. The proposed solution does not cause new problems.
> > > 4. The proposed solution is the best solution to the problem.
> > >
> > > When you apply this set of tests to the usual applications note
> > > recommendation, it often fails test number 1. The proponent has not
> > > acertained that there is a problem and what it actually looks like.
> > >
> > > I put this question to the students in my classes at UC Berkeley, =
> > nearly
> > > always practicing engineers. Can you show me an applications note =
> > with
> > > analysis supporting the use of ferrite beads? So far, there have
been =
> > no
> > > such examples. I keep hoping, but know that most applications notes =
> > are
> > > not prepared that way. Soon as I find one, I'll report it in this =
> > forum
> > > and show it in my classes- and give its author high praise for doing =
> > good
> > > engineering.
> > >
> > > More commonly, they are justified because "we have always done it =
> > this
> > > way." Further, there has usually not been any exhaustive testing to =
> > insure
> > > the solution is valid. That is in no way good engineering. =20
> > >
> > > I'll accept a recommendation that is accompanied by supporting =
> > engineering
> > > analysis. Less, seems to me to be guessing and that's a good way to =
> > get in
> > > trouble with modern e lectronic components.
> > >
> > >
> > >
> > >
> > > =20
> > >> [Original Message]
> > >> From: Joel Brown <joel@xxxxxxxxxx>
> > >> To: <leeritchey@xxxxxxxxxxxxx>
> > >> Cc: Zhangkun <zhang_kun@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> > >> Date: 4/10/2006 10:17:06 AM
> > >> Subject: Re: [SI-LIST] Re: Ferrite bead question
> > >>
> > >> Lee,
> > >>
> > >> After reading the responses and giving some more thought to this,
I=20
> > >> agree that in some situations the use of ferrite beads can be =
> > detrimental.
> > >> In the design I am currently working on, the ferrite beads are
used=20
> > >> mostly on low speed I/O signals and DC power to ICs that are not =
> > running=20
> > >> at very high speeds.
> > >> The highest speed part is a 100 MHz PCI Express clock buffer, and
the =
> >
> > >> manufacturer does recommend to use a ferrite bead.
> > >> While I agree your statements have some merit, I find it
problematic=20
> > >> that you seem to be saying ferrite beads only work by luck.
> > >> Certainly there must be situations in which they can predictably be =
> > used=20
> > >> to reduce noise.
> > >>
> > >> Regards - Joel
> > >>
> > >>
> > >>
> > >> Lee Ritchey wrote:
> > >> =20
> > >>> 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
> > >>> =20
> > > have
> > > =20
> > >>> investigated, ferrite beads have been used to treat a symptom rather
> > >>> =20
> > > than
> > > =20
> > >>> the problem. The problem being that the power delivery system had =
> > too
> > >>> =20
> > > much
> > > =20
> > >>> 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,
> > >>> =20
> > > has
> > > =20
> > >>> made the part perform poorly or below spec.
> > >>>
> > >>> Yes, thousands of applications notes insist that ferrite beads be
> > >>> =20
> > > placed in
> > > =20
> > >>> the power leads and the applications engineers will demad you use =
> > them
> > >>> =20
> > > or
> > > =20
> > >>> 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
> > >>> =20
> > > circuit
> > > =20
> > >>> will work properly if you follow the applications note exactly, the
> > >>> =20
> > > answer
> > > =20
> > >>> is still no! The reason is that there has been no analysis to prove
> > >>> =20
> > > that
> > > =20
> > >>> the use of a ferrite bead is a good idea.=20
> > >>>
> > >>> 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.=20
> > >>> This, even with the manufacturer's recommended capacitors
attached.=20
> > >>> =20
> > > This
> > > =20
> > >>> data was taken from a demo board supplied by the manufacturer who =
> > was
> > >>> =20
> > > not
> > > =20
> > >>> aware of the degradation caused by the ferrite bead.
> > >>>
> > >>> The right solution is to design a power delivery system with power
> > >>> =20
> > > supply
> > > =20
> > >>> noise within the limits of the circuits being supplied and this is =
> > not
> > >>> =20
> > > very
> > > =20
> > >>> 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
> > >>> =20
> > > first
> > > =20
> > >>> 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.
> > >>>
> > >>>
> > >>> =20
> > >>> =20
> > >>>> [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.
> > >>>> =20
> > >>>> =20
> > >>> The decoupling circuit should be considered very carefully.
> > >>> =20
> > >>> =20
> > >>>> In several cases, we use beads and the noise become larger. The
> > >>>> =20
> > > following
> > > =20
> > >>>> =20
> > >>>> =20
> > >>> element should be considered:
> > >>> =20
> > >>> =20
> > >>>> 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 -----=20
> > >>>> From: "Joel Brown" <joel@xxxxxxxxxx>
> > >>>> To: <si-list@xxxxxxxxxxxxx>
> > >>>> Sent: Saturday, April 08, 2006 2:33 AM
> > >>>> Subject: [SI-LIST] Ferrite bead question
> > >>>>
> > >>>>
> > >>>> =20
> > >>>> =20
> > >>>>> I often see recommendations in application notes for ferrite
beads =
> > on
> > >>>>> =20
> > >>>>> =20
> > >>> DC=20
> > >>> =20
> > >>> =20
> > >>>>> power lines. Sometimes a particular P/N is specified and sometimes
> > >>>>> =20
> > > just=20
> > > =20
> > >>>>> an impedance is given.
> > >>>>> Wouldn't I want to choose a ferrite bead that has the highest
> > >>>>> =20
> > > impedance=20
> > > =20
> > >>>>> possible at the frequencies of interest, and that the current =
> > rating
> > >>>>> =20
> > >>>>> =20
> > >>> and=20
> > >>> =20
> > >>> =20
> > >>>>> DC resistance are appropriate? Would the frequencies of interest =
> > be
> > >>>>> =20
> > > the=20
> > > =20
> > >>>>> fundamental switching frequency of the part and possibly the 3rd =
> > and
> > >>>>> =20
> > >>>>> =20
> > >>> 5th=20
> > >>> =20
> > >>> =20
> > >>>>> harmonics?
> > >>>>> Thanks - Joel
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>> -- Binary/unsupported file stripped by Ecartis --
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