[SI-LIST] Re: Ferrite bead question
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>, Charles.Grasso@xxxxxxxxxxxx, CurtM@xxxxxxxxxxx, hassan@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 13 Apr 2006 08:52:25 -0700
Ken,
Another byproduct of designing a low noise power delivery system is EMI
control becomes much easier as much of the EMI has its origin in too much
noise on the power rails.
> [Original Message]
> From: Ken Cantrell <Ken.Cantrell@xxxxxxxxxxxxxxxx>
> To: <Charles.Grasso@xxxxxxxxxxxx>; <CurtM@xxxxxxxxxxx>;
<hassan@xxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 4/13/2006 12:13:12 AM
> Subject: [SI-LIST] Re: Ferrite bead question
>
> Chuck,
> The only time I use them is for analog supply isolation in analog/digital
> applications. I refer to the mfg. recommendations, but I always pick them
> myself. I've never had any issues(digital leakage or emissions), but I've
> also never had the chance to remove them and see what happens. If it
works
> and passes emissions/susceptibility, it's out the door. My guess is that
> Lee is right. Design the system as low noise and you probably won't need
a
> band-aid. However, used selectively, it's a low count and therefore low
> cost part, and many times there isn't enough real estate to isolate
> adequately by other means.
>
> Ken
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Grasso, Charles
> Sent: Wednesday, April 12, 2006 9:34 AM
> To: CurtM@xxxxxxxxxxx; hassan@xxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite bead question
>
>
> Curt - I too used to use the filtered Vcc pin trick. However
> when presented with a BGA with multiple voltages and pins I abandoned
> the idea as unwieldy. After all - there are many BGAs that drive a
> clock
> out and therefore logically should have filtered Vcc pins. Any thoughts
> on how to deal with this issue?
>
> Best Regards
> Charles Grasso
> Senior Compliance Engineer
> Echostar Communications Corp.
> Tel: 303-706-5467
> Fax: 303-799-6222
> Cell: 303-204-2974
> Pager/Short Message: 3032042974@xxxxxxxx
> Email: charles.grasso@xxxxxxxxxxxx;
> Email Alternate: chasgrasso@xxxxxxxx
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Curt McNamara
> Sent: Tuesday, April 11, 2006 6:45 PM
> To: hassan@xxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite bead question
>
> Lots of excellent points in this discussion! A few obsevations "from the
> trenches".
> =20
> Ferrites are used primarily to suppress EMI, mostly at frequencies above
> 100MHz. However, there is another kind of ferrite material effective at
> low switcher frequencies (50-150KHz). Check your datasheet!
> =20
> I used to think (as some have stated) that ferrites were the sign of a
> poor design. However, years of producing Class B telecom designs on a
> monthly basis showed me that in some cases they were effective for EMI
> control. So they became part of the toolbox, and we would often add pads
> allowing us to connect two nets together with a zero ohm jumper,
> inductor, resistor, or ferrite. Then when it was approaching midnight at
> the lab and you needed a result for the morning you had some options.
> BTW I still consider excessive gasketing inside PC cases as a sign of
> poor design practice ....
> =20
> For filtering a supply voltage (for example a PLL pin) I typically use
> an inductor/resistor and dedicated cap(s), not ferrites. This is also
> what I have seen in many app notes.=20
> =20
> Mostly I agree with Lee that people put a ferrite on as some guard
> against the dark forces :-) which they do not understand, and that in
> most cases they cause far more problems than they eliminate. I can't
> count the times I have recommended removing the ferrite and starting
> from that point when debugging some unusual problem ...
> =20
> Curt
> =20
> Curt McNamara. P.E.=20
> Senior Electrical Engineer=20
> Logic Product Development=20
> 411 Washington Ave. N Suite 101=20
> Minneapolis, MN 55401=20
> Tel 612-436-5178=20
> Fax 612-672-0443=20
> curtm@xxxxxxxxxxx=20
> www.logicpd.com=20
> This message (including any attachments) contains confidential
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> delete this message and are hereby notified that any disclosure,
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>
> ________________________________
>
> From: si-list-bounce@xxxxxxxxxxxxx on behalf of Hassan O. Ali
> Sent: Tue 4/11/2006 1:03 PM
> To: si-list@xxxxxxxxxxxxx
> 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 =3D
> > trying to sell stuff, use of beads was considered a serious sign of =
> =3D
> > weakness.=3D20
> > But like anything else, if it works, fine, but you need to understand
> =3D
> > why it works. If you understand why a bead works, often that will lead
> =3D
> > to an insight as to how to fix the problem without using a bead.=3D20
> >
> > Art Porter
> > Agilent =3D20
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] =3D
> > 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=3D20
> > support model.
> > Most have done way with FAEs and factory applications engineers
> unless=3D20
> > maybe if you are a large customer.
> > More often than not support is provided by email and the responses
> are=3D20
> > slow in coming and usually don't adequately address the question.
> > Now when I encounter an application note with a ferrite bead=3D20
> > recommended, the chances of the IC manufacturer providing
> information=3D20
> > that meets all your criteria is slim and none. The suggestion that
> the=3D20
> > IC manufacturers provided a current spectrum is a good one, but it
> seems =3D
> >
> > they are more preoccupied with the inner workings of their chips
> than=3D20
> > supplying information about how their parts effect the power system.
> My=3D20
> > inclination at this point is to include a footprint for a ferrite
> where=3D20
> > I think it might help, then make measurements with various ferrite
> beads =3D
> >
> > and a 0 ohm resistor and see which part results in lowest noise=3D20
> > measurements. The only problem is that a 0 ohm resistor would have
> more=3D20
> > inductance than vias connecting the bypass caps directly to the
> power=3D20
> > plane. This has definitely been a learning experience, thanks to
> all=3D20
> > that posted responses.
> >
> > Joel
> >
> >
> > Lee Ritchey wrote:
> > > Joel,
> > >
> > >
> > > As others have observed, when the solution using a ferrite bead has
> =3D
> > been
> > > engineered to satisfy the following set of conditions it is
> warranted =3D
> > 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, =
> =3D
> > nearly
> > > always practicing engineers. Can you show me an applications note =
> =3D
> > with
> > > analysis supporting the use of ferrite beads? So far, there have
> been =3D
> > no
> > > such examples. I keep hoping, but know that most applications notes
> =3D
> > are
> > > not prepared that way. Soon as I find one, I'll report it in this =
> =3D
> > forum
> > > and show it in my classes- and give its author high praise for doing
> =3D
> > good
> > > engineering.
> > >
> > > More commonly, they are justified because "we have always done it
> =3D
> > this
> > > way." Further, there has usually not been any exhaustive testing to
> =3D
> > insure
> > > the solution is valid. That is in no way good engineering. =3D20
> > >
> > > I'll accept a recommendation that is accompanied by supporting =3D
> > engineering
> > > analysis. Less, seems to me to be guessing and that's a good way to
> =3D
> > get in
> > > trouble with modern e lectronic components.
> > >
> > >
> > >
> > >
> > > =3D20
> > >> [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=3D20
> > >> agree that in some situations the use of ferrite beads can be =3D
> > detrimental.
> > >> In the design I am currently working on, the ferrite beads are
> used=3D20
> > >> mostly on low speed I/O signals and DC power to ICs that are not =
> =3D
> > running=3D20
> > >> at very high speeds.
> > >> The highest speed part is a 100 MHz PCI Express clock buffer, and
> the =3D
> >
> > >> manufacturer does recommend to use a ferrite bead.
> > >> While I agree your statements have some merit, I find it
> problematic=3D20
> > >> that you seem to be saying ferrite beads only work by luck.
> > >> Certainly there must be situations in which they can predictably be
> =3D
> > used=3D20
> > >> to reduce noise.
> > >>
> > >> Regards - Joel
> > >>
> > >>
> > >>
> > >> Lee Ritchey wrote:
> > >> =3D20
> > >>> Joel,
> > >>>
> > >>> I agree with all the respondents who have cautioned you on the use
> =3D
> > of
> > >>> ferrite beads in the power leads of devices. In all of the cases,
> I
> > >>> =3D20
> > > have
> > > =3D20
> > >>> investigated, ferrite beads have been used to treat a symptom
> rather
> > >>> =3D20
> > > than
> > > =3D20
> > >>> the problem. The problem being that the power delivery system had
> =3D
> > too
> > >>> =3D20
> > > much
> > > =3D20
> > >>> noinse or ripple on it.
> > >>>
> > >>> Lately, I have seen many cases where placing a ferrite bead in the
> =3D
> > power
> > >>> lead of a device, very often the power lead of very high speed =3D
> > serdes,
> > >>> =3D20
> > > has
> > > =3D20
> > >>> made the part perform poorly or below spec.
> > >>>
> > >>> Yes, thousands of applications notes insist that ferrite beads be
> > >>> =3D20
> > > placed in
> > > =3D20
> > >>> the power leads and the applications engineers will demad you use
> =3D
> > them
> > >>> =3D20
> > > or
> > > =3D20
> > >>> they won't guarantee the circuit will work properly. The other
> side =3D
> > of
> > >>> that same coin is that when you ask if they will guarantee that
> the
> > >>> =3D20
> > > circuit
> > > =3D20
> > >>> will work properly if you follow the applications note exactly,
> the
> > >>> =3D20
> > > answer
> > > =3D20
> > >>> is still no! The reason is that there has been no analysis to
> prove
> > >>> =3D20
> > > that
> > > =3D20
> > >>> the use of a ferrite bead is a good idea.=3D20
> > >>>
> > >>> I have measured wavforms of a 3.12 GB/S serial link where the eye
> =3D
> > just
> > >>> barely makes the minimum signal amplitude with the ferrite bead in
> =3D
> > place
> > >>> and has large amounts of margin when the ferrite bead is shorted =
> =3D
> > out.=3D20
> > >>> This, even with the manufacturer's recommended capacitors
> attached.=3D20
> > >>> =3D20
> > > This
> > > =3D20
> > >>> data was taken from a demo board supplied by the manufacturer who
> =3D
> > was
> > >>> =3D20
> > > not
> > > =3D20
> > >>> aware of the degradation caused by the ferrite bead.
> > >>>
> > >>> The right solution is to design a power delivery system with power
> > >>> =3D20
> > > supply
> > > =3D20
> > >>> noise within the limits of the circuits being supplied and this is
> =3D
> > not
> > >>> =3D20
> > > very
> > > =3D20
> > >>> 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
> =3D
> > no
> > >>> ferrite beads in them any where and they worked to specification
> the
> > >>> =3D20
> > > first
> > > =3D20
> > >>> time wit hproper margins. So maybe people who want you to use =3D
> > ferrite
> > >>> beads should be challenged with why they want you to add these =
> =3D
> > parts.
> > >>>
> > >>> My fellow SI engineers call ferrite beads "voodoo" beads and "get
> =3D
> > lucky"
> > >>> beads for good reason.
> > >>>
> > >>> This is not likely to make the ferrite bead salesmen happy, I'm =
> =3D
> > sure.
> > >>>
> > >>>
> > >>> =3D20
> > >>> =3D20
> > >>>> [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
> =3D
> > P/N.
> > >>>> =3D20
> > >>>> =3D20
> > >>> The decoupling circuit should be considered very carefully.
> > >>> =3D20
> > >>> =3D20
> > >>>> In several cases, we use beads and the noise become larger. The
> > >>>> =3D20
> > > following
> > > =3D20
> > >>>> =3D20
> > >>>> =3D20
> > >>> element should be considered:
> > >>> =3D20
> > >>> =3D20
> > >>>> 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 -----=3D20
> > >>>> From: "Joel Brown" <joel@xxxxxxxxxx>
> > >>>> To: <si-list@xxxxxxxxxxxxx>
> > >>>> Sent: Saturday, April 08, 2006 2:33 AM
> > >>>> Subject: [SI-LIST] Ferrite bead question
> > >>>>
> > >>>>
> > >>>> =3D20
> > >>>> =3D20
> > >>>>> I often see recommendations in application notes for ferrite
> beads =3D
> > on
> > >>>>> =3D20
> > >>>>> =3D20
> > >>> DC=3D20
> > >>> =3D20
> > >>> =3D20
> > >>>>> power lines. Sometimes a particular P/N is specified and
> sometimes
> > >>>>> =3D20
> > > just=3D20
> > > =3D20
> > >>>>> an impedance is given.
> > >>>>> Wouldn't I want to choose a ferrite bead that has the highest
> > >>>>> =3D20
> > > impedance=3D20
> > > =3D20
> > >>>>> possible at the frequencies of interest, and that the current =
> =3D
> > rating
> > >>>>> =3D20
> > >>>>> =3D20
> > >>> and=3D20
> > >>> =3D20
> > >>> =3D20
> > >>>>> DC resistance are appropriate? Would the frequencies of interest
> =3D
> > be
> > >>>>> =3D20
> > > the=3D20
> > > =3D20
> > >>>>> fundamental switching frequency of the part and possibly the 3rd
> =3D
> > and
> > >>>>> =3D20
> > >>>>> =3D20
> > >>> 5th=3D20
> > >>> =3D20
> > >>> =3D20
> > >>>>> harmonics?
> > >>>>> Thanks - Joel
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>> -- Binary/unsupported file stripped by Ecartis --
> > >>>>> -- Type: text/x-vcard
> > >>>>> -- File: joel.vcf
> > >>>>>
> > >>>>>
> > >>>>>
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> href=3D'http://www.si-list.org'>http://www.si-list.org</a>
> >
> > List archives are viewable at: =3D20
> > <a href=3D'//www.freelists.org/archives/si-
> list'>//www.freelists.org/archives/si-list</a>
> > or at our remote archives:
> > <a href=3D'http://groups.yahoo.com/group/si-
> list/messages'>http://groups.yahoo.com/group/si-list/messages</a>
> > Old (prior to June 6, 2001) list archives are viewable at:
> > <a
> href=3D'http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a>
> > =3D20
> >
> > ------------------------------------------------------------------
> > 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:
> > <a
> href=3D'//www.freelists.org/webpage/si-list'>http://www.freelists.or=
> g
> /webpage/si-
> list</a>
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List FAQ wiki page is located at:
> > <a
> href=3D'http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si-
> list.org/wiki/wiki.pl?Si-List_FAQ</a>
> >
> > List technical documents are available at:
> > <a
> href=3D'http://www.si-list.org'>http://www.si-list.org</a>
> >
> > List archives are viewable at: =20
> > <a href=3D'//www.freelists.org/archives/si-
> list'>//www.freelists.org/archives/si-list</a>
> > or at our remote archives:
> > <a href=3D'http://groups.yahoo.com/group/si-
> list/messages'>http://groups.yahoo.com/group/si-list/messages</a>
> > Old (prior to June 6, 2001) list archives are viewable at:
> > <a
> href=3D'http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a>
> > =20
> >
> >
> ------------------------------------------------------------------
> 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 FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at: =20
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
>
>
>
>
> ------------------------------------------------------------------
> 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 FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at: =20
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
>
> ------------------------------------------------------------------
> 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 FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
> ______________________________________________________________________
> All email being sent to or from SRC Computers, Inc. will be scanned by a
> third party scanning service.
> ______________________________________________________________________
>
> ------------------------------------------------------------------
> 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 FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> 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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
