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 information intended for a specific individual and purpose, and is protected by law. If you are not the intended recipient, you should delete this message and are hereby notified that any disclosure, copying, or distribution of this message, or the taking of any action based on it, is strictly prohibited ________________________________ 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 > >>>>> > >>>>> > >>>>> ------------------------------------------------------------------ > >>>>> To unsubscribe from si-list: > >>>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject =3D > field > >>>>> > >>>>> or to administer your membership from a web page, go to: > >>>>> <a href=3D'//www.freelists.org/webpage/si- list'>//www.freelists.org/webpage/si-list</a> > >>>>> > >>>>> For help: > >>>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >>>>> > >>>>> List FAQ wiki page is located at: > >>>>> <a 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