[SI-LIST] Re: Ferrite bead question
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Istvan Novak" <istvan.novak@xxxxxxx>
- Date: Wed, 12 Apr 2006 08:34:47 -0700
Agreed.
> [Original Message]
> From: Istvan Novak <istvan.novak@xxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>
> Cc: Istvan Novak - Board Design Technology <Istvan.Novak@xxxxxxx>; Joel
Brown <joel@xxxxxxxxxx>; Zhangkun <zhang_kun@xxxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 4/12/2006 5:07:05 AM
> Subject: Re: [SI-LIST] Re: Ferrite bead question
>
> Lee,
>
> We agree that we have to raise the bar. AND both sides have to do their
> part of the homework.
>
> Vendors should work to get the necessary information to the users: current
> signature at the pin to be fed and noise tolerance at the pin. Users
> have to
> accept the fact that vendors cannot (and should not) provide a full
circuit
> schematics for the potential filtering circuit; it is the user's
> responsibility and
> task to make sure that the system design provides the necessary feed
> impedance
> to the active device(s) and provides the necessary isolation against noise
> moving in either direction.
>
> Regards,
>
> Istvan
>
>
>
> Lee Ritchey wrote:
>
> >Istvan,
> >
> >I'd agree with you about some authors of the applications notes not
knowing
> >the user environment if I had not witnessed IC manufacturer's
applications
> >engineers telling customers that they must follow the applications notes
> >exactly or they would not warrant proper operation. Yes, I've been
witness
> >to this dozens of times. Under those conditions, the IC manufacturer is
> >under an obligation to do the proper engineering to support the notes,
and,
> >by an large, that has not happened. It is this class of applications
note
> >that I think most grievously wrong.
> >
> > Imagine if a drug manufacturer published applicaitons notes of this
> >caliber. Would you say they are not accountable if people are harmed by
> >their bad data? I doubt it. Why, then, do we exempt our suppliers from
> >such accountability? Because no one had died? There are occasions where
> >they do.
> >
> >Instead of making excuses for poorly prepared component information, we
> >need to raise the bar.
> >
> >
> >
> >
> >>[Original Message]
> >>From: Istvan Novak - Board Design Technology <Istvan.Novak@xxxxxxx>
> >>To: <leeritchey@xxxxxxxxxxxxx>
> >>Cc: Joel Brown <joel@xxxxxxxxxx>; Zhangkun <zhang_kun@xxxxxxxxxx>;
> >>
> >>
> ><si-list@xxxxxxxxxxxxx>
> >
> >
> >>Date: 4/11/2006 12:21:52 PM
> >>Subject: [SI-LIST] Re: Ferrite bead question
> >>
> >>Lee,
> >>To be fair to the people who write those application notes, they have to
> >>do it blindly: they have no way of knowing the details of the
environment
> >>the chip goes into.
> >>
> >>In those cases, when the use of the ferrite bead is warranted, the
purpose
> >>of the filter is to provide the necessary feed impedance at the filter's
> >>output
> >>AND to provide the necessary isolation between the filter's input and
> >>output.
> >>The necessary feed impedance at the output is something that the person
> >>writing the application note should be able to know. The required
> >>isolation,
> >>however, depends not only on the filter, but also on the noise
> >>characteristics
> >>at the filter input. This means that the necessary filter does depend
> >>on the
> >>circuit environment. There are scenarios, as you pointed out, when
there
> >>is no need for a separate filter; these are the cases when the noise
> >>
> >>
> >before
> >
> >
> >>the 'would-be' filter is low enough so that no additional attenuation is
> >>required AND the main rail's impedance is appropriate for directly
> >>connecting
> >>it to the 'filtered' device. If, however, the main rail has more noise
> >>than
> >>what the 'filtered' device can live with, we have two choices: we could
> >>lower the noise on the main rail, or we can use a filter.
> >>
> >>Devices, which come with a recommended extra filtering, are usually
clock
> >>sources, clock buffers, SerDes analog circuitry. The noise sensitivity
> >>of these
> >>analog pins may require a noise voltage allowance much lower than what
> >>a generic logic may need. For instance, PLL supply points may be very
> >>sensitive
> >>to periodical disturbances around the loop cutoff frequency. Sometimes
> >>
> >>
> >the
> >
> >
> >>noise should not be more than a few mV rms in that frequency range.
> >>DC-DC converter output ripple could be more than this limit, and the
> >>
> >>
> >generic
> >
> >
> >>logic may be OK with it. In such cases reducing the output ripple of
the
> >>converter, though technically might be doable, would probably be more
> >>expensive than requiring a dedicated filtering to the lower-current
> >>analog point.
> >>
> >>You are correct in saying that many of these filters are not designed
in a
> >>proper way. One way of doing it properly would be to get the noise
> >>tolerance and feed-impedance requirement for the chip in question,
instead
> >>of a recommended filter. Based on the noise characteristics of the
system
> >>that goes around the chip, the system designer should be able to
determine
> >>whether a filter is needed, and if needed, should be able to design it.
> >>
> >>Regards,
> >>
> >>Istvan
> >>
> >>
> >>
> >>
> >>
> >>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.
> >>>
> >>>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.
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>>[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
> >>>>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
> >>>>mostly on low speed I/O signals and DC power to ICs that are not
> >>>>
> >>>>
> >running
> >
> >
> >>>>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
> >>>>that you seem to be saying ferrite beads only work by luck.
> >>>>Certainly there must be situations in which they can predictably be
> >>>>
> >>>>
> >used
> >
> >
> >>>>to reduce noise.
> >>>>
> >>>>Regards - Joel
> >>>>
> >>>>
> >>>>
> >>>>Lee Ritchey wrote:
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>>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
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