[SI-LIST] Re: Ferrite bead question
- From: Joel Brown <joel@xxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Tue, 11 Apr 2006 09:46:02 -0700
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
support model.
Most have done way with FAEs and factory applications engineers unless
maybe if you are a large customer.
More often than not support is provided by email and the responses are
slow in coming and usually don't adequately address the question.
Now when I encounter an application note with a ferrite bead
recommended, the chances of the IC manufacturer providing information
that meets all your criteria is slim and none. The suggestion that the
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
supplying information about how their parts effect the power system. My
inclination at this point is to include a footprint for a ferrite where
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
measurements. The only problem is that a 0 ohm resistor would have more
inductance than vias connecting the bypass caps directly to the power
plane. This has definitely been a learning experience, thanks to all
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.
>
> 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.
>>>
>>> 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
>>>>>
>>>>>
>>>>>
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