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Old 16th March 2005, 05:58 AM   #1
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Default Ferrite beads ON chips

A bad or good idea-someone please let me know:

Since ferrite beads are meant to absorb RFI/EMI, does it make any sense to place the beads ON THE TOP of integrated chips so that any radiated RFI/EMI is absorbed? Obviously there will need to be some way to mount the ferrite beads on the chips-blue tack maybe?

I have tried them on leads of inductors and this certainly seemed to lower the noise floor in CD players.
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Old 16th March 2005, 07:28 AM   #2
renes3 is offline renes3  Zimbabwe
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This is a very good idea! It is already common use. Ferrishield in the USA is already making and selling rfi/ emi absorbers, that can be attached on ic's and pcb's.

Look on www.ferrishield.com for more information on the subject.

Best regards,

René
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Old 16th March 2005, 11:02 AM   #3
renes3 is offline renes3  Zimbabwe
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And look at this:


http://www.ferrishield.com/pdf/SB28B2027AT.pdf



René
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Old 16th March 2005, 02:31 PM   #4
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Hi

For the higher frequencies (say several hundreds of MHz) it will do, but below in general the RF currents and supply and or I/O loops will generate most RFI.

With that in mind, it doesn't matter where to place the bead (integrating that material is expensive)

By the way, beads do not absorb, they expose themselves as high RF impedances hence blocking RF currents

On chip measures to do similar jobs are actively switched supply lines (they disconnect when internally generated dI/dt is high)

best regards
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Old 16th March 2005, 04:35 PM   #5
pburke is offline pburke  Germany
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I use Stillpoints ERS paper - just cut it to size for each chip, put a dab of silicone glue on it and stick it on each chip. I even wrap my DAC coupling caps into this stuff. They make the paper also as adhesive tape to wrap cables in it.

It's not cheap, but I bet that it comes out cheaper than a bunch of Ferrishield and it essentially does the same thing.

Peter
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Old 16th March 2005, 05:40 PM   #6
renes3 is offline renes3  Zimbabwe
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Pburke,

Thats true, the ferrishield stuff doesn't come cheap............

I once got a ferrishield evaluation box for €25, but these normally cost more than €100. It was something of a box of chocolate thingie with 20 different ferrite beads in it.

Most of that stuff went into my cd-80.

René
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Old 16th March 2005, 06:39 PM   #7
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Beads work by acting as a resistive element. The reactive portion can usually be ignored. Absorbers are a different beast.

100 Euros for 20 or so beads????? I am in the wrong business. An expensive bead, such as a very large one, doesn't cost more than $1 or so. Unless you buy from the wrong supplier.

Jocko
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Old 16th March 2005, 10:49 PM   #8
Stocker is offline Stocker  United States
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Default Are you guys kidding?

...or did you also pay $100 for your mains power cables? If the latter is the case, please disregard the following commentary.

Ferrite beads will attenuate some high frequency stuff when placed *around* a conductor, creating an inductor with a specific reactance beginning at specific frequencies.

Ferrite beads around the leads of ICs might make some sense, but good luck finding a bead that will fit around adjacent legs of an IC *and* do any good.
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Old 17th March 2005, 01:26 AM   #9
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Default No, it doesn't!

It works because of a frequency dependent resistive component.

Jocko
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Old 17th March 2005, 01:56 AM   #10
Stocker is offline Stocker  United States
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Okay, let's all get on the DIYAudio Semantics Merry-go-round

A resistor has straight resistance.

A cap or inductor has frequency dependant resistance = reactance
They will also have pure resistance, but this will either be very high or very low, respectively.
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