substitute for ferrite

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powerpants
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Posted by powerpants: Sat. Nov 14, 2009 - 07:42 AM
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Hi all....

I'm putting together a little circuit this weekend based around the AD725 NTSC encoder IC for a class I have on monday. I am designing the analog power/ground to digital power/ground part of the board, and the reference design I am using has some parallel capacitors in series with a ferrite bead between the power to the analog and digital parts of the circuit. I don't have any ferrites, but there is a drawer on the lab bench labelled "choke coils" with values from 1.2uH to 1000uH...can I substitute one of these for the ferrite? The smaller the better, right?

Thanks!

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alwelch
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Posted by alwelch: Sat. Nov 14, 2009 - 03:27 PM
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Yes you can filter using an inductor in series with the supply and a cap to ground on the load side.
You might want to download SWCAD from Linear Technology web site and play with the values. You can use one of their jig designs for a switcher and run the simulation and then zoom in on the baseline noise. You should be able to find some noise and then add an inductor and cap to the output and repeat and see the noise reduced.

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ka7ehk
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Posted by ka7ehk: Sat. Nov 14, 2009 - 04:09 PM
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A ferrite is basically a resistor in parallel with an inductor.

I would use a 470 ohm resistor in parallel with the smallest choke.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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powerpants
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Posted by powerpants: Sat. Nov 14, 2009 - 07:37 PM
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I'll give that a try, thanks!

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nleahcim
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Posted by nleahcim: Sun. Nov 15, 2009 - 12:37 AM
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ka7ehk wrote:
A ferrite is basically a resistor in parallel with an inductor.

I would use a 470 ohm resistor in parallel with the smallest choke.

Jim


I thought a ferrite was more of its own beast? I mean it acts like it has inductance as well as resistance... but my memory suggests that its impedance vs frequency curve is different than that of an inductor. Is that accurate?

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ossi
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Posted by ossi: Sun. Nov 15, 2009 - 11:08 AM
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Just one explanation. There are a lotdifferent
types there. One of the main characteristics is,
that these chokes have a very large frequancy range
where the impedance is relatively high.
In the low-frequanc range tis is due to the
inductance, in the high frequency range its due to
the losses. (Correct me, if I am wrong.)

http://www.murata.com/emc/knowho...

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