[sdiy] Choice of ferrite bead inductor

[Eric Brombaugh]

Seb Francis wrote:

> Essentially I want to isolated power spikes caused by digital switching, 
> etc. from sensitive analog parts of the circuit.  I will use a ferrite 
> bead followed by a 10uF capacitor.  The 3 places I'm using this 
> arrangement is:
> - Where the +15V and -15V power comes onto the board.
> - To create a separate clean 3.3V from the dirty 3.3V that powers the 
> uProc, SRAM, etc.
> 
> Don't need a huge current capacity - 200mA should do it.  Needs to be 
> SMD package, something like 0805, or bigger if necessary.
> 
> Could someone give me a quick pointer to a suitable inductor, or perhaps 
> give me a rule of thumb by which I could select one myself?

For low-current RF cleanup applications, I've used these:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=490-1028-1-ND

Only rated to 50ma, so they're no good for your high-current situation, 
but fine for isolating the analog 3.3V from the digital side. Very 
inexpensive though.

Also, I'd recommend that you include a smaller-valued cap (0.1uf) in 
addition to the 10uf in order to clean up the high-frequency crud.

Eric