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Ferrite for GND separation

Former Member

Hi

 

I need to create a few low disturbance voltages  to use with a analog video decoder. I need one 3.3V for digital  circuitry, one 3.3V for analog, one 1.8V for digital and one 1.8V to  drive a PLL. I want thous voltages to have separate GND from the rest  of the circuit, one for digital and one for analog circuitry.

I believe it will be sufficient to use ferrite beads to "isolate" these  voltages from the main voltage and GND, but I'm not sure which  characteristics the ferrite beads need to have.

There are no particular frequencies that I'm aware that I need to  filter. The main 5V is created from a 850KHz DC/DC step down switcher  though.

I'm not sure about the current consumption of the four voltages, but it  can be assumed to be less than 40mA each, and less than 100mA in total.

Price is a factor of course, but also the size is fairly important. I would also much prefer SMD components.

I guess I need fairly low impedance components, but otherwise I'm not sure what to look for.

Does anyone have any components to recommend, or at least some general hints? All help is much appreciated.

 

Kind regards

Albin, Sweden

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  • 0

    Dear Afbin,

     

    thanks a lot for posting the question.

     

    As you already mentioned a chip beat ferrite might be here your best solution since the parts are fairly in expensive with a high level of EMI noise rejection (in differantial mode).

     

    For small DC/DC Converter applications we recommend our WE-CBF Wide Band series:

    http://katalog.we-online.de/kataloge/eisos/index.php?language=xx&pf=WE-CBF

     

    The Wide Band series has the advantage of suppressing EMI noise on a broad frequency spectrum. They all full fill your current requirement and are effective in combination with your dcdc swithcing frequency.

     

    Then it is all depending in which frequeny band you see spikes to make the right selection. Our recommendation is in this case to make some test with different components out of our WE-CBF 0603 or 0805 Wide Band series.

    Instead of buying all components separately the optimum for you would be to get our Chip Beat assortment with the full band of values in 0805, 0603 and 0402 sizes to find the best component for the applcation. The part number is 742790.

     

    I will have Farnell make the kit also available on the web the next days. If there is a need of the kit the next 2-3 days you can either buy a selection of the components or you can also order the kit directly at www.we-online.de

     

    If you have any request please let me know.

     

    regards

    Lars Fahrbach

     

    Würth Elektronik

  • 0

    Dear Albin, I would use our high current version from the WE-CBF series, if you intend to use them in the GND separation, reason the low DCR. It is not easy to design two separate GND without loops. You must be very carefull to not create it. Additional I would separe it in only one point, star distributed.

    Regards,

    Lorandt Foelkel

    Würth Elektronik

  • 0 in reply to lfa555

    Thank you Lars!

     

    I'll choose one or two ferrites from the WE-CBF series to begin with and order a design kit when the need to tweak the filtration.

     

    Kind regards

    Albin, Sweden

  • 0 in reply to Former Member

    Thanks for the input Lorandt!

     

    I'll begin with: 742792023 for the GND separation  (L105, L106 and L104) and 74279265 for the other four ferrites. Then I can  move in ether direction if I need to tweak it.

     

    I was  planning to design a separate GND-plane for the digital circuitry (0V-D)  and to "isolate" that from the main 0V with one ferrite (one point).  The 0V-A GND is then separated from 0V-D in only one point using the  second ferrite, star distributed.

     

    Fortunately there aren't many connections to other circuitry from the circuits driven by 0V-D and 0V-A. I'll do my best not to create any ground loops, might need some fingers crossed.

     

    Kind regards

    Albin, Sweden