[SI-LIST] Re: Decoupling of Oscillator

From: Philip Ross Wellington

> Some provided Pi filters for the
> ground but that didn't work well logically because of the return path
> inductance and Signal Integrity (not even coined back then) induced
> problems.


On datasheets I've seen recommended isolation/filtering networks that look
like this:
(values chosen at random)

RAW_VCC -----------BEAD-------------- VCC'
         |      |          |     |
         |      |          |     |
        22uF  0.1uF      0.1uF  22uF
         |      |          |     |
         |      |          |     |
RAW_GND -----------BEAD-------------- GND'


versus:


RAW_VCC -----------BEAD-------------- VCC'
         |      |          |     |
         |      |          |     |
        22uF  0.1uF      0.1uF  22uF
         |      |          |     |
         |      |          |     |
RAW_GND ----------------------------- GND'


Is there any merit to the first approach, say, for a clock, or a device with
a PLL, or a sensitive analog sub-section?  Both in terms of circuit
operation and EMI/RFI concerns.

I've also seen a couple where the beads are replaced with low or 0 ohm
resistors.


Thanks,


===============================
 Martin Euredjian
  eCinema Systems, Inc.
       voice: 661-305-9320
       fax:   661-775-4876
  martin@xxxxxxxxxxxxxx
  www.ecinemasys.com
===============================








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