[SI-LIST] Re: Decoupling of Oscillator

Maybe someone can explain it, but I've never understood adding _any_
additional impedance to a reference plane (call it "ground") connection.  It
seems I should always be aiming for the common impedance here for reference
purposes.  Creating an isolated power to reference current loop does seem to
show advantage when both high speed digital and low level analog live on the
same board.  Am I missing something?

Regards,

Brent DeWitt
Datex-Ohmeda

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Martin Euredjian
Sent: Wednesday, November 20, 2002 3:31 PM
To: si-list@xxxxxxxxxxxxx
Subject: [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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