RE: [SI-LIST] : Reducing Power and Ground Noise

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From: Tom Dagostino (tom_dagostino@mentorg.com)
Date: Thu May 11 2000 - 08:59:58 PDT


I would look at how you are grounding your scope probe. It sounds like you
have a long ground wire causing a large loop antenna that could be picking
up a lot of clock syncronous noise. You may be better off looking for local
noise (that's what the IC will see) and ground the probe near, within a few
tenth's of an inch of where you are looking at the supply. Though you will
not be able to see the ground moving you should be able to see the total
voltage accross the IC.

Tom Dagostino
ICX Modeling Group
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of sweir
Sent: Wednesday, May 10, 2000 3:47 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Reducing Power and Ground Noise

David,

First, please be sure that your measurement is not lying to you. Things
have got to be really, really bad if you can reliably see your clock riding
on the power supply. It is very hard for a single net to do this. It is
more likely that all the signals triggered by that clock are contributing
to the problem, and / or your are getting crosstalk between your scope
channels due to the test set-up. So, if you only address the clock, you
probably won't correct your symptom as measured.

Even assuming that the measurement is real, and that this one clock signal
is the source of your troubles, I don't think your proposal is a very good
idea. If you successfully isolate the power pin input, the signal current
still has to flow through the signal trace itself through the load and back
through the power / ground plane structure. Isolating one end makes that
return path more of a problem, not less.

It sounds like your problem is some combination of:

         1. Inadequate power / ground decoupling, and
         2. Excessive return path inductance for the high edge rate signals.

Bulk capacitance is good for audio frequencies up to around 1 MHz,
depending on what type of capacitors you are using. It is unlikely that
increasing bulk capacitance will help at 64MHz.

On a four layer, .062 board, you are going to be hard pressed to see a
mounted resonant frequency on the .1 uF decoupling capacitors above 8
MHz. Either use enough smaller capacitors, or lots of .1's to get the
impedance down.

Regards,

Steve.

At 09:12 AM 5/10/00 -0400, you wrote:
>Hi All,
> I'm working on a board currently which has "high" power and ground
>noise. I'm looking for effective methods of reducing this noise.
>
>I spent a bit of time making power and ground noise voltage measurement on
>the current configuration of this board. The results show that the power
>and ground system have noise spikes which are 180 deg. out of phase from an
>output clock on the board. I won't go into the measurement set up too
much,
>but here are the basics:
>Using two FET probes to make the measurements. The ground reference was
the
>digital ground input to the board. Scope was triggered off the suspect
>clock (64Mhz) .
>
>I suspect that an input filter to the device generating the signal (an
ASIC)
>would improve things. I'm not sure about the correct configuration for
this
>filter. Currently there are .1uF caps across each of the input power pin.
>I can think of two configurations that may be effective.
>A ferrite bead in series with each of the input power connections (keeping
>the .1uF caps) may be effective but I'm not sure what "side effects" may
>occur.
>A inductor and resistor in parallel (also keeping the .1uF caps) setting
the
>values .75uH and 5 ohms.
>I'm not sure what the draw backs of these configurations are. Or is there
>are better solutions.
>
>Additionally, I plan on decreasing the power and ground spacing (the board
>is a four layer) to 7mils, increasing the bulk capacitance to the board,
and
>possibly placing .001uF caps in parallel with each of the .1uF caps.
>
>Any comments would be appreciated
>
>Best regards
>
>David Spencer
>
>
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