[SI-LIST] Re: Power plane noise measurement
- From: Larry Smith <Larry.Smith@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx, adsurevv@xxxxxxxxx
- Date: Tue, 20 May 2003 10:49:07 -0700 (PDT)
Virendra - To get a clean measurement of your power distribution
system, you will want to remove the decoupling capacitor from it's pads
before probing them.
If the capacitor is left in place, it will "color" the measurement. The
decoupling capacitor mounted on the pads has a resonant frequency. The
inductance for this resonant circuit is in the vias and mounting pads
and in the height of the capacitor if you attach probes to the top of
it. You will be measuring across the inductance which is highly
involved in the resonant circuit and will measure mostly noise at the
"self resonant frequency".
Usually, there are many small capacitors on a product. Pick one out of
a common value. For example, if there are 10 x .01 uF caps, remove one
out of the 10. It will not have a large effect on the PDS that you are
measuring.
regards,
Larry Smith
Sun Microsystems
> Delivered-To: si-list@xxxxxxxxxxxxx
> Date: Tue, 20 May 2003 10:28:36 -0700 (PDT)
> From: Virendra <adsurevv@xxxxxxxxx>
> Subject: [SI-LIST] Power plane noise measurement
> To: si-list@xxxxxxxxxxxxx
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> Hi all,
>
> i am trying to measure noise on a PCB directly across a decoupling capacitor.
>
The PCB has buried/ blind vias and has no specific power noise test points. My
question is, will there be a difference in the measured noise if i measure
noise
across the capacitor and if i remove the capacitor and probe at the pads? I
cannot remove the capacitor to do a what-if as i have only one board.
>
> thanks in advance,
>
> Virendra Adsure
>
>
>
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