[SI-LIST] Re: Noise measurement
- From: Larry SMITH <Larry.Smith@xxxxxxx>
- To: alan.pfeifer@xxxxxxxx
- Date: Thu, 02 Jun 2005 12:17:21 -0700
Alan - I like single ended 50 Ohm probes. I use a small diameter
semi-rigid coax and solder it directly to decoupling capacitor pads in
such a way that it makes the smallest loop possible. You must remove
the capacitor first because the presence of the capacitor will "color"
your noise measurement.
50 Ohms is much higher than the impedance of your power rail, which is
generally in the 10mOhm range, so you won't be significantly influencing
the voltage you are trying to measure. Be sure and terminate the coax
with 50 Ohms at the instrument or else you will be greatly affected by
the coax cable resonance.
I don't like measurements with active probes because they are bandwidth
limited and the loop for magnetic field pickup will always be larger
than with the simple coax probes.
regards,
Larry Smith
Sun Microsystems
Pfeifer, Alan wrote:
> What arguments, if any, can be made between using a single ended probe or a
> differential probe to measure noise on the power rails? Can the measurement
> be made at the component or should some unused pads be used?
> Thanks,
>
> Alan Pfeifer
>
>
>
>
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