[SI-LIST] Re: Measuring Power supply Ripple and Noise

The use of small solderable coax works well for looking at PDN noise,
however if you set your scope for 50 ohms input Z without a DC block
(perhaps a bias T) be sure your scope's termination resistors can handle
the power dissipated. (at 3.3 volts you'll be dissipating .217watts, so
be sure to verify what is safe for your scope).

Also, with no DC block you'll be drawing 66 mA from a 3.3 supply, so be
sure to verify that you have the extra current to burn. Most likely you
do, but be sure.

Thirdly, if you do use a DC block, be sure it has low enough impedance
to allow you to observe the noise down to the lowest frequency you are
interested in.

-Ray
Xilinx Inc.


> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of DAVID CUTHBERT
> Sent: Thursday, November 13, 2008 11:43 AM
> To: Jaison Fernandez
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Measuring Power supply Ripple and Noise
> 
> Jaiso,
> soldering a coaxial cable directly to the board works very well. The
> common-mode noise rejection is good. Set the 'scope to 50 ohms.
> 
> Power supply boards commonly have CM noise on them and this is a real
> problem with single-ended X10 probes. An active differential probe can
> work
> well.
> 
>    Dave Cuthbert

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