FreeLists / si-list / [SI-LIST] Re: How to Measure Ground Noise

[Istvan Novak <istvan.novak@xxxxxxx>]

Hi pyng,

Beyond the comments you already got from the other list member,
here is one additional item you may want to consider:  taking a
baseline reading of the noise with an oscilloscope in the way you
stated is a good practice to establish the noise floor of the setup. 
If you connect the shorted signal and ground pins of the probe to
 the ground test point near a signal test point, the recorded signal
 level will tell you what is the noise picked up by the various ground
 loops in the setup, and it establishes a level below which you cannot
 trust subsequent measurement data taken at the signal test point.

So the 'ground noise' expression may not be the best choice to
describe of what you were told to do, but nevertheless it is a good
practice.  Make sure that when you take this baseline measurement
you have everything else in the system as close as possible to the
settings of your actual later measurements.  This refers to geometries
and activities alike.  Two simple examples: many active scope probe
heads have very little shielding, so their relative position with respect
to active devices in your system is important.  Likewise activity also
matters: turning on and off subsystems, bus activities, loads on the
power mains (lights) all may create large common-mode noise in the
ground loops.

Best regards,

Istvan Novak
SUN Microsystems
.
ospyng wrote:

>hi,
>I was taught to measure ground noise, by having the ground of the
>probe connect to ground TP of the board and the tip of the probe
>measuring the same TP.
>
>is this the correct where? and Why? 
>is it because of the ground cable (3 inch) cause a loop, therefore the
>reference and the tip of the probe has a different voltage?
>
>thanks
>pyng
>
>
>  
>

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