[SI-LIST] Re: How to Measure Ground Noise
- From: Istvan Novak <Istvan.Novak@xxxxxxx>
- To: zhang_kun@xxxxxxxxxx
- Date: Thu, 07 Apr 2005 14:45:46 -0400
Dear Zhangkun,
If you connect the signal and ground pins of a probe together and
connect both to a ground point on your system, the signal you see on the
oscilloscope could be a combination of several things: close-field
radiated noise picked up by the active probe head, radiated noise picked
up by the cable connecting the probe head to the scope, large
common-mode noise converted somewhere along the path due to saturation
of circuitry, etc., etc. The purpose of such baseline measurement is not
necessarily to identify the sources or these noise contributors one by
one, rather to establish their cumulative effect as a lower limit of the
measured values, below which you need to throw out the data.
Best regards,
Istvan
Zhangkun wrote:
> Dear Istvan
>
> Do you mean connecting the probe to the ground pin? And measure the radiation
> in the environment.
>
> In our measurement, there is often such kind of situation. When the pigtail
> of the probe is long, the radiation will couple into the measured signal. Is
> there any technique about shielding probe?
>
> Best Regards
>
> Zhangkun
> 2005.4.7
> ----- Original Message -----
> From: "Istvan Novak" <istvan.novak@xxxxxxx>
> To: <ospyng@xxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Sent: Thursday, April 07, 2005 8:03 PM
> Subject: [SI-LIST] Re: How to Measure Ground Noise
>
>
>
>>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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--
Istvan Novak Sun Microsystems, Inc.
Istvan.Novak@xxxxxxx Workgroup Servers, BDT Group,
One Network Drive, Burlington, MA 01803
Phone: (781) 442 0340
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