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

[Dimiter Popoff <dp@xxxxxxxxxxx>]

I guess this is why most if not all scope probes have a metal ground
ring close to their tip.
 Everytime I have to bend some piece of wire to make a dual-tip
probe from what I have (thus GND tip being about 6-7 mm) I miss
a Philips scope I had once (>10 years ago...) which had such a
piece as part of the set. Well, I guess mine here may have had it
once, too, but not now. Now I keep bending pieces of wire and swear :).

Dimiter

------------------------------------------------------
Dimiter Popoff               Transgalactic Instruments

http://www.tgi-sci.com
------------------------------------------------------



> Date: Thu, 07 Apr 2005 21:27:56 -0700
> From: Doug Smith <doug@xxxxxxxxxx>
> To: Istvan.Novak@xxxxxxx
> Cc: zhang_kun@xxxxxxxxxx, istvan.novak@xxxxxxx, ospyng@xxxxxxxxx,
>       si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: How to Measure Ground Noise
> 
> Hi Zhangkun, Istvan, and the group,
> 
> I would like to modify Istvan's response somewhat.
> 
> Ground currents flowing from the circuit, through the probe ground 
> lead, and down the shield into the scope chassis typically (although 
> not always) dominates everything else. It is just L(of the ground 
> lead)di/di that creates a voltage across the probe from tip to ground 
> lead attachment point to the probe. The probe reads this voltage like 
> any other. This voltage is typically about 50 to 100 times larger than 
> what leaks through the probe cable shield.
> 
> Doug
> 
> Istvan Novak wrote:
> 
> > 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
> 
> ........lots of lines deleted
> 
> 
> -- 
> -------------------------------------------------------
>      ___          _       Doug Smith
>       \          / )      P.O. Box 1457
>        =========          Los Gatos, CA 95031-1457
>     _ / \     / \ _       TEL/FAX: 408-356-4186/358-3799
>   /  /\  \ ] /  /\  \     Mobile:  408-858-4528
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>

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