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

[Doug Smith <doug@xxxxxxxxxx>]

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


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