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Re: DS1054 LF common mode aberation

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legg wrote:

> Looking at Low-Frequency noise on a 60Hz power supply, using a Rigol
> DS1054 and not getting very far.
> 
> One puzzling issue is, when ch3 or ch4 scope probe grounds are
> connected to a common 0V reference point, a low frequency common mode
> signal shows up in the shorted probe of CH1 or Ch2, when connected to
> the same 0V reference. Disconnecting CH3/CH4 ground clips removes the
> signal in shorted CH1/CH2.
> 
A lot of commodity scopes these days have pretty crummy tolerance to ground 
current.  The way to see theis is to connect the probe's ground clip ONLY to 
some other equipment, such that it might be getting 10's of mA of ground 
loop current.  Turn the attenuator down to some 10's of mV/div and see how 
much signal you get.  Ought to be WAYYY down there around 1 mV, but no, on a 
lot of recent gear, you will see 10's of mV at least.  Instead of the 1/8" 
thick aluminum housings of the old Tek gear, now the BNC shells are grounded 
through PC boards to some other circuitry inside the scope.

Your results above sound EXACTLY like what I'm talking about.

The only hope is to tie big bonding straps from the unit under test to the 
scope, so the ground difference is reduced as much as possible.

Jon

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DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-19 17:04 -0400
  Re: DS1054 LF common mode aberation Frank Miles <fpm@u.washington.edu> - 2017-03-19 23:56 +0000
    Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-19 22:21 -0400
      Re: DS1054 LF common mode aberation Frank Miles <fpm@u.washington.edu> - 2017-03-20 16:30 +0000
        Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-20 14:31 -0500
  Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-19 22:31 -0400
    Re: DS1054 LF common mode aberation Phil Allison <pallison49@gmail.com> - 2017-03-19 20:11 -0700
    Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-20 14:33 -0500
  Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-20 11:58 -0400
    Re: DS1054 LF common mode aberation George Herold <gherold@teachspin.com> - 2017-03-20 09:12 -0700
      Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-20 21:30 -0400
        Re: DS1054 LF common mode aberation Phil Allison <pallison49@gmail.com> - 2017-03-20 18:41 -0700
          Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-21 01:43 -0400
            Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-21 17:04 -0500
          Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-21 17:02 -0500
  Re: DS1054 LF common mode aberation Jon Elson <jmelson@wustl.edu> - 2017-03-20 14:28 -0500
    Re: DS1054 LF common mode aberation legg <legg@nospam.magma.ca> - 2017-03-20 21:36 -0400

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