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Re: DS1054 LF common mode aberation

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On Sun, 19 Mar 2017 17:04:48 -0400, 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.
> 
> Measurement using a current probe show >20mAppk of LF current passing in
> the CH1 or CH2 probe lead, generating a >20mvppk voltage on the display
> for that particular channel, but only when CH3 or CH4 ground leads are
> connected.
> 
> The scope probes are those supplied with the retail unit.
> 
> It's perfectly possible that this unit has ground leakage issues, but
> the safety ground lead on both DUT and scope are secure and share a
> common return point. The 0V reference has a parallel RC connection to
> the frame, so a small scope probe ground current is anticipated.
> 
> The 0V reference effect is only present when the largish 60Hz power unit
> is on. The shape of the voltages and currents look ~capacitive, with
> displacement coincident with line zero-crossing, where most
> low-frequency noise in the DC regulator outputs is anticipated, and so
> cannot be isolated or ignored.
> 
> Is there some issue with DS1054 ground impedance imbalance between
> channels that can be corrected?
> 
> Is there some issue with basic LF CMRR of the DS1054 inputs that needs
> looking into?
> 
> I have other scopes to use for this work, but expected the DS1054 to be
> useful in simple low frequency measurement.
> 
> RL

If the signal of interest is at low frequencies, then by all means get rid
of the extra ground connections.  They're only making matters worse.

Is the origin of the problem in the 'scope?  You should be able to check 
that out without your power supply, testing for proper gain, frequency 
response, input impedance, and leakage currents.  If you're using say, Ch3
and Ch4 as cheapo differential input (via subtraction) you should test its
CMRR too.  Once you're sure your test equipment are working properly you
can measure your power supply properties knowing its limitations.  Until 
then us outside non-observers can only make guesses.

	-F

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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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