Products > Test Equipment

scope differential probes

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tggzzz:
This is for anybody new that comes and reads this thread, not for the OP.


--- Quote from: robint on October 14, 2023, 03:08:58 pm ---I think if I really needed to expose myself and my precious Rigol  :-* then I could power it from my existing standby  Battery/inverter system which might give me a days work/ charge overnight.  Yes a much derided floating scope but the safety issue seems well over blown here (commercial opinions selling).

--- End quote ---

That's offensive bovine excrement. None of us have tried to sell you anything.


--- Quote ---A knowledgeable Tech worth his salt would know the risks and precautions. 

--- End quote ---

What makes you so sure of that?

You seem to have not read things like "Here’s an anecdote about someone that took precautions and still killed themself."


--- Quote ---This board is after all for those with some electrical schooling  :clap:

--- End quote ---

You mean just enough schooling to be dangerous? Yes, the Dunning-Krueger effect is very real, unfortunately.

You are new here. Those of us that have been here a while know some posters have a habit of posting truly hare-brained points.

jonpaul:
Rebonjour:

RE floating scope   had old TEK 212
https://w140.com/tekwiki/wiki/212
Built in NiCd.  But both channel  rtn are common.

Most of the diff proble needs disappear if you  have a good mains isolation trsf.
We used Signal/BelFuse DU series like DU-2, DU-10

Bon soiree,


Jon

Jon

David Hess:

--- Quote from: alm on October 13, 2023, 12:08:27 pm ---Have you tried two channels in subtract mode with the probe grounds connected together (you can wrap electrical tape around the alligator clips so they don't detach and touch the DUT)? What was the limitation you ran into? This mode should be safe for both the user and scope as long as you keep ground away from the DUT and stay within the voltage ratings of probes and scope. And it's very cheap for anyone with a 2+ channel scope and some identical probes.
--- End quote ---

Two probes in add and invert mode works.  The DC common mode rejection must be trimmed with the oscilloscope's variable control, and then the AC common mode rejection can be trimmed with the probe compensation adjustments.  Tektronix made probe pairs with adjustable DC attenuation for exactly this application.

The primary limitation is the oscilloscope's own input range.  The vertical sensitivity must be set to accommodate the input common mode range, which limits differential sensitivity.

Is it worth doing?  You can definitely pick out signals which would otherwise be buried in common mode noise, but the improvement is not quite as good as with a dedicated differential probe.  Digital oscilloscopes, with some old exceptions, do not work quite as well as analog oscilloscopes because they also have to remove quantization noise.

jonpaul:

Hello:  any scope CH1 -CH2 has a common earth between the two probes and forms only a single differential display.
Result  depends on the CMRR and CM V  of the scope and probe balance and matching from DC up to  freq of interest.
Expect CMRR  20:1 or 50:1 By nulling one can improve the DC CMRR 


Old analog TEK mainframe 7000 scopes have some fine diff amp plugins with filters and very good  CMRR. See 7A22, 7A13
https://w140.com/tekwiki/wiki/7000-series_plug-ins#Vertical_plug-ins

Some special probe sets are specified for diff use.

Jon

David Hess:

--- Quote from: jonpaul on October 15, 2023, 05:53:23 am ---Old analog TEK mainframe 7000 scopes have some fine diff amp plugins with filters and very good  CMRR. See 7A22, 7A13
https://w140.com/tekwiki/wiki/7000-series_plug-ins#Vertical_plug-ins
--- End quote ---

The disadvantage of the 7A22 and 7A13 is that they have no built in way to adjust the DC or AC common mode rejection of an attenuating probe.  Attenuating probes with a DC gain trim are required and these have not been made for more than a decade.  This does not apply to a 1x probe except when the high impedance input attenuator is engaged as the attenuators lack a DC trim also.

So these are great differential input vertical amplifiers, however they are not a panacea unless you have probes with DC trims.

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