Author Topic: scope differential probes  (Read 4229 times)

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Offline tggzzz

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Re: scope differential probes
« Reply #25 on: October 13, 2023, 10:02:57 pm »
Is this a techie solution? |O
Half arsed, yes.
But it only offers 1 isolated channel from PE as all BNC's shells share a common connection.

Yup.

And if a probe shield is connected to an "unfortunate" voltage, that voltage will materialise throughout the scope and its case. Then the operator attaches/removes a probe from the scope, or uses the touchscreen or membrane button or similar, and...

As for the UUT, it may not appreciate a large capacitance to ground appearing on one of its nodes, and oscillation or delayed switch on/off may occur with "interesting" consequences.
« Last Edit: October 13, 2023, 10:05:23 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tautech

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Re: scope differential probes
« Reply #26 on: October 13, 2023, 10:09:36 pm »
Is this a techie solution? |O
Half arsed, yes.
But it only offers 1 isolated channel from PE as all BNC's shells share a common connection.

Yup.

And if a probe shield is connected to an "unfortunate" voltage, that voltage will materialise throughout the scope and its case. Then the operator attaches/removes a probe from the scope, or uses the touchscreen or membrane button or similar, and...
Exactly, added with an edit while you were typing.

Again we have these threads about differential probe cost and need when there is a real need for operator safety when they don't fully understand the risks.
To address this basic need we stock the well known and well priced DP-25 which while not impressively spec'd suits most requirements.
Avid Rabid Hobbyist.
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Offline tggzzz

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Re: scope differential probes
« Reply #27 on: October 13, 2023, 10:46:52 pm »
Is this a techie solution? |O
Half arsed, yes.
But it only offers 1 isolated channel from PE as all BNC's shells share a common connection.

Yup.

And if a probe shield is connected to an "unfortunate" voltage, that voltage will materialise throughout the scope and its case. Then the operator attaches/removes a probe from the scope, or uses the touchscreen or membrane button or similar, and...
Exactly, added with an edit while you were typing.

Messages crossing in the æther is a standard problem. It was worse (w.r.t. UK and Australia) before we used the æther for messages :)

Quote
Again we have these threads about differential probe cost and need when there is a real need for operator safety when they don't fully understand the risks.
To address this basic need we stock the well known and well priced DP-25 which while not impressively spec'd suits most requirements.

Yup. Such as https://www.eevblog.com/forum/projects/your-test-equipments-**oscilliscope**-very-best-friend/msg4712102/#msg4712102

But there are the Tek A6902 probes for people that need a couple of isolated channels. Some variants come with standard probes, some with impressively chunky (?manly?) probes.

I ilke to point out the kinds of precautions you need to take to float a scope that wasn't designed to be floated, e.g.


« Last Edit: October 13, 2023, 10:51:01 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline JeremyC

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Re: scope differential probes
« Reply #28 on: October 14, 2023, 02:49:56 am »
So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO.

You should check the DP differential probes from Miscig, on Amazon US they cost ~$190.
https://www.micsig.com/Differential%20Probe04/
 

Offline robintTopic starter

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Re: scope differential probes
« Reply #29 on: October 14, 2023, 03:08:58 pm »
Yes Gents
The bottom line is that these diff probes cost ca £200, just so you can poke around some live DUT  - and - at that price if I needed to do that then clearly it would be better to buy a separate standalone battery operated DSO Nano - I bought one couple years back ca £50, ok for poking around a SMPS - it does the job.

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). A knowledgeable Tech worth his salt would know
the risks and precautions.  This board is after all for those with some electrical schooling  :clap:


Posted by: jonpaul
« on: Yesterday at 08:42:07 pm »

he has the solution so many thanks JP  and Ive learnt so much that I was quite unaware https://w140.com/tekwiki/images/1/19/062-1120-00.pdf.
You could spend 200hrs theory/lab work in a Uni course just to cover all the issues raised.  So I now know what I dont know :phew:

I'm a city shopping car type not a 1000hp EVcar boy racer. Hi spec hi cost solutions are not my bag.
My Rigol has catapulted me from a prewar Austin 7 to a Toyota Aygo (lovely little mover btw)
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Offline tggzzz

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Re: scope differential probes
« Reply #30 on: October 14, 2023, 03:32:19 pm »
This is for anybody new that comes and reads this thread, not for the OP.

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).

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. 

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:

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.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline jonpaul

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Re: scope differential probes
« Reply #31 on: October 14, 2023, 04:44:53 pm »
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

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Online David Hess

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Re: scope differential probes
« Reply #32 on: October 15, 2023, 02:14:27 am »
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.

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.
 

Offline jonpaul

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Re: scope differential probes
« Reply #33 on: October 15, 2023, 05:53:23 am »

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
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Online David Hess

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Re: scope differential probes
« Reply #34 on: October 15, 2023, 09:41:10 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

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.
 

Offline alm

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Re: scope differential probes
« Reply #35 on: October 15, 2023, 10:55:56 am »
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.
The 7A22 and 7A13, or 7000 series scopes, have not been made for decades either ;). Lecroy still sells the DA-1855A which is basically a stand-alone version of the 7A13. They also sell attenuating probes to go with it. I don't know if they would work on a 7A13. They require a specific BNC jack spacing, and the input capacitance may or may not be in range.

Offline jonpaul

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Re: scope differential probes
« Reply #36 on: October 15, 2023, 11:13:58 am »
Bonjour,

The 7A22/7A13 are NOT designed for use  with probes....

 precision matched pair  diff probes were made at one time.

Jon
An Internet Dinosaur...
 

Offline adam4521

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Re: scope differential probes
« Reply #37 on: October 15, 2023, 01:36:52 pm »
For people on the forum learning from this thread, the 'common mode rejection' that members are talking about isn't an esoteric thing that only matters to purists -- having a balanced probe I've found is actually really important when trying to measure across a switching transition on a power supply. That's why floating the scope and using single ended probes is not a great plan even ignoring the safety point: the ground lead and the probe 'pin' have very different impedances from the perspective of the circuit you are measuring and therefore the disturbance you are trying to null out breaks through to your trace and you don't see the truth. So the advice here:
1. Use two identical probes, connect the grounds, use the 'subtract' function or
2. Get a differential probe
are your two practical options.
Option 3, isolate the DUT with an isolating transformer might sometimes be practical but can have similar cost as (2) and will change source parameters that might sometimes be important.
Watch for differential probes on eBay, if you are patient you will eventually find a deal! If you look after them they will hold their value.
 

Online David Hess

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Re: scope differential probes
« Reply #38 on: October 15, 2023, 07:49:32 pm »
The 7A22/7A13 are NOT designed for use  with probes....

They were made to work with either 1x probes, or two of the Tektronix P6055A or later P6135A 10x probes which have DC and compensation adjustments.

There are also some earlier probes which had attenuation adjustments like the 100x P6009, which means they could also have worked with high voltage 1000x probes.

The 7A13 was also intended for comparator applications where there is no requirement for probe matching, and any probe can be used.
 


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