Scope use question

I'd like to use both traces on my scope to look at 2 voltages that are 
without a common reference.  E.g., across R1 & across R3:

________R1____
     |
     R2           ...
     |
     R3
____|______

I know that there are isolation probes to do this ($$$), but I'm 
wondering if there is some clever other way to do it.  In my minimal 
experience I can't see one, but I'm hopeful that someone more clever 
than I has one to share.

Thanks, Bob

On Tuesday, August 15, 2023 at 10:42:33 AM UTC-4, Bob Engelhardt wrote:
> I'd like to use both traces on my scope to look at 2 voltages that are 
> without a common reference. E.g., across R1 & across R3: 
> 
> ________R1____ 
> | 
> R2 ... 
> | 
> R3 
> ____|______ 
> 
> I know that there are isolation probes to do this ($$$), but I'm 
> wondering if there is some clever other way to do it. In my minimal 
> experience I can't see one, but I'm hopeful that someone more clever 
> than I has one to share. 
> 
> Thanks, Bob
You could do it with a four channel cope that allows combining two channels in summing mode, like the cutom probes do.
Can you just use two voltmeters, instead?

On 2023-08-15, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
> I'd like to use both traces on my scope to look at 2 voltages that are 
> without a common reference.  E.g., across R1 & across R3:
>
> ________R1____
>      |
>      R2           ...
>      |
>      R3
> ____|______
>
> I know that there are isolation probes to do this ($$$), but I'm 
> wondering if there is some clever other way to do it.  In my minimal 
> experience I can't see one, but I'm hopeful that someone more clever 
> than I has one to share.
>
> Thanks, Bob

With that exact circuit you could do it because the voltage on R3 is
some fixed multiple of the voltage on R2+R3 but in the general case,
no, an ordinary 2 channel scope can only measure voltage between three 
points (one of them being ground)

-- 
 Jasen.
 &#127482;&#127462; &#1057;&#1083;&#1072;&#1074;&#1072; &#1059;&#1082;&#1088;&#1072;&#1111;&#1085;&#1110;

On 16/08/2023 12:42 am, Bob Engelhardt wrote:
> I'd like to use both traces on my scope to look at 2 voltages that are 
> without a common reference.  E.g., across R1 & across R3:
> 
> ________R1____
>      |
>      R2           ...
>      |
>      R3
> ____|______
> 
> I know that there are isolation probes to do this ($$$), but I'm 
> wondering if there is some clever other way to do it.  In my minimal 
> experience I can't see one, but I'm hopeful that someone more clever 
> than I has one to share.
> 
> Thanks, Bob


You could buy or build a differential probe.

If you only wanted to see one trace (e.g. the voltage across R1), you 
could put the scope into summing mode and invert one channel. This 
doesn't work all that well because the gain of the two channels and 
probes isn't all that well matched.

Could I use a pulse transformer for isolation?  E.g.: 
https://www.digikey.com/en/products/detail/DA101C/3438644

It would put some inductance in the signal path, but I don't deal with 
frequencies higher than audio.  Any other limitations?

On 8/15/2023 4:36 PM, Michael Terrell wrote:

> You could do it with a four channel cope that allows combining two channels in summing mode, like the cutom probes do.
> Can you just use two voltmeters, instead?
> 

I need to see the waveforms.

> You could buy or build a differential probe.
> 
> If you only wanted to see one trace ...


Differential probe:  too much (this is just a lower-level hobby).  Whole 
point is to see both signals.

On 2023-08-16, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
>> You could buy or build a differential probe.
>> 
>> If you only wanted to see one trace ...
>
> Differential probe:  too much (this is just a lower-level hobby).  Whole 
> point is to see both signals.

Is this a repetitive signal, perhaps you could use the trigger input
and probe the other two signals separately?


-- 
 Jasen.
 &#127482;&#127462; &#1057;&#1083;&#1072;&#1074;&#1072; &#1059;&#1082;&#1088;&#1072;&#1111;&#1085;&#1110;

On 2023-08-16, Bob Engelhardt wrote:
>> You could buy or build a differential probe.
>> 
>> If you only wanted to see one trace ...
> 
> Differential probe:  too much (this is just a lower-level hobby).  Whole
> point is to see both signals.

I think a big reason differential probes cost so much is because they are 
usually designed for mains voltage applications and are therefore built to 
meet various electrical safety standards (CAT ratings, etc.).

If you don't care about this safety aspect (e.g., you're probing some low 
voltage electronics) there appears to be some mass-produced ~US$50 option 
readily available on aliexpress (no clue about its quality).

If you are not too fussed about probe performance then maybe you can just 
hack together something on the bench.  Perhaps a very simple opamp 
differential amplifier circuit like this one will fill the bill:

  https://www.electronics-tutorials.ws/opamp/opamp_5.html

On 17/08/2023 6:17 am, Bob Engelhardt wrote:
>> You could buy or build a differential probe.
>>
>> If you only wanted to see one trace ...
> 
> 
> Differential probe:  too much (this is just a lower-level hobby).  Whole 
> point is to see both signals.
> 


I did suggest you might build one. Depending on what your requirements 
are, it might be cheap and easy to do. I made a simple one, I think from 
an AD830.