Isolated USB adapter or differential probe for testing non-floating

[gfmucci]

Question: 

Is it better to use an isolating USB adapter or differential probes to measure a non-floating USB powered device on a stand-alone scope such as the Instek 1054b?  I provide 3 examples of the USB dongle and 1 example of a differential probe linked below.  The USB dongles range in price from $20 to $50, but "best ones" range well over $100.  The differential probe linked is $180 and the cheapest I could find on Amazon.

I would imagine the differential probe would be useful for testing more types of non-floating powered devices than would any isolated USB adapter.  Is that understanding correct?

USB isolation:
https://www.amazon.com/dp/B00QU232UM/?coliid=I2K7I1FJM8W9NG&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_it

https://www.amazon.com/SMAKN-Isolator-Digital-Isolation-Industrial/dp/B00XXPO4UG/ref=cm_cr_srp_d_product_top?ie=UTF8

https://www.amazon.com/dp/B07GB4CQQ3/?coliid=I3DMV60RNKQJY1&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_it

Differential probe:
https://www.amazon.com/dp/B0795XQR68/?coliid=I15ZUNV9NXKO6S&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_it

[Ian.M]

It depends - some tests are very difficult to do with a differential probe as affordable ones tend to lack sensitivity and/or bandwidth, or you may need more channels than you have differential probes.   The best option within your budget (after a full safety assessment) may well be to float the D.U.T, but then you also have to isolate any test signals you may need to apply to it.  (e.g roof-top aerial or the output of your bench signal generator)

[David Hess]

A differential probe is the more universal solution.  Just isolation leaves considerable shunt capacitance on the common input of the probe which may or may not be a problem and can definitely be a safety hazard; do not rely on USB isolation when measuring power line side circuits.

[gfmucci]

What are the specific downsides to the use of the linked Micsig differential probe compared to more costly ones?  Do those downsides relate mainly to safe voltage limitations of items under test, or to other factors?

[David Hess]

Safety is difficult to evaluate outside of the specifications.

I think long term reliability is questionable for all of the low cost high voltage differential probes because of poor substrates for the high impedance dividers.

[gfmucci]

Your concerns sound like if the probe was used a few hours a day, several days a week the this cheapish probe may have a substrate problem sooner.
 
Conversely, if it is "lightly" used for a few hours a month the longevity should not be an issue?

[David Hess]

Your concerns sound like if the probe was used a few hours a day, several days a week the this cheapish probe may have a substrate problem sooner.
 
Conversely, if it is "lightly" used for a few hours a month the longevity should not be an issue?

No, it is related to age and not use, but humid conditions would likely cause problems sooner.  High impedance dividers require special or qualified substrates to prevent "hook".  Common unqualified FR4 is *not* suitable for long term reliable operation.

https://www.eevblog.com/forum/projects/getting-rid-of-_hook_-and-hidden-capacitance/

[gfmucci]

Your concerns sound like if the probe was used a few hours a day, several days a week the this cheapish probe may have a substrate problem sooner.
 
Conversely, if it is "lightly" used for a few hours a month the longevity should not be an issue?

No, it is related to age and not use, but humid conditions would likely cause problems sooner.  High impedance dividers require special or qualified substrates to prevent "hook".  Common unqualified FR4 is *not* suitable for long term reliable operation.

https://www.eevblog.com/forum/projects/getting-rid-of-_hook_-and-hidden-capacitance/

How about if the user is 74 years old?  "Long term" then equates to just a handful of years.  Please define "long term" which is a relative term.

[Ian.M]

@David Hess,
Unfortunately the drop-box'ed document in the topic you linked has gone AWOL.  Some quick googling leads me to think it may have been:

Doeling, W., Mark, W., Tadewald, T., and Reichenbacher, P., "Getting Rid of Hook: The Hidden PC-Board Capacitance." Electronics, October 12, 1978
but I cant find it online (other than possibly behind academic paywalls).

@gfmucci,
I think David's implying that if your lab's not air conditioned with relatively low RH, storing budget HV probes in a sealed container with an active desiccant sachet would be prudent.  I don't know how anal about it you need to be, its probably climate dependent.

[gfmucci]

Thanks, Ian.  My bench is in my attached garage.  In the Central Florida summers, the garage is hot as hell enough to keep the humidity around 50%.  In winter its probably a bit less.  I guessing humidities above 70% will become problematic.

[David Hess]

How about if the user is 74 years old?  "Long term" then equates to just a handful of years.  Please define "long term" which is a relative term.

I have heard of them drifting out of specification within months, but most users would not notice.

Unfortunately the drop-box'ed document in the topic you linked has gone AWOL.  Some quick googling leads me to think it may have been:

Doeling, W., Mark, W., Tadewald, T., and Reichenbacher, P., "Getting Rid of Hook: The Hidden PC-Board Capacitance." Electronics, October 12, 1978
but I cant find it online (other than possibly behind academic paywalls).

That is the article.  I have a copy but last time even after compression, it was a bit too large for the forum to accept.  I will try including it again.

I think David's implying that if your lab's not air conditioned with relatively low RH, storing budget HV probes in a sealed container with an active desiccant sachet would be prudent.  I don't know how anal about it you need to be, its probably climate dependent.

That could delay the problem.

[Ian.M]

Interesting . . .
Thanks for the article.