Electronics > Beginners

Nead an insulated probe for oscilloscope

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Electro Detective:

--- Quote from: Nerull on June 05, 2019, 03:50:44 pm ---
A poor mans diff probe is to use two channels simultaneously and a math function.

If you're worried about accuracy this won't be nearly as good as a diff probe, though.


--- End quote ---

No contest on that  :-+

BUT.. with a bit of fiddling about getting both channel gains to match  :horse: and 'square as square can be' 10x probes calibration
one can get a lot of poor persons non critical tasks done in dual channel diff/invert/ADD mode,
with no wallet biting probe croc clips fitted or BANG potential earth/ground BNC/scope chassis connections necessary.
 
I've had good enough results doing this with 15 to 200mhz scopes, and on mains*** with extreme caution  :scared:

OP may get some serious bandwidth doing this with a 350mhz killa scope  8)

Best to practice this stuff*** on very low AC and or DC voltages FIRST before every session,
and take physical notes of the setup for repeatability and or future use,
before diving in and over-driving the probes and or channels into oblivion with higher voltages,

..whilst the scope screen displays no signal, due to misadjusted or bumped controls  =  :-//  :-BROKE  :-[  = $ $ $ $  :palm:


EDIT:

It would be handy to have some sort of knockup external box or easy internal mod (switchable) that can invert the signal on channel 4 on a four channel DSO,

so an impoverished user  :'(  can have two poor persons differential channels using four calibrated x10 probes

or is this pushing imagination and hope beyond DIY (+cheap n easy) feasabilty?   :-// :-[

..assuming this comment made any sense  ;D

FriedMule:
great replyes! :-)

Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?
How would you do that?

tautech:

--- Quote from: FriedMule on June 06, 2019, 12:30:33 am ---great replyes! :-)

Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?
How would you do that?

--- End quote ---
Why ?
What could you possibly learn from looking at the LV AC ripple from the transformer. We all know it will be at mains frequency.
What matters is the ripple at the PSU output under various states of load and for which you'd just connect across the output terminals (after checking 0V doesn't create ground loop issues with a DMM) and use a 1x probe for the additional sensitivity it offers.
For a low impedance source like a PSU rail the capacitive loading of a 1x probe is never an issue.

Electro Detective:

OP may wanna look at both while he tweaks   :-/O   :-//

David Hess:

--- Quote from: FriedMule on June 05, 2019, 11:48:01 am ---Am I understanding right if I am saying that I can either build a crappy probe that will be of no use anyway, or dig deep in my pocket once for all and get a decent one?
--- End quote ---

That pretty much sums it up.  The cheapest way if you have the time is to build a specialized probe into the circuit at the measurement point.

The old way was to use an isolation amplifier between the probe and oscilloscope:

http://w140.com/tekwiki/wiki/A6902


--- Quote ---But what about that 500X attenuation, would that not eliminate the 2uV noise I may look for in an circuit?
--- End quote ---

A high voltage differential probe will not be useful in that case because of its input attenuation but there may be other ways if that is your requirement.  An isolated probe will not measure 2 microvolts noise either.

An active differential probe might be able to resolve 2 microvolts under the right circumstances.


--- Quote from: AndersJ on June 05, 2019, 01:01:18 pm ---You will most likely never see 2 uV signals on a oscilloscope, regardless of which probe you use.
--- End quote ---

A Tektronix 7A22 differential amplifier can resolve 2 microvolts over a range of +/-1 volt but only at a bandwidth significantly lower than its rated 1 MHz.  Doing better than this even 40 years later would be a significant challenge but it could be done.

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