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Old 13th May 2012, 06:45 AM   #11
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Floating Oscilloscope Measurements …And Operator Protection
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Old 13th May 2012, 07:15 AM   #12
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by akis View Post
I will try to fix it. Seeing as differential probes cost thousands of dollars.
Not necessarily. Check this one out.

Your diy creation might work fine, but basically it's unproven and untested safety-wise. I'd strongly suggest you limit its usage to separated extra low voltages only!
According to IEC, separated extra low voltages (SELV) are < 50 Vrms and < 120 VDC. These are high enough to give you a shock, though...

Disconnecting a scope's safety earth or protective earth (PE) is potentially dangerous. Those that know the dangers may do so at their own risk, but they must never recommend such procedure. When measuring high floating voltages, the only thing that can be recommended is a certified differential probe and that rules out diy. This may sound exaggerated, but when it comes to safety, it's "safety first".

Last edited by jitter; 13th May 2012 at 07:45 AM.
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Old 13th May 2012, 08:03 AM   #13
akis is offline akis  United Kingdom
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I have tried 4 different op-amps with the exact same results: LM833, LM4562, TL082 and MC33072.

The problem seems to be around U3A - it is very sensitive to any parts "moving" around it, and placing a small cap across the 100K feedback resistor has dramatic, but uncontrollable effect.

The problems seem to start at about 50KHz and peak at 500KHz. Anything below 50KHz measures very correctly. I will see if this is "simulate-able" , else I am giving up
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Old 13th May 2012, 08:26 AM   #14
akis is offline akis  United Kingdom
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Quote:
Originally Posted by jitter View Post
Not necessarily. Check this one out.

Your diy creation might work fine, but basically it's unproven and untested safety-wise. I'd strongly suggest you limit its usage to separated extra low voltages only!
According to IEC, separated extra low voltages (SELV) are < 50 Vrms and < 120 VDC. These are high enough to give you a shock, though...

Disconnecting a scope's safety earth or protective earth (PE) is potentially dangerous. Those that know the dangers may do so at their own risk, but they must never recommend such procedure. When measuring high floating voltages, the only thing that can be recommended is a certified differential probe and that rules out diy. This may sound exaggerated, but when it comes to safety, it's "safety first".
OK, but still hundreds of pounds It is fascinating that what I am trying to do has already been "built" commercially since I had no idea that active, differential probes even existed.

There are a few reasons to try and build this circuit at home :

1) saves one hundreds of pounds (or thousands of dollars)
2) much preferable to removing the earth lead from your scope
3) I do not really care for 100MHz - just 500KHz will do nicely - of course if the circuit is good for higher frequencies then so much the better.
4) I do not care to measure thousands of volts - just 100 Volts AC peak is also fine. Anyway, my scope has a limit at 400V - same precautions as to when using the passive leads apply.
5) I do not care about super fine accuracy down to a few mV at GHz - I do not need to make a "certified" laboratory instrument.

So I think that when I meantioned "floating voltages" some people instantly thought "many hundreds or thousands of volts in high tension photovoltaic circuits, or tube amplifiers" but I say again this is not the reason to try and do this.
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Old 13th May 2012, 08:31 AM   #15
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"So I think that when I meantioned "floating voltages" some people instantly thought "many hundreds or thousands of volts in high tension photovoltaic circuits, or tube amplifiers" but I say again this is not the reason to try and do this."

Then why didn't you say so? Accuracy in asking will get you much more appropriate advice!
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Old 13th May 2012, 09:39 AM   #16
akis is offline akis  United Kingdom
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Quote:
Originally Posted by cliffforrest View Post
"So I think that when I meantioned "floating voltages" some people instantly thought "many hundreds or thousands of volts in high tension photovoltaic circuits, or tube amplifiers" but I say again this is not the reason to try and do this."

Then why didn't you say so? Accuracy in asking will get you much more appropriate advice!
I am trying, I am trying
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Old 13th May 2012, 11:16 AM   #17
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If interested, here's one more good reference:
A Primer on Differential Measurements, Types of Amplifiers, Applications, and Avoiding Common Errors
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Old 13th May 2012, 12:00 PM   #18
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Good link! Thanks!
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Old 13th May 2012, 12:21 PM   #19
PChi is offline PChi  United Kingdom
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In my limited experience the differential probes where the amplifier is close to the tip have good common mode rejection and have been useful for looking at noise. Though very expensive for what they are. The types which have a seperate box with BNC connectors for conventional probes haven't worked much better than setting the Oscilloscope to add Channel 1 and Channel 2 and invert Channel 2.
You have used rather high value resistors. Your frequency response irregularities may be due to stray capacitance and the input capacitance of the amplifiers. A few pF across R3, R9 R7 and R8 might help.
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Old 13th May 2012, 12:44 PM   #20
gpapag is online now gpapag  Greece
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Akis
You may have a look on this one.

Implementing a “Y Out” on an Oscilloscope

It works for me.
Check out the IC datasheet to see if it is OK for your requirements

George
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