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By MichaelN
#118804
fll-freak wrote:...But if I do this with my elderly Lecroy, the ground clip drags the V/2 reference to ground. (The V/2 being created from a voltage divider). And I get sparks if I clip to the 5 volt rail. And yet, I have never experienced these issues with much more newer and expensive scopes at work. If all scopes tie the probe tips to ground, then I am at a loss to understand how this could have worked.
If the circuit you’re measuring has no connection to main / protective Earth, then you should be able to clip the ground clip of the scope probe anywhere you like without issue.

Circuit “ground” in most cases is NOT the same as mains / protective Earth. Not sure what's happening in your case , but as I said I have never seen a scope that didn't bond the ground clips to mains earth...
By mac
#118809
All scope I used had ground to earth (at least when AC mains operated).
But on most scope you can add a differential input probe, which are input to a differential amplifier.
In all case inputs are not isolated from ground/earth and have limitation in maximum voltage.
By fll-freak
#118826
It would appear that the power supplies I use at work are fully isolated from earth ground. This is why I have been able to pick an arbitrary location for the probe's ground clip. These are expensive Agilent type power supplies. At home I am using computer power supplies modified to run as lab supplies. The grounds of these are most definitely tied to earth.

And with this a small light bulb has formed over my head. This explains numerous small strange things that have happened to me over the years.

And the light buld has formed a small dark cloud. How am I going to measure a small signal with a large DC offset if I can't reference an arbitrary ground? I would normaly just AC couple, but my signal is at 2 Hz, well bellow the AC filter cutoff. Sounds like finding a scope I can run of batteries might be a good strategy, or running my circuit of batteries during test. Or, is there such thing as an isolation 'thing' I can use to isolate by power supply or scope from each other?
By waltr
#118827
You could use an isolation transformer to power the scope or power supply from the AC mains.

Or used two scope probes and then do a difference trace of the two inputs if your scope has this function.
By AndyC_772
#118991
fll-freak wrote:It would appear that the power supplies I use at work are fully isolated from earth ground. This is why I have been able to pick an arbitrary location for the probe's ground clip. These are expensive Agilent type power supplies. At home I am using computer power supplies modified to run as lab supplies. The grounds of these are most definitely tied to earth.

And with this a small light bulb has formed over my head. This explains numerous small strange things that have happened to me over the years.

And the light buld has formed a small dark cloud. How am I going to measure a small signal with a large DC offset if I can't reference an arbitrary ground? I would normaly just AC couple, but my signal is at 2 Hz, well bellow the AC filter cutoff. Sounds like finding a scope I can run of batteries might be a good strategy, or running my circuit of batteries during test. Or, is there such thing as an isolation 'thing' I can use to isolate by power supply or scope from each other?
Almost all scopes have their ground leads earthed, though there are one or two that don't. The Tek TPS2000 series, for example, has isolated channels and is intended for people working on power supplies.

Do bear in mind that even if you have dc isolation between the scope's earth and the device you're measuring across, there may still be significant capacitance which will affect your measurement. For example, I always have the scope itself earthed but sometimes use a lab supply which is floating so I can earth an arbitrary point without dc current flowing. But that's not so say there isn't capacitance between the PSU output and earth even if there's no dc connection, so the circuit under test still sees the scope ground connection as a capacitor to earth. At high frequencies there's no substitute for a true differential measurement using two channels of the scope, or better still, a true differential probe.

To measure your 2Hz signal, just leave the scope in dc coupled mode and fit a capacitor between your signal point and the probe tip. If you have a x10 probe then you have a time constant of 10MOhms*C, so a 10uF cap should allow you to measure it without significant degradation (100 sec time constant), and 1uF might even be more suitable. Bear in mind that you'll need to connect the signal to the scope for some time before the cap charges to the correct dc level and the trace appears on screen.
By fll-freak
#120905
Just received my DS1102D (Mixed signal 100 MHz DSO and a 16 Digital Channels logic analyzer) from Saelig. I use "lunch pail" scopes at work, but this guy shocked me for how small it was. At first glance it looked like a toy. After 30 minutes of playing with it I realized it was not. Certainly not a $200,000 10GHz Lecroy put this little engine has some guts.

Had to cut my play time short because I had to build an emergency "octave vent remover". Anyone want to guess what one is and does? (Clue: it has nothing to do with electronics).
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