Re: Differential probes


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> a écrit dans le
message de news:oq7h9298fct69o6683rijcer3ne327somh@xxxxxxxxxx
On 20 Jun 2006 15:53:10 GMT, Jim Yanik <jyanik@xxxxxxxxx> wrote:



We just got a new Tek TPS2024 scope. It's a 4-channel, 200 MHz color
scope and all four channels plus the trigger inputs are isolated. So
you can use regular 1:1 or 10:1 probes for off-ground measurements
at millivolt sensitivity. Slick.

The 50:1 attenuation of the P5205 turns low-level stuff to mush.

John


It won't have the CMRR of a true diff amp.


The TPS2024 is truly isolated to 600 volts RMS

So what? That has nothing to do with differential amp performance.
All that means is the front ends are isolated from the case to 600 V.


That sure sounds differential to me. And it sure lets me measure stuff
that nothing else can.


*almost* differential.
You can surely count on unmatched input impedance.
It'll probably work ok on a low impedance circuit, but try to measure a diff
voltage in a 10K impedance circuit. You might have some surprise.



The 7A13 has a wide input overdrive capability,has matched attentuators
carefully calibrated for low CMRR,and a true low CMRR differential
amplifier.

But it can't usefully look at signals on an opamp that's running 180
volts off ground, which is specifically why I bought the scope.

AFAIK,NONE of the TDS series were designed to have true differential
capability,just the simple "invert&add Ch2" sort like the older analog
scopes.

and goes down to 2
mv/cm, which is pretty extreme. It would be nice if it had the 10
uv/cm sensitivity and switchable bw of a 1A7A/7A22.

Many of the TDS series have an adjustable BW-limit feature.

But clear down to 100 Hz would be nice. At 10 uV/div and 20 MHz bw,
all I'd ever see would be radio stations.


I recently learned that the 7K plug-ins (only the vertical ones) work in my
DSA601 scope.
Just cut the polarizing finger and voilà, the only "inconvenience" being
that there's no dialog between the mainframe and the plug-in, so you have to
manually inform the mainframe.
So I bought 2 7A22, and having them along with another 4 inputs plug-in in a
8 traces DSO is a _very_ valuable debugging tool.




Maybe I'll make a little battery-powered preamp box to front-end the
2024 and get down to microvolt levels; switchable bandwidth would be
handy, too. It could be single-ended, since the scope provides the
isolation.


It still would make a lousy differential amp.

I don't want to play with words, I want to measure things.


Try this:

2x10 k matched
___
100 kHz sine wave >-+-|___|----> IN1
|
| ___
`-|___|----> IN2


The asymetrical impedances of a floating single ended probe will do wonders
:-)

Of course, if in your case the 180V offset is DC or,only LF, then, no pb.

--
Thanks,
Fred.


.

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  • Re: Differential probes
    ... scope and all four channels plus the trigger inputs are isolated. ... at millivolt sensitivity. ... That has nothing to do with differential amp performance. ... isolation. ...
    (sci.electronics.design)