Re: Differential probes

On 20 Jun 2006 05:08:46 GMT, Jim Yanik <jyanik@xxxxxxxxx> wrote:

John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
news:ioge92lem7883ifqr24k90tet34se42dqp@xxxxxxx:

On Mon, 19 Jun 2006 16:49:57 -0700, Chris Carlen
<crcarleRemoveThis@xxxxxxxxxxxxxxx> wrote:

Hi:


I have a Tek P5205 1:50/1:500 100MHz probe. I just tested it with a
1MHz square wave and it gives nice clean edges with little ringing.

However, a Fluke DP120 20MHz 1:20/1:200 probe loaned to me by an Agilent
sales rep. gives miserably ringing edges. The Fluke is kind of silly in
having 4 ft. long heavy cables with huge probes that look suitable for
heavy duty power distribution probing. Not very convenient for little
stuff.

But worse, the Fluke gives horribly ringing edges. I tried making the
test cables into a twist-pair, which helped a little but not much. Do
the designers ever really think that the thing can give meaningful
measurements of 20MHz signals with such a huge pickup loop area! Also,
the FP120 is about 4x more noisy than the Tek. The Fluke must have a
power supply in there to generate a negative supply, whereas the Tek
just pulls clean power from the scope.

Ugh!

The problem with the Tek is it can only work with the scope. But I need
to look at a filtered signal to get better RMS measurements of PWM BLDC
motor terminal voltages.

Although the FLuke gave a little better 1MHz CMRR of about -66dB vs. Tek
-49dB.


There are also a bunch of probes on the market that look like this:

http://www.probemaster.com/activedifferential.html
http://www.linkinstruments.com/adf25.htm

I wonder how they perform?


Oh well, just deliberating in public.

Thanks for input.



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 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.

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.

Does anybody make male BNC connectors with plastic (insulated)
connector shells?

John



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Relevant Pages

  • Re: Differential probes
    ... the Fluke gives horribly ringing edges. ... the FP120 is about 4x more noisy than the Tek. ... The problem with the Tek is it can only work with the scope. ...
    (sci.electronics.design)
  • Re: Differential probes
    ... the Fluke gives horribly ringing edges. ... the FP120 is about 4x more noisy than the Tek. ... The problem with the Tek is it can only work with the scope. ...
    (sci.electronics.design)
  • Re: Differential probes
    ... heavy duty power distribution probing. ... the Fluke gives horribly ringing edges. ... the FP120 is about 4x more noisy than the Tek. ...
    (sci.electronics.design)
  • Re: Differential probes
    ... The Fluke is kind ... the FP120 is about 4x more noisy than the Tek. ... The problem with the Tek is it can only work with the scope. ... There are also a bunch of probes on the market that look like this: ...
    (sci.electronics.design)
  • Re: Differential probes
    ... 1MHz square wave and it gives nice clean edges with little ringing. ... The Fluke is kind ... the FP120 is about 4x more noisy than the Tek. ... there is also a crowd who make the best damn diff probe there is. ...
    (sci.electronics.design)