[Test-Equipment] Fluke current clamp probes & scopes

[J. Forster]

There is NO requirement that the source impedance (20 Ohms) match the load
impedance (1 MOhm) UNLESS you are dealing with RF where there is a need to
"match" the impedances to reduce reflections.

The Load Impedance spec of 1 MOhm is to maintain the calibration of the
probe (V/mA).

The probe, most likely, has a current transformer and a resistor across
its secondary inside.

The Fluke info is entirely reasonable.

Best,
-John

===================



> I've been looking at some Fluke current clamps for use with
> oscilloscopes. These output typically 1mV/1A and have a BNC
> connector. With a banana adapter it can be used with a DMM.
>
> I looked at the specs for a couple of these clamps (i410, i200s) and
> was quite surprised to see that the output impedance is spec'd at <20
> ohms (not Kohms, or Mohms). The "load impedance" was spec'd at 1 Mohm.
>
> See here for the i410 & i1010 PDF user guide:
>
> <http://assets.fluke.com/manuals/i4101010iseng0200.pdf>
>
> here for the i200 & i200s PDF user guide:
>
> <http://assets.fluke.com/manuals/i200s___iseng0500.pdf>
>
> I'm confused about how these mV output clamp probes can be used to
> obtain an accurate signal level on an oscilloscope if the output
> impedance is not 1 Mohn (the commonly-accepted norm input for scopes).
>
> Fluke tech support was no help in determining if this was a misprint
> or to even understand how such a non-standard output impedance could
> impact measurement.
>
> I could go on the premise that Fluke wouldn't be selling any of these
> "scope" type current probes if there was such an impedance mismatch,
> but I'd really like to understand if/how well such a combination
> works.
>
> Is anyone using such a probe with a scope? Is the amplitude viewed
> correct? What model probe do you use?
>
> Thanks,
> Dave
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