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ludwik kowalski wrote:_______________________________________________
Digital Multi Meters (DMM) sample waveforms and display updated
avarages.
1) Does anybody know what the sampling frequencies are in cheap DMMs?
Long ago Rat Shack sold one that used a dual-slope integrator with
a one-second cycle time. Ewwwww.
2) What are these frequencies in more expensive DMM's?
I have a handheld Fluke (10+ years old) that is good to 100kHz,
true RMS.
I have (relatively) cheap National Instruments boards lying around the
lab that are good to 20MHz.
4) Any potential traps in using DMMs?
More than I know how to count.
Perhaps the biggest one is Dragon Breath: The instrument injects weird
noise out of its input into the device under test.
There are many types of Dragon Breath. One kind arises if the
nominally-floating (i.e. gauge invariant) probes aren't quite,
i.e. there is some nontrivial impedance (usually capacitive) to
the rack and/or the "ground" of the power line.
Then there's aliasing at high frequencies.
Then there's eyeball-aliasing, if you only look at it intermittently,
as opposed to logging *all* of the data.
Sensitivity.
Linearity.
Multiplicative calibration errors.
Additive DC offsets.
Non-infinite input impedance.
Thermocouple effects, if the thing you're measuring isn't at room
temperature.
etc. etc. etc.