Anthony said:
The meter doesn't change accuracy based upon the scale it's using, it only
changes resolution. It remains .03% accurate. Whether reading Amps,
milliamps, or microamps.
Take a look at the specs. They most certainly
do change, depending on the scale you are using.
Read the Fluke app note Understanding DMM Specifications.
Noise becomes a significant factor at the low end of a
range within the meter, and in general when measuring very
small voltage or current. And the specs, regardless of
noise, vary from range to range.
http://assets.fluke.com/datasheets/2153ExtSpecs.pdf
http://us.fluke.com/usen/support/ap...ukeProducts)&parent=APP_NOTES(FlukeProducts)#
I guess I wasn't clear or we're not understanding each other. The meter
will be in a ranger where 500uA is the full scale reading. 10's of nA is
two decimal places.
Ok, I see what you are saying. To get accuracy on
that scale, you would need at least 6 digits displayed,
and that's before you consider any error in the
circuitry. But I now undertsnd what you have in
mind based on what you said at the bottom of your
note, where a reading of anything from .03 ua
to .07 ua will meet your needs for your .05 ua
current. That's not the accuracy I thought you
were talking about. A +/- 20 nA variation on a
50 nA measurement is an error of 40 percent - which
I call innacurate.
But now that I understand what you have in mind,
I see your point. The way I was thinking about
it was too stringent for the example you posted,
so your example does prove the case of a kind
of measurement that fits into the under tens of
mA that I was talking about. Now that I understand
what you are saying, I think the confusion was at my end.
Your error calculation is assuming a full scale reading. The error
(neglecting the count uncertainty) at 50nA is only .125nA, it wouldn't even
show on the display.
But at 50nA it would read .03 to .07uA on my meter including the 2d
uncertainty, plenty good enough for me.
That statement clears it up for me, as I mentioned above.
To me it's a 40% error, but for what you are doing it
is accurate.
Try looking at the Extech I just ordered. .1%+2d 50000 count.
I'd like to - if you have a handy url, please post it.
If not handy, don't go digging for it. All ths talk
has piqued my interest in buying yet another DMM
(that I don't need - too many DMM's not enough time)
or at least drooling over the specs.
Is there an antidote for "test equipment lust"?
That's why I didn't buy the Fluke. The meter I bought will give me 10nA
resolution. I know it won't be dead on when reading 50nA, but it will be
close enough that I know that I didn't leave some pull-ups turned on or some
other peripheral pidling away the juice. In current mode the Extech will be
good enough for me to be sure of what's happening. Any worse accuracy, and
I couldn't be sure.
Yes, I have done time-wasting methods like this before, that's why I want a
new meter, DSO and a logic analyzer.
The 10% error is due to your technique not the DMM, you said so yourself,
and I quote:
"> For 11 uA, put a 10K .01% resistor in series with
By my calculations, a 5nA error on a 50nA reading is a 10% error or did I
miss something?
No, I did. I thought you were talking about meter accuracy
when you said 10% - you were talking about measurement
error.
I agree that these techniques are valid and worthwhile at times, but I will
stick with the convenience and accuracy of a $200 meter instead of buying $5
resistors.
I've got a tracking number and it should be here tomorrow,
I can't wait. I reall can't wait til my scope gets here.
))))))
Go ahead - make me drool! Enjoy the meter.
And the scope.
Ed
