ehsjr said:
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)#
According to the specs, the accuracy is a percentage of the reading that is
being observed. I take that to mean that at small readings, you have an
equivalently small error since it's a percentage (well...+ uncertainty
digits which never changes and is certainly a large part of the reading when
trying to measure 50nA). Hence the 30-70nA expected reading. Now that I've
read the manual, I see that their marketing material was a little optimistic
on the number of digits in current mode. Turns out to be 20 and not 2 like
the marketing slick says, hmmm..... is there no truth in advertising
anymore? At least there is a delta button to erase away the noise reading
of about 10nA after it settles down.
I would hope that the days of needing to keep things in the upper third of
the scale went out with the analog meters. But aparently they haven't. I
was looking at the manual and they mention that true RMS readings are
specified over 5% - 100% range. Tell me what this means: "Maximum Crest
Factor <5:1 at full scale, <10:1 at half scale" and then they add some extra
that makes it sound like they are only referring to AC signals that are
non-sinusoidal with that.
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.
And you're right, it is inaccurate when you look at it like that.
Fortunately by using the delta button or just a little quick math to
subtract the idle reading, I can obtain what I need. Sort of.... ;-)
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.
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.
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.
Sorry about that, here you go:
http://www.extech.com/instrument/products/alpha/MM560_570.html
Is there an antidote for "test equipment lust"?
Not that I can see.
No, I did. I thought you were talking about meter accuracy
when you said 10% - you were talking about measurement
error.
Go ahead - make me drool! Enjoy the meter.
And the scope.
Got it today, I like it so far.
See the new thread on the Extech vs.
Micronta shootout.
