ehsjr wrote:

Anthony Fremont wrote:

ehsjr wrote:

Too_Many_Tools wrote:

I have a well stocked test bench at home containing a range of
analog, digital and RF test equipment as I am sure most of you also
do. Well the question I have is how do you handle the calibration of
your
equipment? What do you use for calibration standards for resistance,
voltage, current and frequency?

Links to recommended circuits, pictures and sources would be
appreciated.

Since this is a need for anyone who has test equipment, I hope to see
a good discussion on this subject.

Thanks

TMT


The real question is how much precision do you
really need in the home "lab"? How often have
you needed to use your DMM with how many
*accurate* significant digits? 100 minus some
*very* small percent of the time, 2 significant
digits is all you need. Do you _really_ care
if your 5.055 volt reading is really 5.06 or 5.04?



Oh hell yes, I want to puff out my chest like everyone
else and think I have *accurate* equipment.

But I'm curious as to what home circuits need meters
that can read voltage accurately to 3 decimal places?
2 decimal places? The question for current measurement:
in what home brew circuit design/troubleshooting do you
need accuracy below the tens of mA digit ? *Need*, not



You surely didn't mean tens of _mA_, did you?


I surely meant tens of mA.

I build stuff with PICs as

you know, and some of it is designed to run on batteries and needs to
go for long periods of time unattended. The current draw for a 12F683
running at 31kHz is 11uA, sleep current is 50nA. If I could only
measure current to "tens of mA", I'd never know if the PIC was setup
right for low current draw and I certainly couldn't have any idea of
expected battery life. I wouldn't even know if it was sleeping until
it ate thru some batteries in a few days instead of six or eight
months. I think I have a need to measure fractions of a uA.


You may, but not accuracy below the tens of _mA_ digit.
When you need accuracy below tens of mA, you measure
voltage across a resistance. It doesn't make a lot of
sense to look for your meter to be accurate to 8 decimal
places for your .00000005 amp reading.

Here's how you do it with accuracy at the tens of _mV_ digit:

For 11 uA, put a 10K .01% resistor in series with
the supply and measure .11 volts across it. The voltage
would range from 0.109989 to 0.110011. Keep only
2 decimal places. Your computed current, worst case,
would be off by 1 uA

For 50 nA, use a 2 meg 1% resistor and measure .10
volts across it. The voltage would range from .099
to .101 taking the 1% into account. Throw out the
last digit. Your current computation would be off
worst case, by 5 nA.

With a voltmeter accurate to 2 decimal places.
I don't know why you would



*want*. Do you even trust your DMM on an amps setting
for those measurements, or do you measure the current
indirectly? How about ohms? Would you trust any
DMM, regardless of who calibrated it, to measure
down in the miliohm numbers?

To me, the design of the circuit being mesured has
to take care of all of that crap. If it is so
poorly designed that a 10 mV departure from nominal
(that is missed by my innaccurate meter) will keep
it from working, that suggests other problems.
Yes, the home "lab" person wants extreme accuracy
to as many decimal places as he can get. But when does
he ever really need it?



When he needs it he needs it, what can I say?


I asked, looking for concrete cases. Your case
with the PIC is an excellent example of when a
person needs to know about really small currents.
It definitely fits into the difference I had in mind
between "needs" and "wants". But it does not mean he
needs accuracy out to 8 decimal places. He needs it to
2 decimal places, as was shown. Three decimal places
would be nice. :-)

Do I really "need" a new DSO?


I have no opinion on that, and it would be irrelevant
if I did. I don't know what your situation is.

Well I've managed to get by all this time without one, so maybe you
think I don't really "need" one. I see it like this though, I don't
get allot of time to tinker anymore. I'd like to spend it more
productively. Instead of fumbling around and trying to devise silly
methods to make my existing equipment do something it wasn't designed
to (like going off on a tangent to build a PIC circuit that will
trigger my scope early so I can try to see some pre-trigger history).


None of this is to argue against having the best
instrumentation you can afford, or references to
check it against, or paying for calibration and so



I don't know if I really agree with that. ;-)


Well, you're free to argue against having the best
instrumentation you can afford, or having references
to check it against or getting it calibrated or
whatever, if that's how you feel. I tend to err on
the side of wanting the best even when it is
not the best fit for what I really need.

Ed



forth. But for myself, I need a dose of reality
from time to time when I start drooling over some
accuracy specs that I will never need at home. My
bet is that most of us are seduced by that same muse.

Ed




Please see my earlier post regarding the use of a shunt box for a DVM.