ehsjr wrote:
Anthony Fremont wrote:
ehsjr wrote:

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

Isn't that exactly how my DMM does it?

sense to look for your meter to be accurate to 8 decimal
places for your .00000005 amp reading.

Now come on, the 8 decimal places is only assuming that the scale is in an
Amps range. The meter would be in the 500uA full scale range where 50nA is
only 2 decimal places.

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.

Those are fine ways to measuring static current levels, but they will not
work for me. Until the PIC goes to sleep, the current draw is much higher.
So much so that it would never power up thru a 2M resistor.

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

If your volt meter has a 1V maximum at full scale and one can live with 10%
error, then I agree. If it has a 100V range, then you need .01% accuracy on
your equipment to make your measurements, right?



*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. :-)

Aren't you arbitrarily relocating your base measurement scale to uA or nA
and then claiming that you're only being accurate to two decimal places?
You are still measuring current to the same "8 decimal places" in terms of
whole Amps, you just moved the decimal around.

IMO, it's not about decimal places at all, that's just a matter of scale.
It's about accuracy. 10% ain't good enough, and that's only accounting for
the error in your shunt resistors. :-)

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

Actually, I'm all for that part.

the side of wanting the best even when it is
not the best fit for what I really need.

And this is what I do as well. I'd rather have a margin of overkill than to
be constantly living with sacrifice by saving a couple of bucks on a NRE.
What I wasn't "sure" about was whether _you_ really felt that way. ;-)


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