Anthony Fremont wrote:
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?
I don't know what your meter does. I assume it's
like any other. If so, it uses a shunt and develops
a voltage across the shunt so it is the same principle
as what I'm taking about, but not the same values.
AFAIK, they don't use a megohm neighborhood shunt
for low current - but then, I don't have any
meters with an nA scale.
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.
Perhaps I did not make the point clearly.
When you are using your DMM and measuring something in
the neighborhood of 8 decimal places, like tens of nA,
your meter, regardless of scale, will be less accurate
than when it is measuring something in the 2 decimal
place neighborhood. The meter itself is more susceptable
to uncertainty the lower you go.
AFAIK, the current
shunt even for low current scales has a much lower
resistance than the 2meg or 100 k I mentioned. That
means that the meter has to work with a lower level
than the 110 mv those resistors produce.
Regarding scaling - DMM's have tens of mV in 2 decimal
places. Most DMM's do not have tens of nA in 2 decimal
places. To get an 8th decimal point current reading into
the 2 decimal point range, convert it to mV with a resistor.
To put it in another perspective, consider a Fluke 187.
It will give .01 uA resolution (2 digits after the decimal)
on the 500 ua scale at a claimed accuracy of +/- .25%.
We'll ignore the further 20 count uncertaincy. That's
a +/- 1.25 uA error. That measurement is useless for the
55 nA current measurement you need. The meter could show
500.00 or 500.05 or 501.25 or whatever and you would not
know whether you had 55 nA or not. On that scale, the
meeter cannot be accurate to 2 decimal places. And you
cannot throw away the third digit after the decimal - it
doesn't exist on the meter, the resolution is too poor.
The same meter, on the 3 volt (3000mV) scale is accurate
to within +/- .025% which is +/- 75 uV - again, ignoring
the further 5 count uncertainty. On the 3 volt scale
with the technique I mentioned where you throw away the
third digit after the decimal, the error is meaningless.
That digit happens to be accurate on this meter and scale,
so the error is meaningless, even if you keep it.
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.
So I guess you're stuck with a need that the fancy Fluke
mentioned above cannot meet. How _do_ you measure the
55 nA?
What I would do is bypass the resistor with a switch so
the PIC can power up and run, and monitor it while it
is active by whatever technique you choose, so that you
know it is active. When it goes inactive, open the switch
to measure the voltage across the 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?
Anyone who is not smart enough to turn his meter
range down from the 100V scale to measure mV
is not smart enough to need nA measurements.
Measuring mV with the range set to 100 is stupid.
And 10% error for a DMM is stupid. I know you are
*not* stupid. So what is your point?
Ed