Calibration Of Electronic Equipment In The Home Workshop

It drops it like any other shunt would. If you were designing precision equipment, would you attempt to measure nA by using a .01Ohm resistor? Read Ed's post, he is right about being able to measure low currents that way, just as Robert is here. They just don't work if the current is varying over a wide range. But for static current measurements, THEY WILL WORK FINE.

You've got to be kidding! Don't you design/build HV supplies? Tell me that your feedback circuits sense the full output voltage. HA HA HA I guess you don't use any micros, huh? Or do you just use a 100Ohm and .01Ohm resistive divider to sense that 50KV output? I'd really like to see that.

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.

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.

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.

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

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.

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.

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.

Try looking at the Extech I just ordered. .1%+2d 50000 count.

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?

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. :-)))))))

We expect internal pics to be posted ASAP!

I'm curious about the build quality of the Extech...

Dave :)

Well, I guess while I'm in there butchering up the knob spring and resetting the cal using my Micronta, I might as swell snap some pics. ;-)

Me too, for that much money it better be as good as a Fluke.

The scope wont be here for a couple of weeks. :-( I hope it's as good as the manual makes it sound. What do you think of this:

No fair waving the red flag at the bull now, he must be tired from all that charging!

this:

Love the Jinglish: "Digital popularization storm-the best substitute of EDU series digital oscilloscope for analog oscilloscope

EDU digital storage oscilloscope is the best choice to substitute for traditional analog oscilloscope though it don't coming with surprising function and specification. Thanks to the creative R&D thoughts and strict cost control, EDU series is outstanding when compete with analog oscilloscope even in price. EDU digital storage oscilloscopes display by STN LCD big screen in compact design, it is available at bandwidth of 20MHz to 40MHz and real sample rate of 100Ms/S.The series satisfy the detection of electric manufacturing production lines as well as basic mathematics needs.

Hesitation is wasting! We highly expect the "Digital popularization storm "will bring the reliable measurement instruments which "Meet your best needs" for you."

Hilarious!

25MHz bandwidth and 100MS/s ain't exactly going to blow the lab down. It sucks for sure, but the question is only buy how much?

Dave :)

Oops wrong one, try here:

60MHz, TFT 6K depth, all the normal bells and whistles (USB memory stick, printer etc) and I think it might have a 7.8" screen. Not bad for under $600. It seems to be a match for the current generation of thin (6") depth units from China. For only $70 more, you get a 3hr battery and charger.

Check that other one at Saelig, it's made by Owon. 60MHz 250Msa/s 6K PDS6062T not bad looking.

Yeah that's what I loved about the Rigol manual, not a trace of engrish anywhere. I wonder if a customer wrote it for them. ;-)

this:

..."basic mathematics needs" ?????

Presumably they are talking about cursors and the auto-math features like Vavg, Vpp, Vrms, duty cycle etc.... Act now, hesitation is wasting! ;-)

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.

(FlukeProducts)&parent=APP_NOTES(FlukeProducts)#

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.

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.

Is there an antidote for "test equipment lust"?

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. :-)

Ed

(FlukeProducts)&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 For 11 uA, put a 10K .01% resistor in series with

Got it today, I like it so far. :-) See the new thread on the Extech vs. Micronta shootout.