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AN8008 US $19, 9999count, 1uV, 0.01uA, 0.01Ohm, 1pF resolution meter

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IanB:

--- Quote from: ocw on July 16, 2017, 07:31:17 pm ---Let's take a specific example.  I took my voltage reference which has a 5.00377 volt output and connected my 100 ohm 0.005% resistor (Y1453100R000V9L) in series with it.  On my Agilent 34401A meter I measured 47.4017 mA.  Under identical circumstances my AN8008 measured 49.9 mA!  That's because the shunt resistance on the 34401A is 5.022 ohms as compared to 0.0301 ohms on the AN8008.  If you put both meters in series I would expect both meters to read about 47.4 mA due to the 34401A's shunt resistance.  Which method is more meaningful and/or accurate?  I want to know how accurate the current measurement is compared to the current without the meter inserted (50.0377 mA).

In the above situation the AN8008 was more accurate than my relatively expensive 34401A.  However, if you correct the 34401A's reading, its accuracy is -0.016% as compared to the AN8008's accuracy of
-0.2452%.  I'm trying to find the meter which has the best accuracy without requiring me to use my "slide rule."

--- End quote ---

I think you have many people here mystified and scratching their heads.

For one thing, how do you know that your voltage source is stable when subjected to a 50 mA load?

The correct thing to do in this situation is to measure the voltage across the 100 ohm resistor. Firstly this is an accurate and already inserted current shunt. Secondly this minimizes any influence the meter has over the circuit under test. And thirdly this will indicate whether the voltage reference is maintaining its specified value.

MBY:

--- Quote from: boffin on July 16, 2017, 08:06:09 pm ---
--- Quote from: stj on July 16, 2017, 06:05:20 pm ---
--- Quote from: boffin on July 16, 2017, 05:50:36 pm ---The AN8008 looks great except the two missing mA ranges.  Does anyone know if the AN8002 suffers from the same fault?

--- End quote ---
take a look

--- End quote ---

Many thanks, looks like no missing ranges. 

* 0-60mA (10u)
* 0-600mA (100u)
* 0-6A (1m)
* 0-10A (10m)I'll probably pick up a couple of these as giveaways for my next course at work.

--- End quote ---
For clarity: AN8002 suffers from the "opposite" problem, no µA-ranges. 10µA is the lowest resolution (as you see in your table). Just pointing that out because it's easy to assume that even an el cheepo meter has like a 600 µA-range.

ocw:

--- Quote ---For one thing, how do you know that your voltage source is stable when subjected to a 50 mA load?
--- End quote ---

I measure it as you suggest.  I have either 0.005% or 0.01% resistors with values of 10, 50, 100, 1k, 2.5k, 9k, 10k, 49.9k, 100k, 1M and 10M.  When using the higher resistances for lower current verification there's not much need to worry about voltage drop.  Voltage drop is a serious concern with the lower resistances/higher currents.  That is why my 10 ohm resistor is a four terminal resistor.  The same resistors can be used for resistance measurement accuracy.

My method confirms current measurement accuracy for the way that I typically measure current.  By accounting for the meter's shunt resistance the same reading will show compliance with the manufacturer's ratings.  The resistors are not inexpensive but my method can better point to which meter will be more accurate for a particular task.  As past pointed out, there can be times when the AN8008 will provide a more accurate current measurement than my 34401A will when I only want to look at the meter's display.

alm:
Or instead of going through all that, just feed it close to the maximum current of the range and measure the voltage across the terminals. This measures the actual burden voltage, which is the specification you are comparing here (and something you can compare to the datasheet value). It is well known that even expensive high-resolution meters do not necessarily have a low burden voltage. That is why Dave developed the µCurrent, and why there are instruments with feedback ammeters that reduce the burden voltage to almost zero. See the the Keithley low level measurements handbook for details.

MacMeter:

--- Quote from: stj on July 16, 2017, 06:05:20 pm ---
--- Quote from: boffin on July 16, 2017, 05:50:36 pm ---The AN8008 looks great except the two missing mA ranges.  Does anyone know if the AN8002 suffers from the same fault?

--- End quote ---

take a look

--- End quote ---

Thanks, interesting I could never find that manual online!

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