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

[stj]

interesting thing in the manual for people who bitch about the fuses,
the maximum voltage for current readings is 36v DC or 25v AC.

[ocw]

Two meters in series Carry current the same

Yes, but Ohm's Law says that the current through both will down from what it was before the meters were inserted.  Again, I'm talking about the typical currents which I measure during repair and alignment.  Those are almost always not constant currents.

The attachment shows how I label all of my similar meters.

[Mark Hennessy]

Putting two meters in series makes the limitations of both meters influence the other.

Howcome? 

Instead of just the shunt resistance of the "tested" meter limiting the current, you will have the two shunt resistances in series limiting the current.  I want a meter which won't change the actual current when it is inserted.  The added shunt resistance will lower the current unless you are measuring a constant current.  I don't often measure that when I am using the meter for repairs or alignment.  That's when I want it to be accurate.

That isn't the "limitations of both meters influencing the other". Rather, that's the extra resistance of the measurement meter slightly reducing the current in the circuit to the DUT.

But how is that a problem when the other meter is telling you the test current?

I've no idea what the voltage is, and I've no idea how accurate the 50 ohm resistance of my Wavetek 162 is. But it doesn't matter. As I said, I simply want to see how the DUT compares to my existing meters.

I adjust the amplitude of the Wavetek until the measurement meter says something close to the sort of current I want. Then I write down both numbers, and calculate how close they are. Simple. I'm aware of the limitations, but I'm no voltnut.

BTW, how does your method account for the non-linear resistance of the fuse?

[ocw]

That isn't the "limitations of both meters influencing the other". Rather, that's the extra resistance of the measurement meter slightly reducing the current in the circuit to the DUT.

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."

[boffin]

The AN8008 looks great except the two missing mA ranges.  Does anyone know if the AN8002 suffers from the same fault?

take a look

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.

[Mark Hennessy]

That isn't the "limitations of both meters influencing the other". Rather, that's the extra resistance of the measurement meter slightly reducing the current in the circuit to the DUT.

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 49.9mA!  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."

Sorry, I feel like I'm clearly missing something really obvious.

Why does the actual current matter? Why go to the effort of using a precise voltage and a precise resistance in the hope of achieve a particular current when the total resistance added by the meter will be affected by the fuse (which is non-linear) and connectors/wires in addition to the shunt?

Or to put it another way, what is wrong with what I'm doing?

As I say, I don't care about the actual value of current - I just want to see how it compares to my known-good meters at certain approximate points, and to achieve that goal, I can't see any flaws with putting two or more meters in series. No slide rule required.

A current meter is specified in terms of the current passed through it, not the reading that results when some arbitrary voltage source is connected to it via some arbitrary resistance, no matter how precise those might be.

Obviously any current meter will have a shunt (and fuse, etc), so the current will be affected when you insert it into a circuit. The value of this resistance is just a fact of life, and is hardly a "figure of merit" given that all current meters must have a shunt to work. Just about any measurement will change things to a small extent; it's up to the operator to consider the effect of the particular shunt given the context: often it simply doesn't matter; other times, the burden voltage might indeed be a problem. But all that is quite separate from determining the basic accuracy of a current meter.

[Fungus]

The AN8008 looks great except the two missing mA ranges.  Does anyone know if the AN8002 suffers from the same fault?

No, it suffers from completely different ones.

[Fungus]

interesting thing in the manual for people who bitch about the fuses,
the maximum voltage for current readings is 36v DC or 25v AC.

You mean the maximum 10 amps will pass at 36V?

That's how Ohm's law works, yes, and you'll find similar values for other meters, even Flukes!

The usual workaround is to put something else in series with the multimeter. That way the voltage drop across the multimeter will only be a tiny fraction of the total voltage from the supply and (hopefully) much less than 36V. 

[ocw]

Or to put it another way, what is wrong with what I'm doing?

I thought that my example made it obvious.  Doing it your way both of my meters connected in series read about 47.4 mA.  Doing it my way one meter reads 49.9 mA while the other reads 47.4 mA.  It's clear which one is more accurate in measuring current when you do not want to go to the trouble of correcting for the meter's shunt resistance.

Let's say that my 5.00377 is a critical voltage supply which can't dip below 5.0 volts.  Connecting the 34401A will lower the voltage to about 4.7658 volts while the AN8008 insertion will only cause the voltage to drop to 5.00227 volts.

While my expensive electrometers can measure current with insignificant insertion resistance (or burden voltage), my relatively inexpensive ADA4530-1 current to voltage converter can do the same thing.  Because of that I want the manufacturers to start adding that feature in more moderately priced meters.  If you shrug your shoulders and say that you are happy the way things are nothing will change.

[The Soulman]

interesting thing in the manual for people who bitch about the fuses,
the maximum voltage for current readings is 36v DC or 25v AC.

You mean the maximum 10 amps will pass at 36V?

That's how Ohm's law works, yes, and you'll find similar values for other meters, even Flukes!

The usual workaround is to put something else in series with the multimeter. That way the voltage drop across the multimeter will only be a tiny fraction of the total voltage from the supply and (hopefully) much less than 36V. 

It has everything to do with the voltage rating of the fuse, if the fuse fails the voltage that is across your supply now also is across the both ends of the fuse, just a couple of millimeters won't hold back a high-voltage.
That is why high voltage fuses are physically larger, and have a higher burden voltage.

[Fungus]

interesting thing in the manual for people who bitch about the fuses,
the maximum voltage for current readings is 36v DC or 25v AC.

...

It has everything to do with the voltage rating of the fuse, if the fuse fails the voltage that is across your supply now also is across the both ends of the fuse, just a couple of millimeters won't hold back a high-voltage.

Sure, but 37V isn't a high voltage. It's not going to jump that fuse.

FWIW most of those 10mmx3mm fuses are rated at 250V. In theory they'll stop 240V AC mains.

http://www.newark.com/c/circuit-protection/fuses-fuse-accessories/fuses/cartridge-fuses?fuse-size-metric=3.6mm-x-10mm

(not that I'd use this meter for mains work, and do we really have to keep laboring that point?)

[Mark Hennessy]

Or to put it another way, what is wrong with what I'm doing?

I thought that my example made it obvious.  Doing it your way both of my meters connected in series read about 47.4 mA.  Doing it my way one meter reads 49.9 mA while the other reads 47.4 mA.  It's clear which one is more accurate in measuring current when you do not want to go to the trouble of correcting for the meter's shunt resistance.

Let's say that my 5.00377 is a critical voltage supply which can't dip below 5.0 volts.  Connecting the 34401A will lower the voltage to about 4.7658 volts while the AN8008 insertion will only cause the voltage to drop to 5.00227 volts.

While my expensive electrometers can measure current with insignificant insertion resistance (or burden voltage), my relatively inexpensive ADA4530-1 current to voltage converter can do the same thing.  Because of that I want the manufacturers to start adding that feature in more moderately priced meters.  If you shrug your shoulders and say that you are happy the way things are nothing will change.

Yes, obviously, a current meter has a burden voltage (or resistance, if you prefer). And sometimes we have to take it into account... And yes, it would be lovely if manufacturers could minimise it somehow. I'm not shrugging my shoulders or otherwise; I'm simply not worrying about it right now because it's a completely different issue to the question of how closely a DUT meets its published specification for current accuracy.

To validate the manufacturer's specifications, you have to measure the way they did. So do you think they did what you're doing? I'm pretty sure that a DMM calibrator uses a current source for this, not a voltage source and low value resistor. Just a thought experiment: how about trying 1000V and 20k - what would that do for your numbers?

As I say, burden voltage, is quite separate from the basic accuracy as quoted in the spec. By all means complain about burden voltage - it's a noble cause - but you can't conflate the two

I appreciate your time here, but in the context of reporting whether or not the AN8008 meets its specification, I think I'm happy with what I'm doing

[The Soulman]

interesting thing in the manual for people who bitch about the fuses,
the maximum voltage for current readings is 36v DC or 25v AC.

...

It has everything to do with the voltage rating of the fuse, if the fuse fails the voltage that is across your supply now also is across the both ends of the fuse, just a couple of millimeters won't hold back a high-voltage.

Sure, but 37V isn't a high voltage. It's not going to jump that fuse.

FWIW most of those 10mmx3mm fuses are rated at 250V. In theory they'll stop 240V AC mains.

http://www.newark.com/c/circuit-protection/fuses-fuse-accessories/fuses/cartridge-fuses?fuse-size-metric=3.6mm-x-10mm

(not that I'd use this meter for mains work, and do we really have to keep laboring that point?)

There is a big difference between breaking ac or dc as ac (mains) inherently goes to o volts 100 or 120 times each second and therefore will remove the arc, after that there is enough space and material to prevent another arc-over.
DC on the other hand is much harder to brake, as the first arc doesn't die out by itself there is much more space and material needed to prevent it.
Also switches and relays have much lower dc than ac ratings.
I couldn't find a dc rating at all for the fuses you've linked to, not surprising as they are intended for ac mains only. 

[alm]

I'll probably pick up a couple of these as giveaways for my next course at work.

If you do that, at the very least put a label over the CAT ratings indicating not for use over 50 V or something like that.

[MacMeter]

The 8002 was pretty nice for the price.   For low voltage, low energy it's way ahead of the free meters from HR.

Because it failed at a low enough level, it made it a prime candidate to run on my half cycle generator.  Still not a lot of energy with this setup.   
https://youtu.be/HrcxnbkkhYg?t=1793

That's the one I bought, the ZT102, same as the 8002. This 8008 looks good too for the price.

[IanB]

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."

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]

The AN8008 looks great except the two missing mA ranges.  Does anyone know if the AN8002 suffers from the same fault?

take a look

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.

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]

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

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]

The AN8008 looks great except the two missing mA ranges.  Does anyone know if the AN8002 suffers from the same fault?

take a look

Thanks, interesting I could never find that manual online!

[ocw]

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.

I don't understand why people like to refer to the burden voltage.  There are an infinite number of actual burden voltages which obviously depend on the actual current being measured rather than the maximum current of the range.  There are typically two or three shunt resistances.  That's why I have their values listed on the meter.  With those two or three figures you can calculate the actual burden voltage.  For the lower currents where one of the resistors is used, it probably has a relatively insignificant burden voltage.  However, at the high current limit of that one resistor the readings may have an accuracy way out of its rated specification.  Some lower price meters have protection diodes across those resistors which start to conduct even when measuring a current within its ratings.  I find that in my tests.

I already own two of the Keithley electrometers for low current measurement with insignificant shunt resistance/burden voltage.  I've made my own version of Dave's uCurrent using an ADA4530-1 (which I don't believe was available when Dave designed his circuit).  My circuit can measure femtoamps with an accuracy exceeding my ability to guarantee its accuracy.  While a commercial equivalent would need greater protection than my design, it should not add that much to the price of the mid to upper level multi-meters.

[janekm]

I've made my own version of Dave's uCurrent using an ADA4530-1 (which I don't believe was available when Dave designed his circuit).  My circuit can measure femtoamps with an accuracy exceeding my ability to guarantee its accuracy.  While a commercial equivalent would need greater protection than my design, it should not add that much to the price of the mid to upper level multi-meters.

Even if it had been available, that one chip seems to cost several times the uCurrent BOM

[ocw]

Even if it had been available, that one chip seems to cost several times the uCurrent

There are currently 67 ADA4530-1's available for $23.49 each from Mouser as compared to $69.66 USD for a uCurrent.  That was Google's translation for AU$89.00

[alm]

I don't understand why people like to refer to the burden voltage.  There are an infinite number of actual burden voltages which obviously depend on the actual current being measured rather than the maximum current of the range.

Well yes, the burden voltage is a maximum per range. A worst case that you can take into account as you design your setup. It will likely be very close to 0 V at 0 A . You can quantify it for intermediate values if you are so inclined, but no need to confuse it with accuracy.

There are typically two or three shunt resistances.

Which is why there may be a burden voltage spec per current range.

However, at the high current limit of that one resistor the readings may have an accuracy way out of its rated specification.

Is that rated specification for a current source with such a low output impedance? Are you also measuring voltage with a 1 MOhm series resistor? Then your electrometer would definitely be much more accurate .

Some lower price meters have protection diodes across those resistors which start to conduct even when measuring a current within its ratings.  I find that in my tests.

The effect at full scale currents should be included in the burden voltage specs.

While a commercial equivalent would need greater protection than my design, it should not add that much to the price of the mid to upper level multi-meters.

The fuses will still represent a significant resistance, so I am not so sure how much you would gain by reducing the shunt resistance. This is also why the µCurrent does not have a fuse (and is not marked CAT III ).

Even if it had been available, that one chip seems to cost several times the uCurrent

There are currently 67 ADA4530-1's available for $23.49 each from Mouser as compared to $69.66 USD for a uCurrent.  That was Google's translation for AU$89.00

You left off the word BOM in the quote. If the µCurrent sold for US$ 70, then I would expect US$ 23 to be close to the BOM costs.

[Mark Hennessy]

I express burden voltage as volts-per-amp, or millivolts-per-milliamp, or whatever.

However, it can only be approximate because - as I've mentioned at least once already - the fuse is non-linear.

Take a typical 20mm 500mAT fuse - the sort found in cheap multimeters for the mA range - and measure the voltage across it for a range of currents. Last time I did this, the value of resistance at ~500mA was practically double what it was at ~100mA. I've no idea if "proper" HRC fuses are better or worse; at ~£15 a go, that's a lot to risk if something goes wrong. Someone else can run that experiment