Author Topic: Documentation for uCurrent Gold  (Read 15553 times)

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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #25 on: April 06, 2017, 07:24:13 am »
I will report back on AC+DC voltage plug AC voltage only.

Thank you.

How much will the uCurrent Gold's resistor TOLERANCE will affect nA measurement accuracy

Dave uses +/- 0.05% resistors (10ppm) for R2 & R9.  You suggested a change of R2 from the current 10K value to 100K (so as to get better accuracy for nA measurements).  I was able to find one SMD resistor that has a +/- 0.01% (5ppm) spec here:

http://venkel.com/part/UPTF2512-2W-P-1003UT

Do you think the difference in tolerance between 0.05% & 0.01% for a replacement 100K resistor be largely irrelevant when taking nA measurements?  (My guess is that it would be irrelevant if we are fighting this much AC noise.) 
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #26 on: April 06, 2017, 07:29:48 am »
I will report back on AC+DC voltage plug AC voltage only.

Thank you.

How much will the uCurrent Gold's resistor TOLERANCE will affect nA measurement accuracy

Dave uses +/- 0.05% resistors (10ppm) for R2 & R9.  You suggested a change of R2 from the current 10K value to 100K (so as to get better accuracy for nA measurements).  I was able to find one SMD resistor that has a +/- 0.01% (5ppm) spec here:

http://venkel.com/part/UPTF2512-2W-P-1003UT

Do you think the difference in tolerance between 0.05% & 0.01% for a replacement 100K resistor be largely irrelevant when taking nA measurements?  (My guess is that it would be irrelevant if we are fighting this much AC noise.)

I think at your interested current range, resistor's tolerance doesn't matter at all. You won't get even 0.1% accuracy from uCurrent, even with the best resistor. It is limited by offset voltage, parasitic oscillation, and a lot of bizarre things when you go down that 100nA range. Thermo couple effect on output terminals can easily give you a few hundreds of uV or even a few mV.

If you want to get 1% accuracy with calibrated system (offset RELed out), then may be you will have the luck. After all nA measurement is tricky, especially considering uCurrent is not really meant to used to measure tens of nA.

Changing 10k to 100k will help, but don't expect offset voltage will magically go away. Also if you can, increase C3 and C4 to suppress parasitic oscillation.
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #27 on: April 06, 2017, 07:36:02 am »
...uCurrent is not really meant to used to measure tens of nA.  Changing 10k to 100k will help, but don't expect offset voltage will magically go away. Also if you can, increase C3 and C4 to suppress parasitic oscillation.

C3 & C4 are 10pF now.  What larger value do you propose?

uCurrent Gold SCHEMATIC:
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #28 on: April 06, 2017, 07:37:25 am »
...uCurrent is not really meant to used to measure tens of nA.  Changing 10k to 100k will help, but don't expect offset voltage will magically go away. Also if you can, increase C3 and C4 to suppress parasitic oscillation.

C3 & C4 are 10pF now.  What larger value do you propose?

uCurrent Gold SCHEMATIC:


If you don't do high frequency things, and only wants to measure DC current, 1nF will be a starting point.
 
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Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #29 on: April 07, 2017, 07:49:47 am »
I did a test, this time with everything warmed up for 15 minutes, and output of uCurrent grounded.
Result: ~+1nA offset and <+-1% linear error. 100PLC, D-filter on.
In ACV mode, 8846A reads ~170mV AC in both 20MHz, 200MHz and 3db filter mode. This ACV voltage varies while I move my hands, so it is highly stray capacitance sensitive.
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #30 on: April 07, 2017, 08:01:09 am »
Just to clarify...

You sourced 100nA of current and then measured it with a stock (unmodified) uCurrent Gold using the 8846A in Volts DC Mode, measuring 100mV (which means "100nA") and you saw an error of only +/-1nA? 

You then switched your 8846A to AC mode and measured 170mA (170nA)?  Is that correct?

And you also used your 8846A when set to 100uA mode to measure that 100nA directly and got an accurate reading too?  Correct?  If so, I am not sure what is wrong with my 8845A.  However, I don't have the means to source accurate current like you do.  Like I said, I am using large resistor values to limit current, and perhaps that is simply something the 8845/8846 cannot handle.
 

Offline FrankBuss

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Re: Documentation for uCurrent Gold
« Reply #31 on: April 07, 2017, 08:14:43 am »
Test setup: SMU sourcing 100nA, while uCurrent is set to 10K mode, output of uCurrent is grounded with scope's ground lead, scope used here is a Keysight MSOX6004A, bandwidth is unlimited, using 700MHz 10:1 kit probe, low frequency calibrated.
Result: 500mV p-p 37.5kHz distorted sine wave noise, plus 1500mV p-p mains frequency spikes.

See this posting:

https://www.eevblog.com/forum/testgear/problem-with-ucurrent-gold/

I solved the problem with a capacitor at the input. Of course, not a solution if you want to measure AC, but works for my applications (mostly power consumption of microcontrollers and other DC things). Without it, you have to really be careful with long wires and feedback of the output to the input.
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Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #32 on: April 07, 2017, 08:20:41 am »
Just to clarify...

You sourced 100nA of current and then measured it with a stock (unmodified) uCurrent Gold using the 8846A in Volts DC Mode, measuring 100mV (which means "100nA") and you saw an error of only +/-1nA? 

You then switched your 8846A to AC mode and measured 170mA (170nA)?  Is that correct?

And you also used your 8846A when set to 100uA mode to measure that 100nA directly and got an accurate reading too?  Correct?  If so, I am not sure what is wrong with my 8845A.  However, I don't have the means to source accurate current like you do.  Like I said, I am using large resistor values to limit current, and perhaps that is simply something the 8845/8846 cannot handle.

1. Yes. I saw 1%+1mV over a range of input values, from 10nA to 100nA. Particularly on 100nA, if my memory serves me correctly, 102mV. However, the error strongly depends on grounding and stray field. Without grounding or with my hands too close to test setup, the 1% can go to 10%.
2. Yes, everything are the same, just the DMM is set to AC mode.
3. As said in my previous post, 8846A can handle nA measurements, and is more accurate than uCurrent, without oscillating nor need of warming up. I don't have a 8845A, so I can't test it. I think I can find a DMM4020 somewhere in the lab, that should also be a Fluke rebrand, likely 8845A. I can test on that one.
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #33 on: April 07, 2017, 08:39:17 am »
...8846A can handle nA measurements, and is more accurate than uCurrent, without oscillating nor need of warming up. I don't have a 8845A, so I can't test it. I think I can find a DMM4020 somewhere in the lab, that should also be a Fluke rebrand, likely 8845A. I can test on that one.

The problem is that our test cases are not the same.  You are sourcing current.  I cannot do that so I am using large resistors to ground and inserting my 8845A probes between the resistor and ground.  That is how I created this data:



It could be that your 8846A would give you the same Percent Error as I discovered, if you repeated my test case using resistors instead of sourcing current.  But that is only a guess.  (I did not have a single 100M-ohm resistor, which is why I used 10pcs of 10M-ohm instead.  As you can see from my data, I measured nA and then measured uA.  The uA test was more accurate, however it uses a smaller resistance value and higher voltage.)
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #34 on: April 10, 2017, 01:16:46 am »
Dear uCurrent users,

If you have 10pcs of 10M-ohm resistors, could you please conduct the same nA current test I did (see results in my previous post) and then post your results?

I don't have a uCurrent and that is why am seeking your assistance.  I want to know if my $1000+ Fluke 8845A is more or less accurate than the uCurrent.  But the only way to know is for me and you to conduct the same exact test.  blueskill kindly conducted tests in this thread, but he used a Current Source which I do not have access to.  Therefore, the only way to confirm my findings is if we all conduct the same exact test.  That means using 10pcs of 10M-ohm resistors.

I look forward to your kind replies.  Thank you!
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #35 on: April 11, 2017, 12:41:24 am »
The dearth of replies indicates that none of you possess 10M-ohm resistors.
I guess they are hard to find.   :palm:
 

Offline FrankBuss

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Re: Documentation for uCurrent Gold
« Reply #36 on: April 11, 2017, 01:05:06 pm »
Actually I didn't have 10 meg resistors, but bought some from eBay. I can't measure the whole resistance, but measured the individual resistors and the sum was 100.31 meg ohm. Now I have a 10 meg to 100 meg kind of variable resistor:



With 1 V the uCurrent shows 9.83 mV, with 2 V it is 19.82 mV and with 3V it is 29.8 mV. That's without my capacitor. If I carefully avoid output cables coupling to input cables, I can see the same result without the input capacitor (i.e. unmodified uCurrent). All measured with a BM257s, and not on a breadboard to avoid additional parallel resistance and getting too much noise because of the long traces in the breadboard.
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #37 on: April 12, 2017, 12:23:15 am »
Bless you for repeating my 10M-ohm x 10pcs test, FrankBuss!

That seems to prove the uCurrent, even without any hacks and without your capacitor is extremely accurate for nA measurements down to 9nA!

My US$1k+ Fluke 8845A doesn't hold a candle to that accuracy.  I actually wrote to Fluke last week.  I sent them the same "nA Measurement" data I posted in this thread.  Today they replied as follows:

Quote
Well looking at the spec at that range - percent of measurement (reading) + percent of range  that percent of range alone will add 25nA of error on the 100uA range.  So that is going to make it very shaky down there.   So I would say - no - not quite accurate enough for what you are trying to do.  Unfortunately that next jump in accuracy is an exponentially larger price tag.. :(

So it would seem quite safe and reasonable for me to purchase a uCurrent.

Many thanks!
 

Offline FrankBuss

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Re: Documentation for uCurrent Gold
« Reply #38 on: April 12, 2017, 03:57:36 am »
Quote
Well looking at the spec at that range - percent of measurement (reading) + percent of range  that percent of range alone will add 25nA of error on the 100uA range.  So that is going to make it very shaky down there.   So I would say - no - not quite accurate enough for what you are trying to do.  Unfortunately that next jump in accuracy is an exponentially larger price tag.. :(

So it would seem quite safe and reasonable for me to purchase a uCurrent.

Looking at your measurements, this makes sense. If you subtract 20 nA for your last two measurement series as an offset error, then it looks like it gets quite accurate. But I don't understand why the 100 uA range has errors in the nA range, doesn't make sense. Can you measure the 20 nA with shorted leads, too? This would allow you to measure the offset.

Of course, buying a uCurrent solves this, too, without any manual offset calculations and reasonable accuracy.
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #39 on: April 12, 2017, 04:29:11 am »
Can you measure the 20 nA with shorted leads, too? This would allow you to measure the offset.

You mean "shorter" leads?  I have more than 10pcs of SMD 10M-ohm resistors and a set of jumper pins like you.  (In my first test I used 10pcs of axial-lead resistors and I put them in a breadboard.)
 

Offline FrankBuss

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Re: Documentation for uCurrent Gold
« Reply #40 on: April 12, 2017, 07:57:50 am »
No, I meant short-circuited, but I might be wrong, maybe better to not connect the leads at all (to avoid any noise). My assumption is, that in this case it shows only the offset, and if you note this and then subtract this when measuring your circuits, then it will be pretty accurate.
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #41 on: April 12, 2017, 08:51:05 am »
Here's a video for you, Frank, showing probes connected and disconnected, with various PLC settings and AVERAGING:

https://youtu.be/ehCxeM5_ou0
 

Offline FrankBuss

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Re: Documentation for uCurrent Gold
« Reply #42 on: April 12, 2017, 10:04:21 am »
The higher value with shorted probes might be because you are inducing something with your hands or the probes loop receives more noise compared to when it is open. But it is really not very stable and maybe not good to trust that the offset is always the same. Better buy a uCurrent. It might have less burden voltage as well compared to the Fluke.
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Online 2N3055

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Re: Documentation for uCurrent Gold
« Reply #43 on: April 12, 2017, 01:12:07 pm »
Hello!

Let me just chime in a bit. First, if you don't have it, buy (or make) a shortcut plug like the one in photo.
You can use it to check zero volt offset, zero ohm offset and also small current range offset.

What you see here is probably induced interference, it's nano amperes... Everything should be shielded, your 1V source should be battery powered, shielded cables and such.

And than, last two digits on ANY 6.5 voltmeters are a finicky bunch.

Look here on Metrology section there a several topics of measuring and sourcing nano and even femtoamps...

All the best,

Sinisa
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #44 on: April 12, 2017, 09:50:45 pm »
First, if you don't have it, buy (or make) a shortcut plug like the one in photo.
You can use it to check...

Buy it where? You didn't provide any link to the place that sells them.

You also seem to be talking exclusively about the uCurrent even though we've also been discussing the Fluke 8845A.  I wish to point that out for clarity, for those who later come and read through this thread.

As to testing nA currents on battery voltage alone, consider well that such is not practical in that you may need to test small currents when connected to a AC powered source at times.  In other words, just as is the case with any handheld or desktop meter, there are times when we need to test circuits powered by batteries and times when we need to connect them tested to a wall socket because that's the environment in which they will be used.
« Last Edit: April 13, 2017, 12:43:28 am by JDW »
 

Offline casinada

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Re: Documentation for uCurrent Gold
« Reply #45 on: April 12, 2017, 11:12:42 pm »
https://www.digikey.com/products/en/test-and-measurement/accessories/613?k=4%20terminal%20shorting
http://www.newark.com/fluke/884x-short/4-wire-short/dp/55M2652
or you can make one yourself  :)
Check Joe Smith's last video where he modified his multimeter to reduce the burden voltage  :)
« Last Edit: April 12, 2017, 11:15:37 pm by casinada »
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #46 on: April 13, 2017, 12:47:46 am »
https://www.digikey.com/products/en/test-and-measurement/accessories/613?k=4%20terminal%20shorting
http://www.newark.com/fluke/884x-short/4-wire-short/dp/55M2652

We're talking about the Fluke 8845A here.  Got it.  Thank you for the links.  $36 for that.  Typical Fluke!

Check Joe Smith's last video where he modified his multimeter to reduce the burden voltage  :)

A Fluke 8845A?  Can you please provide a direct link to that video?

Thanks.
 

Offline Vgkid

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Re: Documentation for uCurrent Gold
« Reply #47 on: April 13, 2017, 01:42:31 am »
Quote from: casinada link=topic
We're talking about the Fluke 8845A here.  Got it.  Thank you for the links.  $36 for that.  Typical fluke.
[/quote
Could be worse. Check out Keysights 200$ offering.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline casinada

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Re: Documentation for uCurrent Gold
« Reply #48 on: April 13, 2017, 05:21:09 am »


He runs extensive test on multimeters. His channel is very interesting. He is also an active eevblog member JOEKSMITH :)
 

Online 2N3055

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Re: Documentation for uCurrent Gold
« Reply #49 on: April 13, 2017, 06:50:03 am »
First, if you don't have it, buy (or make) a shortcut plug like the one in photo.
You can use it to check...

Buy it where? You didn't provide any link to the place that sells them.

You also seem to be talking exclusively about the uCurrent even though we've also been discussing the Fluke 8845A.  I wish to point that out for clarity, for those who later come and read through this thread.

As to testing nA currents on battery voltage alone, consider well that such is not practical in that you may need to test small currents when connected to a AC powered source at times.  In other words, just as is the case with any handheld or desktop meter, there are times when we need to test circuits powered by batteries and times when we need to connect them tested to a wall socket because that's the environment in which they will be used.

Sorry, I didn't provide link, you are literally on the other side of the planet, so I thought you might be able to find something closer to where you live.

And no I didn't even mention uCurrent. I was talking in general about microvolt and nanovolt  and microamperes and nanoamperes measurement with any instrument.

At those voltage and current levels, impedances are so high and measured signal is so low that parasitic emissions, static fields, magnetic fields, triboelectric and thermal effects are sometimes greater than signal you are trying to measure.

Here is the link to a good read.. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwisiuXE56DTAhWmIpoKHeVJDFoQFgg6MAA&url=http%3A%2F%2Fwww.tek.com%2Fdocument%2Fhandbook%2Flow-level-measurements-handbook&usg=AFQjCNGVEOdo-RW87qH4qowipnt8wWqH_A&sig2=eV4eDeg3TGedyhA5b0A9jg

It is the "Low measurement Handbook" by Keithley (says Tektronix but that's corporate crap), a very good overview of the low level measurements and common pitfalls..


As for shorting bars, you can also buy two of these from Pomona:

http://www.mouser.com/ProductDetail/Pomona/5165-2/?qs=%2fha2pyFaduiB6fjNcFfbfg0LjqdcPIiv05%252bYHAJJ%252b9E%3d&utm_source=octopart&utm_medium=aggregator&utm_campaign=565-5165-2&utm_content=Pomona

They are shorts, and can be used in more than one combination.... Including two or more of them connected together..

As for your question, yes, sometimes you need to measure with power grid powered equipment. And then you have complications... Additional filtering, shielding....
You should try to avoid it, if you can..

Hope that helps.

Regards,

Sinisa

 


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