Author Topic: Documentation for uCurrent Gold  (Read 15542 times)

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Offline JDW

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Documentation for uCurrent Gold
« on: March 30, 2017, 02:31:17 am »
I've not yet purchased a uCurrent Gold and am now browsing this forum to see what the caveats are.  I see some basic info here:

http://www.eevblog.com/projects/ucurrent/

But where is the complete documentation? 

I've read that at least one person added diodes for greater protection, so it's pertinent that documentation explains what pitfalls to avoid when using the uCurrent.  Surely someone must have put together some documentation on the uCurrent by now, but I am not finding it.  Where is the documentation?

Thank you.
 

Offline BroMarduk

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Re: Documentation for uCurrent Gold
« Reply #1 on: March 30, 2017, 04:26:47 pm »
There is a link to the original Alternate Zone article here: http://alternatezone.com/electronics/ucurrent/uCurrentArticle.pdf that was linked to from the eevblog site.  It does cover a lot of the internal workings.  Or were you looking for something more specifically published as a user guide?   I'd be interested if that exists too...

(Link to the diode protection forum thread in case others were interested: https://www.eevblog.com/forum/crowd-funded-projects/current-gold-on-kickstarter/msg448208/#msg448208
 
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Offline guenthert

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Re: Documentation for uCurrent Gold
« Reply #2 on: March 30, 2017, 05:16:50 pm »
Wished I'd seen this earlier.  Not sure about the diodes, but the modification to the on-off-short switch I should have done too.  I was earlier already wondering why the uCurrent should be on, when the connection is shorted.  That doesn't make any sense.  :-//  Unfortunately, as it is designed, the input is not shorted in the off position (why, oh why?).   |O While testing a current supply (with a hearty compliance voltage), I managed to thoroughly fry my uCurrent which I forgot to be in the circuit (turned off)  :palm: - yes, cracking sound of bursting SMT resistors and all.  :-BROKE
  I tried replacing the broken resistors, but it seems at least one of the OpAmps didn't take the abuse lightly either -- FUBAR.
 

Offline MLXXXp

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Re: Documentation for uCurrent Gold
« Reply #3 on: March 30, 2017, 09:07:28 pm »
Unfortunately, as it is designed, the input is not shorted in the off position (why, oh why?).

When I modified mine, I could have shorted the inputs when off, instead of using back to back diodes, but I though there might be cases where an "off" position that didn't affect the burden voltage (within the measurement range) would be useful. My other mod turns off the unit in the "short" position, so I can select either "off with shorted input" or "off with burden voltage".
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #4 on: March 31, 2017, 12:38:40 am »
With all this talk of critically important mods, it would seem only reasonable that there be a fire sale to clear out existing uCurrent Gold inventory so as to release an updated model with the aforementioned mods!  For truly, who wants to pay nearly $100 (shipping included) for a device that we might accidentally fry very quickly?  I know some of you out there have never experienced that, but the rest of us are not electronics gods -- we are but flawed mortals who err.
 

Offline MLXXXp

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Re: Documentation for uCurrent Gold
« Reply #5 on: March 31, 2017, 02:44:59 pm »
For truly, who wants to pay nearly $100 (shipping included) for a device that we might accidentally fry very quickly?

A primary purpose of the µCurrent to is provide current measurements with a very low burden voltage. It's difficult and/or expensive to add circuitry which would properly protect the inputs, without increasing the burden voltage, at least on the higher current ranges.

If your situation doesn't require a low burden, then you could either add external protection to the µCurrent or use other equipment with the desired built in protection.

On the nA range, a forward biased diode for DC, or back to back diodes for AC, across the inputs, plus a fuse in series, would probably suffice. The diode's impedance below its forward voltage conduction point would be high enough, compared to the 10k shunt, to not affect the accuracy of the readings. If the current gets too high, the voltage across the shunt would exceed the diode's forward voltage point, causing the diode to conduct and the fuse to blow if the fuse's rating is exceeded.

On the nA range, the fuse's resistance would likely be low compared to the 10k shunt, so it also wouldn't affect the burden voltage much. On the other ranges, the fuse's resistance could affect the burden.
« Last Edit: March 31, 2017, 02:48:11 pm by MLXXXp »
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #6 on: April 01, 2017, 08:19:44 am »
Question...

uCurrent Gold specs say this about Burden Voltage:

    3 Current ranges:
    +/- 0-1250mA (20µV / mA burden voltage typical) * switch contact resistance plays a role here. 10uV due to the shunt resistor.
    +/- 0-1250µA (10µV / uA burden voltage)
    +/- 0-1250nA (10µV / nA burden voltage)

Fluke 8845A 6.5-digit multimeter (>US$1k device) specs:



Take Note: 100uA range is <0.015 V burden voltage, and Resolution for 6.5 digits is 100pA

So in terms of the Burden Voltage spec, it would appear that my Fluke meter has the uCurrent Gold beat, hands down.  But even with the probes disconnected I see 23nA on the display of my Fluke 8845A.  And when I try to take low current readings, I get lots of fluctionations in the nA range.  I originally thought the uCurrent might benefit me, but looking at the specs for Burden Voltage, I don't see how.  Do you?
« Last Edit: April 01, 2017, 08:30:28 am by JDW »
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #7 on: April 01, 2017, 08:26:20 am »
I originally thought the uCurrent might benefit me, but looking at the specs for Burden Voltage, I don't see how.  Do you?

Not everyone can afford a $1000 DMM, is it? That's where uCurrent comes handy.
Frankly I didn't find my uCurrent very useful either since I also use an 8846A (particularly, a Keithley rebrand, DMM4050), but for its price, if it benefits even only a single project, it is worth all the money you've paid for it.
It's not like a $1000 DMM that has to prove its usefulness all the time in order to recover its cost.
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #8 on: April 01, 2017, 08:29:42 am »
...I also use an 8846A (particularly, a Keithley rebrand, DMM4050), but for its price, if it benefits even only a single project, it is worth all the money you've paid for it.

The current measurement features of my 8845A and your 8846A should be exactly the same.  How well can you measure nA?  Can you measure something down to 1nA or lower with good accuracy?  I am not able too, which is what led me to uCurrent and this forum in the first place.
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #9 on: April 01, 2017, 09:02:00 am »
...I also use an 8846A (particularly, a Keithley rebrand, DMM4050), but for its price, if it benefits even only a single project, it is worth all the money you've paid for it.

The current measurement features of my 8845A and your 8846A should be exactly the same.  How well can you measure nA?  Can you measure something down to 1nA or lower with good accuracy?  I am not able too, which is what led me to uCurrent and this forum in the first place.

I don't go down to that far. For me, low current usually mean a few hundreds nA. I've never had a chance to do nA measurements.
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #10 on: April 04, 2017, 04:47:28 am »
For me, low current usually mean a few hundreds nA. I've never had a chance to do nA measurements.

I made time today to do the following nA measurements, which include uA measurements for comparison:



I do not have a uCurrent to repeat the same test.  As such, it would be appreciated if those of you with (a) a uCurrent gold, (b) 10pcs of 10M-ohm (5% tolerance) resistors, and (c) a benchtop power supply could do the same test and let us know your Percent Error.  I am curious if your US$100 device is better, equal or worse that this >$1000 Fluke Meter.

Thank you.
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #11 on: April 04, 2017, 05:26:29 am »
I have a Keithley 2410 SMU, which is rated 0.035%+600pA accuracy. I can use it to easily generate 10nA reference current and used to test my uCurrent Gold. I will report back if I can find my uCurrent. At least I will test my 8846A with it, see how much of error it has.
 
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Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #12 on: April 05, 2017, 12:22:16 am »
So I did a quick test with KEI2410 SMU and KEI4050 DMM, at 100PLC, D-filter on, 6 digits (LSB=100pA), both devices were NOT warmed up (I did this in a hurry), and DUE calibration for more than 2 years.
The KEI brothers agree with each other by the DMM reads ~1% plus ~200pA higher than SMU set point, consistent across 10nA to 200nA (test points: 10nA, 20nA, 30nA, 40nA, 50nA, 100nA, 200nA).

uCurrent is also measured, and it doesn't look good. Using the same SMU and DMM (in voltage mode, 6 digits, 100PLC, D-filter on), now the DMM reads up to 11% different (always lower) than set point, and the error curve does not fit a first order function, i.e. it is not in a  "x% rdg+y dgt" form.

With all due respect to Dave, if I see a $1500 Keithley disagrees with a $100 uCurrent, especially when the Keithley brothers agree with each other, I trust the Keithley more.
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #13 on: April 05, 2017, 01:23:02 am »
Thank you for your time in testing.

For the sake of our readers who may not understand, the "SMU" you refer to is your Keithley 2410 which is defined as a high-impedance multimeter that has a built-in low-noise Power Supply.  (In comparison, the Fluke 8845A used in my test is high-impedance but lacks a built-in Power Supply.)  The documentation for the 2410 shows it can, in theory, measure from 50pA to 1.05A, whereas my Fluke 8845A can, in theory, measure only as low as 100pA when using the 100uA range setting.

In your test, rather than using resistors, you set your meter to generate a 10nA reference current, then used that same meter to measure it, and you repeated for 20nA, and so on, up to 200nA.  You then said your meter gives you an error of only 1%+200pA.  You then tested the uCurrent and found its error to be up to 11%.

I myself have no means to generate a stable and trustworthy "reference current" and so in my test I used resistance values (see my earlier post).  And you can see that I am getting huge errors in the nA range, but small errors in the uA range.  I am therefore curious if this is some problem with my Fluke 8845A, or a problem with using resistances in the test?  To know the latter, I would be curious to know what kind of current measurements you get using the same resistance values I did.

Without being forced to buy a new meter (my Fluke works fine otherwise), I am seeking to know how to get the most accurate nA measurements possible.  Do I really need a uCurrent, or is there something I need to do to adjust my Fluke meter?

Thank you.
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #14 on: April 05, 2017, 01:31:27 am »
In your test, rather than using resistors, you set your meter to generate a 10nA reference current, then used that same meter to measure it, and you repeated for 20nA, and so on, up to 200nA.  You then said your meter gives you an error of only 1%+200pA.  You then tested the uCurrent and found its error to be up to 11%.

No. I used 2 different equipment, an SMU (2410), and a DMM (4050, aka Fluke 8846A). I compared between the 2410's set point and 4050's reading. FWIW, the 2410's read back agrees with 4050's reading.

Without being forced to buy a new meter (my Fluke works fine otherwise), I am seeking to know how to get the most accurate nA measurements possible.  Do I really need a uCurrent, or is there something I need to do to adjust my Fluke meter?

I have no idea about this, but nA measurement is a tricky business.
 

Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #15 on: April 05, 2017, 01:49:52 am »
...nA measurement is a tricky business.

That brings me back to square one. 

I originally discovered uCurrent and ultimately this forum because I began Googling the issue of nA measurements due to my Fluke 8845A not giving me reliable nA measurements.  And yet my Fluke works fine otherwise!  So I have been trying to determine if buying a uCurrent is worthwhile.  At this stage, I honestly do not know.

In your case, it seems that there is no need for a uCurrent insofar as your meter is giving you highly accurate readings (relative to my Percent Error, and relative to your own uCurrent). 

I'm not sure what to do at this point.  And it seems no one else has an opinion, seeing that only you and I are in this discussion.   |O
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #16 on: April 05, 2017, 02:14:42 am »
I'm not sure what to do at this point.  And it seems no one else has an opinion, seeing that only you and I are in this discussion.   |O

My 2 cents: modify your uCurrent and increase its burden resistance, so that offset voltage is less significant than signal voltage. Of course, you lose low burden feature, you then you can reliably measure lower current. The preset one is 10K, which you can set it to higher for higher element voltage, which eventually gets you to higher SNR. At 100nA, 10K gives you 1mV. With a 0.1mV offset amp, that easily gives you 10% error.
If you mod the sensing resistor to 100K, then you get 10mV for the same current, while 10mV burden is not a big thing on power supply for most devices.
Another idea is to use an ultra low offset opamp with inverting IV converter topology. Consider ADA4530-1 or similar low offset, low input bias current opamps.
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #17 on: April 05, 2017, 03:00:45 am »
...modify your uCurrent and increase its burden resistance... The preset one is 10K... If you mod the sensing resistor to 100K, then you get 10mV for the same current...

I haven't purchased a uCurrent yet, mainly because I have been trying to ascertain if it would be a benefit to me at all in light of the fact I have a Fluke 8845A.  But thank you for that suggestion about changing the uCurrent's 10K resistor to 100K.

I found the following post in this forum which speaks about AC riding on the measurement cables and negatively impact nA measurements using the uCurrent:

https://www.eevblog.com/forum/testgear/ucurrent-not-acurate-on-na-range/msg882779/#msg882779

Could that have contributed to the 11% error you cited?
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #18 on: April 05, 2017, 03:15:15 am »
I haven't purchased a uCurrent yet, mainly because I have been trying to ascertain if it would be a benefit to me at all in light of the fact I have a Fluke 8845A.  But thank you for that suggestion about changing the uCurrent's 10K resistor to 100K.

From my experiment, it is still better than your test data. Imaging it gets 90% less offset by increasing burden voltage. Not a bad deal.

I found the following post in this forum which speaks about AC riding on the measurement cables and negatively impact nA measurements using the uCurrent:

Could that have contributed to the 11% error you cited?

Didn't bother to hook up a scope. Will see if clipping happens or not and let you know tomorrow. ATM I'm at home, and I don't want to go to my lab at night.
However I don't think there is a path for AC leakage current since I didn't touch anything when doing the test, everything in the test loop are floated, while the test equipment are grounded.
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #19 on: April 05, 2017, 08:34:17 am »
Didn't bother to hook up a scope. Will see if clipping happens or not and let you know tomorrow.

I appreciate that.

In the meantime, I am curious how much the resistor TOLERANCE will affect the accuracy of the uCurrent Gold.  (I still don't have a uCurrent Gold to test, but I may purchase one at some point.)  Right now Dave uses +/- 0.05% resistors (10ppm) for R2 & R9.  You suggested a change of R2 from the current 10K value to 100K.  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 feel that the difference in tolerance between 0.05% and 0.01% for the replacement 100K resistor be largely irrelevant for the nA measurements we are talking about?
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #20 on: April 06, 2017, 04:08:26 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.
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #21 on: April 06, 2017, 04:32:07 am »
Under that condition, when you read the output of the uCurrent with your Voltmeter (set to DC), what is the measurement?  (How close is it to the 100nA?)
 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #22 on: April 06, 2017, 04:40:44 am »
Under that condition, when you read the output of the uCurrent with your Voltmeter (set to DC), what is the measurement?  (How close is it to the 100nA?)

Weird enough, today the DMM reads higher than set point, this time 11% higher. But today I forgot to set PLC number, so it is using its default 10PLC.
I will test at 100PLC tomorrow, replicating exact same test setup as I did yesterday, just with uCurrent's negative output grounded by o'scope.
 
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Offline JDW

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Re: Documentation for uCurrent Gold
« Reply #23 on: April 06, 2017, 04:51:58 am »
...today the DMM reads higher than set point, this time 11% higher... it is using its default 10PLC.

Which means you measured 111nA instead of the 100nA you are sourcing, tested with your DMM set to 10PLC (and I assume your DMM was set to measure DC voltage, not AC).  Curious what voltage value you get when trying the AC setting.  If it is noticeably different than when using the DC setting, that would imply noise is impacting the reading.

 

Online blueskull

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Re: Documentation for uCurrent Gold
« Reply #24 on: April 06, 2017, 04:54:25 am »
Which means you measured 111nA instead of the 100nA you are sourcing, tested with your DMM set to 10PLC (and I assume your DMM was set to measure DC voltage, not AC).  Curious what voltage value you get when trying the AC setting.  If it is noticeably different than when using the DC setting, that would imply noise is impacting the reading.

I will report back on AC+DC voltage plug AC voltage only.
 
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