uCurrent Gold Measurement Errors

[Cdngreybeard]

I have a standard uCurrent, but I wanted more bandwidth so I ordered the GOLD version and it arrived today.  However, very quickly ,I discovered what I think is a severe problem with the unit.

Using a Fluke 289 and with a shorted input the standard uCurrent reads about about 40 uV offset, but the GOLD version reads about 240 uV offset. With a 1 ma constant current source and with both units set for a 1 mV / ma: the standard unit measures 0.9998 mV one way and -1.0002 mV leads reversed; the GOLD version measure 0.745 mv one way and -1.245 mv leads reversed.

I'm assuming my uCurrent GOLD is FUBAR'd. Comments?

[rstoer]

Dave did a video describing possible OOT resistors getting into a recent batch of uCurrent Golds.
Did you contact him about it or are you relying on him seeing this?

[Cdngreybeard]

His website hinted that the forum may be faster than email, but I was planning on emailing him when I had carried out more exacting tests (now completed).

I must have missed the video somehow.  Do you remember the episode number?

[SkyMaster]

Cdngreybeard,

You are not alone, my uCurrent Gold (serial # 001452), when switched to SHORT, is showing:

-0.65 mV offset on the 1mV/mA range
-0.90 mV offset on the 1mv/uA range
-4.75 mV offset on the 1mV/nA range

[EEVblog]

I picked a random unit still in the bubble wrap (serial #1916)
Results are as expected.
 

[Cdngreybeard]

I ran some more tests this morning on the uCurrent Gold s/n 003907 and using the Agilent 34461A, and I was reading 115uV with a  500+ sample size.  Distribution of readings follow a normal curve.

[EEVblog]

I ran some more tests this morning on the uCurrent Gold s/n 003907 and using the Agilent 34461A, and I was reading 115uV with a  500+ sample size.  Distribution of readings follow a normal curve.

So you got 240uV before and now get 115uV?

[EEVblog]

The problem is the GOLD is a two stage unit to get the extra bandwidth, so the 2nd stage amplifies any x10 offset from the first stage.
The MAX4239 is 2uV max, so 20uV out max. So final output could in theory be 200uV (220uV actually).
I've found that typically it is under 50uV on the output though.

[Cdngreybeard]

Good catch!
That was right out of the box using the Fluke 289 and not using a coax connection between the meter and the uCurrent.  With the 34461A, the DMM was powered for an hour before testing, the uCurrent  and 1ma source (DMMCheck) were on for about 10 minutes before testing. 

[Cdngreybeard]

Yes, I was assuming there would be more noise, I just wasn't expecting such a high value.  What are the typical reading differences between a -/+ connection and a +/- connection?  It seems to be non-linear and gets worse as the input level decreases, but I haven't nailed it down yet.

[EEVblog]

Yes, I was assuming there would be more noise, I just wasn't expecting such a high value.  What are the typical reading differences between a -/+ connection and a +/- connection?  It seems to be non-linear and gets worse as the input level decreases, but I haven't nailed it down yet.

When you are down at the 1mV output level, you are down into properties of the opamps.
I get a few people saying they are measuring and expect 1mV output and it's reading 1.1mV or something, and they wonder why it's not 0.05% accurate and giving 1.0005V output. It just doesn't work like that.
Measuring at the extreme bottom of the range as you have done here is not a good indicator of it's performance.

[EEVblog]

What are the typical reading differences between a -/+ connection and a +/- connection?

Exactly as you'd expect.
If you are are able to carefully null it out then it drops to zero.
If I do the same at closer to full scale I get +1.0000V and -1.0001V for 1mA test current on the uA range. Basic least significant bit error stuff.

[Cdngreybeard]

Dave,
Thanks for the help and advice.  I'm going to re-do the tests. Based on your data, I may have blown my testing procedures.  I'll let you know how it goes.

[EEVblog]

Thanks for the help and advice.  I'm going to re-do the tests. Based on your data, I may have blown my testing procedures.

With the reports I get it's almost always something to do with the setup or test procedure.
Especially for very low level current in the order of 10's or 100's of nA. You really have to know what you are doing to measure the sorts of currents, especially when connected to a scope.
Keithley have an entire low level measurements handbook for a reason 
http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf