Author Topic: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range  (Read 1338 times)

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Offline synoptica

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Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« on: September 17, 2020, 12:42:08 am »
Hi all!

This one might be better suited to the beginners forum, sorry if not appropriate for here!

I've recently finished building a tinyCurrent to help with some ultra-low power Arduino/ESP8266 projects I'd like to keep battery powered for several months.

mA and uA ranges work just fine by all tests, however the nA range does not give accurate values.

Using known inputs for a predictable output within ranges that should be measurable in both the uA and nA ranges, I'm seeing 1uA = ~450nA, and 0.5uA = ~150nA.  As mentioned, given my known inputs the uA values are assumed accurate.

In all honesty, this doesn't bother me much as anything in the single-digit uA ranges is more than enough to know my project will live a long time on battery, but of course I'd still like to get it working.

I'm just hoping somebody might be able to offer some guidance on where I might look to find the source of the issue.  Given I'm usually only on this forum looking for help, if anybody can help I've still got 5 spare PCBs for this so happy to post these out to anybody who can help me out  ;)

Schematic for tinyCurrent can be found here: https://github.com/nfhw/tinycurrent/blob/master/Altium/Project%20Outputs%20for%20tinyCurrent_Version1/Schematic.PDF

Any help massively appreciated (and potentially rewarded as mentioned!)

Edit: I've checked R2 which seemingly would be the first obvious step, but it's fine.

« Last Edit: September 17, 2020, 01:04:27 am by synoptica »
 

Offline bob91343

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #1 on: September 17, 2020, 01:21:32 am »
Does zero current produce zero output?
 
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Offline synoptica

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #2 on: September 17, 2020, 01:37:33 am »
Hi mate - thanks for replying!  Yeah both open circuit and direct short across the current input produces 0v0 across all three ranges.
 

Online bdunham7

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #3 on: September 17, 2020, 01:45:21 am »
What exactly are your known accurate current sources?  Does the tinyCurrent use a 10K shunt resistor?

EDIT:  I see that it is 10K.  Can you try putting an ohmmeter on the inputs and seeing what it measures when you are in that range?  If it is accurate, the most likely culprit would seem to be some leakage causing the apparent shunt resistance to be too low.  Perhaps the switch?
« Last Edit: September 17, 2020, 01:49:18 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline synoptica

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #4 on: September 17, 2020, 03:05:09 am »
Hi mate,

I might be using "known accurate" a bit loosely - it's my bench PSU (which is not of the highest quality) - I'm using my DMM to measure 0.1v output from it and a 1M resistor for a result of 100nA.

Testing across current input in situ, getting 10.5 ohms on the mA range and 10.12k on the nA range, which seems to check out?

It's worth mentioning that this is not a high spec DMM, just a basic UNI-T 3.5 digit.
« Last Edit: September 17, 2020, 03:08:47 am by synoptica »
 

Online bdunham7

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #5 on: September 17, 2020, 03:16:35 am »
Hi mate,

I might be using "known accurate" a bit loosely - it's my bench PSU (which is not of the highest quality) - I'm using my DMM to measure 0.1v output from it and a 1M resistor for a result of 100nA.

Testing across current input in situ, getting 10.5 ohms on the mA range and 10.12k on the nA range, which seems to check out?

It's worth mentioning that this is not a high spec DMM, just a basic UNI-T 3.5 digit.

I can tell you that your method of producing 100nA won't work with my picoammeter, I'm not sure about a uCurrent or tinyCurrent.  You might try greatly increasing your drive voltage, say 10V with a 100M resistor.

 What exact model DMM do you have?  Many of them have fairly accurate precision current sources in the Ohms range that goes down with higher ranges.  Fluke 8842A, for example, uses 500nA in the 20M range with over 10V of compliance voltage (drive).  I don't know about UNI-T models.  Do you have just the one DMM? 
« Last Edit: September 17, 2020, 03:24:08 am by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline synoptica

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #6 on: September 17, 2020, 04:05:47 am »
I can tell you that your method of producing 100nA won't work with my picoammeter, I'm not sure about a uCurrent or tinyCurrent.  You might try greatly increasing your drive voltage, say 10V with a 100M resistor.

Yeah, it's probably not the world most accurate approach, but it measures accurate in the uA range (DMM shows 0.1mV) but then shows ~450mV when measured in the nA range - so while the method is not perfect, it seems accurate for nA-level currents when measured in the uA mode?

What exact model DMM do you have?  Many of them have fairly accurate precision current sources in the Ohms range that goes down with higher ranges.  Fluke 8842A, for example, uses 500nA in the 20M range with over 10V of compliance voltage (drive).  I don't know about UNI-T models.  Do you have just the one DMM?

It's a UNI-T UT136B, pretty entry-level but does the job  :-DMM  I do have other DMMs but they're all about the same level, however I'll try and report back!

Update: Other DMM agrees, nA range still being funky.  Am now measuring 0.5v with 1M resistor, uA range gives the expected 0.5mV on DMM, nA range gives ~150mV.
« Last Edit: September 17, 2020, 06:12:07 am by synoptica »
 

Offline synoptica

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Re: Troubleshooting tinyCurrent (uCurrent Gold fork) nA range
« Reply #7 on: September 17, 2020, 06:27:25 am »
Found the answer, and it lies in this post:

https://www.eevblog.com/forum/testgear/problem-with-ucurrent-gold/msg1145919/#msg1145919

Adding the capacitor across the input has resolved this!
 


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