Beginner level: DIY an accurate picoammeter (measure picoamps!)

[NeverDie]

Hi,

The advertisement for uCurrent Gold says:

"Measure the standby and sleep current of the latest generation of microcontroller and other digital electronics, energy harvesting devices etc, down to the nanoamp (or even picoamp!) level with superb accuracy."

https://www.kickstarter.com/projects/eevblog/current-gold-precision-multimeter-current-adapter

Great!

Question #0:  What exactly must I do to get the advertised "superbly accurate" picoamp measurements?

I've tried using the uCurrent Gold in conjunction with a 500,000 count DMM, and I'm not getting superbly accurate picoamp measurements.

Background:
I was hoping to use the uCurrent Gold to measure picoamps for some energy harvesting circuitry that's under development:  https://forum.mysensors.org/topic/10812/the-harvester-ultimate-power-supply-for-the-raybeacon-dk/190
I recently acquired a 500,000 count DMM (an Amprobe AM-160-A) to complement the uCurrent Gold so as to accomplish picoamp measurements.

However, even after taking anti-static precautions, which seem to have the millivolt readings on my DMM under control, the uCurrent Gold measurements below 1 nanoamp are all over the map.  I've grounded my body and have a grounding mat underneath the DUT, the uCurrent Gold, and the DMM and test leads.  However, when I move my hands and arms anywhere near it, the picoamp measurements shift  around (but stop when I stop moving).  The problem is: every time I move, the measurements settle on a different value, so there's no repeatability. 

Question #1:  Have I diagnosed the problem correctly?  Does this phenomena, as I've described it, sound as though static charge is the problem, or is it likely to be something else?  Unfortunately, to get the right offset, I have to first measure with uCurrent Gold in shorted mode, and then I have to physically switch the uCurrent Gold to measurement mode, so there's no way I have avoid either touching it or moving near it.

Question #2: Is nanoamp accuracy about as good as I can expect for taking manual measurements using the uCurrent Gold, or would additional anti-static mitigation allow me to get clean, repeatable, manual measurements down to 1 picoamp?  If picoamp accuracy is feasible with the uCurrent Gold, what are the minimum prerequisites required to attain it?  I need to know so that I can decide whether to spend more on upgraded anti-static measures (if it will work) or else improvise some instrumentation so that I can take the uCurrent Gold picoamp measurements wirelessly, so that I (and my static charge) are nowhere nearby when the picoamp measurements are taken.  Would that work, or would even that not be enough?  Is there something else I should do/try first?

[AVGresponding]

With all the static precautions you're taking, it sounds more like a noisy environment issue (EMI-wise).

Do you have a mobile phone/wi-fi/LED light fittings/SMPS anwhere near the test setup?

[NeverDie]

With all the static precautions you're taking, it sounds more like a noisy environment issue (EMI-wise).

Do you have a mobile phone/wi-fi/LED light fittings/SMPS anwhere near the test setup?

Nothing nearby.  No SMPS.  I'm using just a battery for power.

[schmitt trigger]

It seems that this could be RF related.
Therefore: a wild, wild, wild idea:

So how about AC grounding yourself?
The normal grounding straps have a 1 Meg resistor in series for safety purposes. How about you put a capacitor in parallel with said resistor? The capacitor value should be calculated to provide a reactance of no less than 100k at your power line frequency for safety purposes.

Otherwise you’ll perhaps require a small Faraday cage.

[NeverDie]

Is anyone here able to get superbly accurate, repeatable picoamp measurements using their uCurrent Gold?  Has anyone here besides me even tried?  It would help my troubleshooting to know if the problem is unique to me  or whether others here are either having no problems at all or else either are experiencing or have experienced the phenomena I've described.

I'm having no problems getting repeatable measurements down to about 1 nanoamp of current.  It's below that current level where it gets problematic.

[SilverSolder]

First, measuring at the picoamp level is difficult - even if you have an electrometer which is specifically designed for the job, you have to be careful about what you do - shielding your experiment, using triaxial cables, etc. etc., to get reliable results.

I like the uCurrent Gold and have used it to get ridiculously sensitive and repeatable measurements in the picoamp range, but I used it in an AC bridge to do it - not DC.   

By using AC to test, you can filter everything (noise) out of the result except the exact frequency of your test current (for example, 400Hz or 1KHz).   All worries like thermal voltages at wire junctions etc. disappear when you use AC. (A bunch of new worries crop up, of course, they always do.)

I might try to see how low I can get with DC some time this week, just for fun, so we have something to compare.

[NeverDie]

Thanks.  So, if it's a given that shielding is required, which from your post it sounds like it is, then I guess my plan will be to configure an  arduino to autonomously do the measurements as well as log them.  Once that's working I'll have to put the whole thing inside a sealed metal can while the measurements are taken.  If that doesn't solve the issue, then I don't know what will.

I guess the cleanest way would be to use a latching relay to switch between the uCurrent's Shorted Mode (to get the offset) and the uCurrent's Masurement Mode.  That way the arduino can let the relay settle after switching and between doing the measurements without creating possible interference during the measurement itself.

I'll try using a 24 bit ADC to read the uCurrent Gold's millivolt output.    The ADC I have on hand is this one:  https://www.amazon.com/gp/product/B071P16YCS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Hopefully it will be at least as good (?) for that purpose as my DMM would have been. 

I'd run the arduino directly from a battery with no LDO.  I'd rather not hack the ADC, but I suppose if it proves necessary I could make it run directly  from either a battery or a supercap.  However, I'm guessing the ADC probably generates its own reference voltage, and that just is what it is.  All I need are two measurements, plus redoing them a few times to see if the measurements are repeatable or whether they vary.

Does this plan sound like it should work?  Does anyone foresee any problems with this approach?

[SilverSolder]

I had a play with it using DC and found things got very noisy below about 10nA, meaning that I got to about the same limit as you -  1nA just about discernible, but not really a satisfying high quality measurement.  We probably have to face the fact that the uCurrent is not designed to be a picoammeter and we are not going to be able to make it 1000x better by shielding or anything else. 

I suspect that getting significantly further will require something like an electrometer op amp circuit (there are plenty of threads here on the EEVblog).  You might be able to build that inside a shielded box together with your experiment, battery powered, and capture the output with your ADC.

[NeverDie]

I have a sinking feeling that you're probably right.

I found this: https://www.eevblog.com/forum/projects/picoammeter-design/150/
However, just throwing something together in an altoid can... not sure about that.  I suspect board layout will be important.  For instance, TI says, "a "Guard" trace is recommended when designing sub-nanoamp systems."    Even the author of a DIY picoammeter admits that a proper PCB is the way to go:  http://circuitcellar.com/wp-content/uploads/2014/03/CC_042010_Lacoste_ReprintedwPermission-web.pdf

There's a for-profit fully assembled PCB made by a UK company that I suppose is an option.  It got a nice review here:  http://physicsopenlab.org/2019/09/23/picoammeter/
It sells for about $150 delivered to the US, but the shipping from the UK takes 3 weeks.
However, I see no way to hook it up to an oscilliscope.  If so, that would severely limit its usefulness.

It doesn't make sense for us all to be reinventing the wheel.  If there exist gerber files for some open source PCB design which had already been vetted... but I'm not finding it.

[NeverDie]

I just ran across this video, which uses the uCurrent Gold and essentially the same OEM 500,000 count multimeter as what I have:



At time index 9:35 he's waving his hands near the multimeter and uCurrent Gold, and you can see the numbers jump around.  In my case they jump around more than his, but apparently it's par for the course when attempting to measure picoamps.

[SilverSolder]

Electrometer circuits are actually very simple, with only a few components.  For a home lab project they can be made using "dead bug construction" techniques in order to reduce leakage as much as possible, better than pretty much any PCB board.  To calibrate the circuit, maybe a suitably large resistor and a voltage divider to drive it... you can probably do some "close enough" work with standard components.

E.g. https://www.eevblog.com/forum/projects/picoammeter-design/msg790045/#msg790045

[NeverDie]

Which would be better?  His design or TI's LMP7721 eval board (I posted a link to the gerber files here: https://www.eevblog.com/forum/projects/picoammeter-design/msg3065852/#msg3065852 )

[Gyro]

The LMC662 has very similar typical bias current (around 3fA) as the LMP7721, is available in dip package (LMP is SO8 only) and is much cheaper. The 662 has a wider Vos spec. but for a one-off, it's easy to trim out, especially as it is a dual.

Chris (him)


P.S. The 662 also has significantly lower input referred current noise.

[NeverDie]

Cool. 

So, it looks as though a way to minimize external influences is to put the picoammeter part in its own sealed case and then ground that to an old computer case, which, has the DUT and test leads, such as here (with the lid off):

https://www.semanticscholar.org/paper/Low-cost-picoammeter-for-dielectrics-Epure-Belea/c97ebfd01a1be99c876b938bf5bd8e102b401e82

Is that sufficient, or should the whole shebang also sit on a grounded anti-static mat?

Any particular specs or ratings to look for in an anti-static mat?  I have a cheap one, but it doesn't have the conductive rubber on the back side of it that the more expensive ones have.

[Gyro]

As long as you have the whole setup in a Faraday shielding metal enclosure, it doesn't really matter what you have underneath it. Just making sure that you ground yourself to the case using a proper ESD strap while making adjustments to anything ESD susceptible (soldering iron tip too if relevant).

[SilverSolder]

As long as you have the whole setup in a Faraday shielding metal enclosure, it doesn't really matter what you have underneath it. Just making sure that you ground yourself to the case using a proper ESD strap while making adjustments to anything ESD susceptible (soldering iron tip too if relevant).

Looking at the circuit diagram in the other thread,  I'm not sure I understand the location of the 1M resistor at the input...  is there a "typo",  should it be on the other side of the junction with the 1G resistor, to form an inverting op amp circuit with amplification 1,000x ?  Or is this just me not getting how it works...

Looking at the pictures, it seems it is me that doesn't get how it works.   -  I suppose the idea is that with a really small current flowing into the input terminal, the op amp will balance that current by flowing an equal current backwards through the 1G resistor -  so it is actually a kind of inverting amplifier, but without an input resistor as you'd expect with a normal voltage driven inverting op amp?

I should really build one too...

[Gyro]

If you mean the one on my picoammeter, the 1M is purely there as an ESD protection resistor (limiting the current at high over-voltages to within the capability of the LM662's internal bootstrapped protection diodes). The circuit operates as a transimpedance amplifier, using the 1G resistor as the feedback to the input (giving 1mV/pA), and keeping the input at 0V.

Edit: Good idea, every home should have one. 

[NeverDie]

Which parts do you recommend for these?

The reason I ask is that the guy who wrote http://circuitcellar.com/wp-content/uploads/2014/03/CC_042010_Lacoste_ReprintedwPermission-web.pdf said:

Be really careful. These
diodes can’t be generic because their
reverse current must remain far lower
than the measuring range

Of course, he was referring to the non-zener diodes in his circuit, not the zener diodes in your circuit, and he wires his diodes differently than you do yours, but...  Did you pick zener diodes and wire them the way you did because that way they have lower leakage?

The guy in the video above picked a jfet for that purpose and said he's getting single digit picoamp reverse leakage.  Is that better or worse than the leakage from the two zener diodes?

[NeverDie]

For anyone who's interested, the moving hand problem described in the OP and shown in the video  is a symptom of "electrostatic coupling," as explained here:

If the interference, say power line noise, increases as you move your hand close to the circuit, and reduces if you touch ground, then you are suffering from electrostatic coupling.

The most common effect of electrostatic coupling is the “proximity” effect, or the ability for a sensitive circuit to “see” your hand moving from centimeters away, or the “antenna” effect, where the circuit can “sense” another signal (power line noise) from a distance.

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-2-component-selection/

It goes on to say that the solution is to surround the input stage with conductive shielding.

[Berni]

Yes shielding is vital for such sensitive measurements.

Your body moving around it can electrostaticaly attract electrons in your test setup and these electrons create a current that gets measured. So measurements like this are typically done in shielded boxes (Large diecast metal enclosures are quite popular).

The shielding doesn't need to be any anything fancy (Like RF tight) Just covering the thing with aluminum foil connected to ground will do since it brings everything around it to a fixed voltage potential. Tho if you do have a lot of RF it might need more, for example that USB microscope could be spewing out some RF, so perhaps poke a small hole in your shield for the microscope to look trough.

[NeverDie]

FWIW, the lowLevelHandbook recommends the attached.  For D1 and D2 it suggestions using 1N3595.  Unfortunately, it didn't suggest any particular zener diodes.

[Gyro]

Which parts do you recommend for these?

The reason I ask is that the guy who wrote http://circuitcellar.com/wp-content/uploads/2014/03/CC_042010_Lacoste_ReprintedwPermission-web.pdf said:

Be really careful. These
diodes can’t be generic because their
reverse current must remain far lower
than the measuring range

Of course, he was referring to the non-zener diodes in his circuit, not the zener diodes in your circuit, and he wires his diodes differently than you do yours, but...  Did you pick zener diodes and wire them the way you did because that way they have lower leakage?

The guy in the video above picked a jfet for that purpose and said he's getting single digit picoamp reverse leakage.  Is that better or worse than the leakage from the two zener diodes?

Those two zener diodes on my circuit are on the output terminals. They were an afterthought (probably totally unnecessary) to provide a bit of ESD protection on the output leads.

The reason that they're air wired on the terminals?  They were an afterthought. 

[NeverDie]

   Gestalt error on my part.   

Thanks, Gyro! (aka his/him) 

I'll order the parts (and a few 1N3595 "for good measure") and build it.

BTW, I found a picoammeter circuit in the datasheet for the CA3420: https://www.mouser.com/datasheet/2/698/ca3420-1528689.pdf
What do you think about  having a range switch?  Any advantages to that, or not? 

[Gyro]

BTW, I found a picoammeter circuit in the datasheet for the CA3420: https://www.mouser.com/datasheet/2/698/ca3420-1528689.pdf
What do you think about  having a range switch?  Any advantages to that, or not?

A 1G resistor gives a maximum range of at least +/-2.5nA at 1mV/pA, assuming reasonable utilisation of the 9V battery discharge life. This was good enough for me, there are other ways of measuring greater than 1nA currents. I have a 1uV resolution bench meter, so I have no need for greater sensitivity.

It really depends how sensitive you want to go.

[NeverDie]

Makes sense.  I ordered an aluminum enclosure for it.  That way I can turn it on-off with a reed switch on the inside and a magnet on the outside.