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

[magic]

Doesn't that just move the hassle somewhere else?  (I.e. now your reference has to supply a higher current and may lose precision instead?)

Fair point. More importantly, it caused me some unexpected hassle with the switches when I calculated how little resistance they need to have. For single digit ppm it could be hell, my target was more modest.

As for output impedance of vrefs, meh, most of them seem to be single digit mΩ. Easily the cables and connectors may dominate.

[NeverDie]

Build-plan executed.  Assembly completed:


Next step: dial-in trim pots. 

Afterward: measurements!

Edit: The 10 megaohm resistor (bottom module) is supported by teflon standoffs so as not to lose current from leakage.

[NeverDie]

I added a BNC connector to the tester current output.  Also shown is a shorting cap made by Pomona so that the output can be shorted to ground for calibration purposes:


There are two pushbuttons shown in this picture.  One is power-on/off, and the other reverse polarity, as per Kleinstein's earlier suggestion further above.

The test points I received from amazon were so flimsy that I didn't install them.  Instead, I made some much beefier DIY test points using a loop of 22AWG wire for each test point and mounting it to the PCB.

In addition, I added a green jumper  connector so that the first stage module can be isolated from the last stage module during calibration.  I'm not sure that I'll ever need it, but it's there now.



This side view is better at showing the various stand-off heights:


Aside from the teflon standoffs, all the other standoffs on the test  setup are nylon.  The supportive board that all the different modules are sitting-on/attached-to is 1/8" thick uncladd FR=4 board.

The red output current wire is UV-glued to the last teflon offset so as to provide some strain relief for the wire.  I ended up using solid core 22 AWG gauge wire for the current output wire, but maybe I should have chosen stranded wire, which is more resilient, instead.  Anyhow, I did order some thicker gauge wire to replace the 22 AWG wire if/when it ever breaks.

[NeverDie]

I made an attempt at the calibration.  I was able to dial the first stage in to exactly 1v:

[NeverDie]

So far, so good.  Then I attempt to calibrate the second stage, but the trim pots have no effect and it seems stuck at 0.00950v:


The target is 0.01v (for divide by 100 stage).  Rather than troubleshoot and fix it, though, I decide to proceed with picoammeter measurements since it's only off by 0.5%.

[NeverDie]

For the sake of clarity, as I continue to describe the readings, we're going to pretend that stage 2 really did read 0.01000v and not 0.00950v.

With that in mind, the device was configured to send 1000pa to the picoammeter, which should read it as 1.0v exactly.  Instead, it reads a negative 0.9611:
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which, except for the change in sign, is consistent with the actual (non-pretend) stage2 voltage reading.

[I would show a photo here, but the forum just now started to block my photo uploads.  I guess they're too big for its liking.  So, no more photos.]

I'm not sure why it reads negative, but I do have a switch-polarity button, so I push that and continue with the measurements.

Anyway, long story short, I then went on to take the following measurements:
What should be 500pa reads as 0.47930 volts (again, consistent with the actual stage 2 division).
10pa, 8.749mv
4pa, 2.5mv
2pa, 0.8mv
0pa reads as 0.6mv, so it seems something is wrong with that measurement.

Now, importantly, even though this was supposed to be the bulletproof enclosure test setup, I'm *still* able to influence the DMM millivolt readings by moving my hand toward the test enclosure, even when it is closed up.  So, go figure.  Nonetheless, I seem able to get decent DMM measurements down to about 4pa, which means I can now read most of the single digit picoamps.  At this point, to go further, I see no choice but to put the DMM inside the enclosure to do its measurements.  Either that, or just be happy that it's doing as well as it seems to be.   

[SilverSolder]

Is the test enclosure grounded?  That may make things less sensitive.=

[Kleinstein]

The trans-impedance amplifier is inverting. So a negative output with current from a positive source is normal.

Getting still some 0.6 mV at the output suggests that the OP in the TIA has some offset. Assuming 1 GOhms at the TIA and 10 M at the current source this would be a voltage gain of 100. So the 0.6 mV at the output correspond to some 6 µV of offset for the OP. Of cause there could also be some bias current of leakage current.

Another point could be an EMI effect, if the case is not grounded (relative to the TIA circuit). It can also be necessary to have some series resistance / inductance at the TIA input. Otherwise the TIA can be unstable with to much capacitance at the input (e.g. cable). 

[Gyro]

It sounds as if the picoammeter isn't completely nulled. Have you repeated the Null adjustment procedure (-ve out linked to input)? It may be that it has 'settled' a little since you built it.

[NeverDie]

It's not earth grounded, if that's what you mean.  There is no enclosure around the DMM, so I'd wager that's the weak link.  ESD protection would undoubtedly help mitigate.  I just recently received 32 feet (65 pounds) of Hakko ESD matting that I haven't yet installed....

However, short of an all-encompassing enclosure, I think it's likely there's always going to be some electrostatic influence, not nothing.  I mean, even a perfectionist like Marco Reps is abused by it.  For a good laugh, consider time index 5:44 of:



I'm confident we'll find a way to make this thing accurate all the way down to a single picoammeter.  This set of measurements was already an improvement over the last set.  I'll start by troubleshooting the second stage voltage divider, and then try again.

[SilverSolder]

It's not earth grounded, if that's what you mean.  There is no enclosure around the DMM, so I'd wager that's the weak link.  ESD protection would undoubtedly help mitigate.  I just recently received 32 feet (65 pounds) of Hakko ESD matting that I haven't yet installed....

However, short of an all-encompassing enclosure, I think it's likely there's always going to be some electrostatic influence, not nothing.  I mean, even a perfectionist like Marco Reps is abused by it.  For a good laugh, consider time index 5:44 of:

Earth grounding the metal boxes can help (but I have also seen it increase noise...  there is an element of black art to this) - suck it and see!

An electrometer is much less sensitive to the effects seen in the video when the DUT is inside a metal box, grounded by the triaxial cable that connects to the instrument.  Everything has to be shielded, as you said...   the DUT, the cable, and the instrument!

When you see how difficult it actually is to measure picoamps, you begin to see the achievement in measuring atto-amps! 

https://www.eevblog.com/forum/blog/eevblog-1017-enter-the-world-of-atto-amps/

[NeverDie]

Maybe the following would work as a cheap hack: find a multimeter with a "hold" button on it.  Hook it up to the picoammeter but wrap it in aluminum foil, making a solid connection between the aluminum foil and the shared ground.  After the system settles, press the "hold" button through the aluminum foil.  Then unwrap the foil and read the measurement. 

Anyway, I fixed the second stage.  It was just a wiring mistake on my part.  I'll try taking another set of measurements tomorrow and, @gyro, this next time I will be sure to re-calibrate the picoammeter too.

[Kleinstein]

I don't know the details if the circuit. It could help against an effect from the DMM at the output, if there is some resistance (e.g. some 100 Ohms -  1K) at the TIA output, so there is less effect of EMI and cable capacity. Normally the Meter at the output should not effect the reading.

[NeverDie]

When it gets to the single digits on the next set of measurements, I'm going to use the Fluke 87V and turn on averaging and then step away.     I was reading in the 87V manual that the 87V can DC average over a period as long as 36 hours!    i.e. a lot more than long enough.     Perhaps averaging will be a workable solution to noise at the single digit picoamp level.

I'll read the digits on the DMM from a distance using a telescope....   

[NeverDie]

Mission Accomplished!

If I take into account the zero current offset, use my lunchbox enclosure, quality BNC cables and BNC connectors, and turn on DMM averaging, then Gyro's picoammeter gives accurate measurements--accurate to much less than a picoamp at single digit picoamp currents!   

Congratulations to everyone on this thread who helped with this.   

And this time I didn't even need to step away from the table.  The averaging handled the noise beautifully. 

What follows are the details.

First of all, I did all checks and calibrations with the equipment in their final positions.

Answering gyro's earlier question: this time I double checked the picoammeter's calibration, and the picoammeter was still zeroed from the previous time that I calibrated it, so no adjustment needed there.

Here is the current source calibration photo:

It shows calibration of the second stage of the current source.  Exactly 10mv are being produced, which means that 1000pa are being sent to the picoammeter, which registers that as 1.004v (equals 1004pa).  You can see the 10mv is positive, but the picoammeter is showing a negative 1.004v.

I did meticulous measurements, which follow:
1000pa, -1.004v
900pa, -0.905v
800pa, -0.805v
700pa, -0.704v
600pa, -0.605v
500pa, -0.503v
400pa, -0.403v
300pa, -0.302v
200pa, -0.201v
100pa, -0.101v and -100.4mv (averaging now turned on)
90pa, -90.3mv
80pa, -80.2mv
70pa, -70.2mv
60pa, -59.7mv
50pa, -49.8mv
40pa, -37.0mv
30pa, -29.3mv
20pa, -19.7mv
10pa, -9.3mv
8pa, -7.0mv
6pa, -5.4mv
4pa, -3.3mv
2pa, -1.5mv
1pa, -0.3mv
0pa, 0.5mv 

 

[Gyro]

Nice result, I admire your perseverance (and I suspect that you learned a lot in the process).  Well done. 


P.S. Try uploading the image again, it looks as if a few folks have found it glitchy today. It would be a shame to lose it due to a broken link in the future.

[SilverSolder]

Mission Accomplished!

If I take into account the zero current offset, use my lunchbox enclosure, quality BNC cables and BNC connectors, and turn on DMM averaging, then Gyro's picoammeter gives accurate measurements--accurate to much less than a picoamp at single digit picoamp currents!   

Congratulations to everyone on this thread who helped with this.   

And this time I didn't even need to step away from the table.  The averaging handled the noise beautifully. 

What follows are the details.

First of all, I did all checks and calibrations with the equipment in their final positions.

Answering gyro's earlier question: this time I double checked the picoammeter's calibration, and the picoammeter was still zeroed from the previous time that I calibrated it, so no adjustment needed there.

Here is the current source calibration photo:

It shows calibration of the second stage of the current source.  Exactly 10mv are being produced, which means that 1000pa are being sent to the picoammeter, which registers that as 1.004v (equals 1004pa).  You can see the 10mv is positive, but the picoammeter is showing a negative 1.004v.

I did meticulous measurements, which follow:
1000pa, -1.004v
900pa, -0.905v
800pa, -0.805v
700pa, -0.704v
600pa, -0.605v
500pa, -0.503v
400pa, -0.403v
300pa, -0.302v
200pa, -0.201v
100pa, -101v and -100.4mv (averaging now turned on)
90pa, -90.3mv
80pa, -80.2mv
70pa, -70.2mv
60pa, -59.7mv
50pa, -49.8mv
40pa, -37.0mv
30pa, -29.3mv
20pa, -19.7mv
10pa, -9.3mv
8pa, -7.0mv
6pa, -5.4mv
4pa, -3.3mv
2pa, -1.5mv
1pa, -0.3mv
0pa, 0.5mv 

 

Nice work!  That is definitely awesome, getting to this level of measurement is not easy (as you have discovered, LOL!)

I managed to attach the photo.

[NeverDie]

Nice result, I admire your perseverance (and I suspect that you learned a lot in the process).  Well done. 

I'm very happy with the results.  i feel like we just broke the 4 minute mile.  When I started this I thought we'd lose at least a few picoamps just to leakage alone or that the noise might  be too loud to get both resolution and accuracy.    I'm just impressed that for so little money we were able to measure such tiny quantities so accurately, and without even a second all-encompassing enclosure! 

P.S. Try uploading the image again, it looks as if a few folks have found it glitchy today. It would be a shame to lose it due to a broken link in the future.

Done.  Now it's good for all eternity.

[NeverDie]

Nice work!  That is definitely awesome, getting to this level of measurement is not easy (as you have discovered, LOL!)

Well, that's the great irony of it all.  It actually is easy once you know what to do.  Now that it has been done, as documented in this thread, it will be easy for anyone else who wants to do it.  The hard part was getting to the point where it was finally easy.

[SilverSolder]

Yes, setting up the experiment is 99% of the way to a good result.

[NeverDie]

Follow-up question:

I'm measuring a timer circuit that supposedly consumes picoamps worth of current while it slowly charges a capacitor, but when the capacitor voltage hits threshhold the circuit turns on more and very suddenly consumes a lot more (not sure yet if its nanoamps, microamps, or maybe even milliamps) for about 40ms before the whole cycle starts over again.

To protect against cases like this one, and potentially far worse ones, would it be possible to build a protection circuit into the picoammeter that could react fast enough and disconnect fast enough to protect the picoammeter from damaging overcurrent?  A reference voltage and a comparator could certainly detect the problem, but could it disconnect fast enough?  On the other hand putting in an inductor might "buy time" while the disconnect is happening, but it would obviously distort time varying signals as well if the picoammeter output is being monitored by an oscilloscope.  What about a line delay?  Any different?  Is there  a solution at all?  My multimeter manages to autorange without blowing up during sudden shifts, so there are clearly solutions that are "good enough" for multimeters.

How to best approach this?

[Kleinstein]

The usual pA meter will have protection (resistor in front of the OP input). So if the circuit tries to draw more current than the circuit capacity, the resistance see will go up (e.g. from some 100 Ohms to some 100K). In most cases this current will thus not go up to much, but the circuit will see a dropping voltage. This protection can be good up to a few 100 V, how much depends on the details of the circuit.

The multimeter with shunts and for a larger current range usually have a different type of protection. There are relatively large diodes (often 2 or 3 in series each)  in parallel to the shunts. So if the drop at the shunt goes over some 1 V the current would flow through the diodes in stead of the shunts. For gross over-current there is a fuse. This type of protection could also be be used for a TIA. However it would add to the error and noise. So not that good for the very low currents, but maybe OK for 10 or 100 pA resolution.

[NeverDie]

The usual pA meter will have protection (resistor in front of the OP input). So if the circuit tries to draw more current than the circuit capacity, the resistance see will go up (e.g. from some 100 Ohms to some 100K). In most cases this current will thus not go up to much, but the circuit will see a dropping voltage. This protection can be good up to a few 100 V, how much depends on the details of the circuit.

Well, I won't be going to hundreds of volts.  So, basically, I don't need to worry about it then?

[Kleinstein]

One should still have the resistor for protection (directly in series with the inverting input) in the circuit. Diodes to divert the residual current are usually inside the CMOS OPs.

[Electro Fan]

Nice work!  That is definitely awesome, getting to this level of measurement is not easy (as you have discovered, LOL!)

Well, that's the great irony of it all.  It actually is easy once you know what to do.  Now that it has been done, as documented in this thread, it will be easy for anyone else who wants to do it.  The hard part was getting to the point where it was finally easy.

”It actually is easy once you know what to do.”

Heroic:  tremendous vision, collaboration, skill, perseverance, and very humble too.  Amazing accomplishment.  Really, Really Outstanding