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

[SilverSolder]

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.

Can you explain how the offset trim works in your circuit, using the second op amp?

[NeverDie]

Can you explain how the offset trim works in your circuit, using the second op amp?

+1

[Gyro]

Can you explain how the offset trim works in your circuit, using the second op amp?

+1

It looks as if I'd better copy the schematic over from the Picoammeter thread to supply some context here...




The offset adjustment is really quite simple (once you get it). The resistor network between +B and -B (100k, 15R 15R, 100k) is used to establish the mid rail point for the rail splitter, and for the offset adjustment. The two 15R resistors in the middle provide a voltage drop of a few mV (I'll let you do the sums). The 100k pot across this 30R span allows the main opamp's ground reference (In+) to be shifted slightly relative to mid rail, to compensate it's offset voltage. Because the span is so small, it can use a single turn preset and still get decent zero adjustment.  The second opamp (the rail splitter) simply takes it's mid-rail reference from the junction of the two 15R resistors. The two 100n capacitors drop the AC impedance of the midpoint to keep noise down.

Remember that in this case, the rail splitter is driving the case, so it effectively floats the battery to be +/- 4.5V relative to the case. Because there is a reasonable amount of capacitance involved (battery casing to case, board to case, switch wiring etc.) it needs some damping to prevent oscillation - hence the 10n/1M feedback network and the 100R on the output.


P.S. the two 1k resistors in series with the output terminals are there for stability purposes too - to isolate the main opamp and rails splitter from stray capacitance on the DMM and its leads (and a bit of protection).

[NeverDie]

Today I received all the parts in Gyro's picoammeter BOM, so I hope to put the picoammeter together soon.  I expect it will look similar to Gyro's but with some small differences.  I'll post photos when I finish it.

Meanwhile, I have a related question: since the DUT and picoammeter and all related measurement equipment is supposed to be enclosed together under a conductive shield prior to measurements being taken, how is it that I can use the picoammeter in conjunction with my Rigol 1054Z oscilliscope?  I mean, if the Rigol is under the conductive shield too (?), then how do I see what's on the Rigol's screen?  Do I need to connect my PC to the Rigol over a fiberoptic cable or something?  Or, must I configure the equipment to broadcast measurement data to me via soundwaves through the metal shielding?  Or, do I hire a psychic to remote view the oscilliscope?     Or, ....? Seriously, how should it be done?

"In for an inch, in for a mile."

[Gyro]

Looking forward to seeing another implementation. Remember cleanliness!

You can treat the output connectors of the picoammeter as the external interface of the shielded enclosure (you certainly don't want a noisy Rigol scope inside! ).

Measuring single digit picoamps isn't really scope territory, well I guess it can be at low timebase speeds, but remember that the picoammeter circuit has a bandwidth of about 0.5Hz (due to the 330pF feedback capacitor). Assuming that you have the conductive shield referenced to mains ground (reasonable), then you can connect the  scope probe ground clip to the output +ve terminal (which is effectively connected to the picoammeter case and shielded enclosure). Connect the probe tip to -ve output terminal.

The scope will read increasingly negative voltage with increasing (+ve) current - set the scope channel to 'invert' if this bothers you.


P.S. It's probably about time for you to download a copy of the Tek (formerly Keithley) Low Level Measurements Handbook... https://www.tek.com/document/handbook/low-level-measurements-handbook

[SilverSolder]

Picoamp measurements can take a while to settle down to a steady reading.

Overall, you are probably better off with a DMM for this measurement.

On the scope, you could try its Chart Roll mode (if the Rigol has that?),  where it basically acts like a chart recorder, taking maybe a minute to trace across the screen.  That might be kind of cool, you'd certainly see when the measurement was settled!

[NeverDie]

Picoamp measurements can take a while to settle down to a steady reading.

Even when the DUT is completely shielded?  If so, is there anything that can be done in addition to the shielding to reduce/eliminate the settle-down time without adversely affecting accuracy?

[SilverSolder]

Picoamp measurements can take a while to settle down to a steady reading.

Even when the DUT is completely shielded?  If so, is there anything that can be done in addition to the shielding to reduce/eliminate the settle-down time without adversely affecting accuracy?

Once the system has settled, I would expect that small changes in the signal would not take long to propagate down the line.  But if there is a large step in the signal, it takes time to settle to the new value.

[NeverDie]

I'm nearly done putting it together.  Just waiting for a missing part to arrive, hopefully tomorrow.

the picoammeter circuit has a bandwidth of about 0.5Hz (due to the 330pF feedback capacitor).andbook/low-level-measurements-handbook]https://www.tek.com/document/handbook/low-level-measurements-handbook[/url]

Sounds like may I'll need to build something else for the oscilliscope.  Are there any schematics you can recommend?

[SilverSolder]

How fast is the signal you are looking at?

[Gyro]

You can reduce the value of the 330pF capacitor. Bandwidth will increase, but so will the noise level. Don't expect miracles though.

[Gyro]

How fast is the signal you are looking at?

... and what current range?

[NeverDie]

Not the best example,

but maybe being able to use the picoammeter to show the AM3 current flow on a oscilliscope?  This is a SPICE simulation on TI TINA.  At the right it shows a virtual oscilliscope with voltages and currents, and so it provides a target of sorts for the picoammeter (if connected to an oscilliscope)  to match up against.

Edit1: If the current picoammeter can do this, then great.  But if the frequency were greater....?

[Gyro]

Remember that a Picoammeter measures current to ground (or at least case for the battery operated one). You might get away with carefully insulating the case and portable DMM from ground using a decent insulating sheet (PTFE or Polystyrene). It doesn't really tie in with the screened enclosure though.

As soon as you attach a scope, you are definitely talking about a to-ground measurement. A Picoammeter doesn't work as a differential current probe.

[exe]

JFYI a cheap DMM with 10M input impedance in mV mode works remarkably well as a micro|nano-ammeter. I read about this advice on this forum, I tried it, and I got much less noisy results than with uCurrent Gold. With 0.01mV resolution it maps into 1pA current resolution. Of course, in reality it won't measure current of a few pA, nor it will be very accurate because input impedance is rarely exactly 10M, but still very capable approach.

[NeverDie]

@exe
You mean like this?  https://hackaday.com/2016/02/01/need-a-nano-ammeter-you-already-have-one/

[exe]

@exe
You mean like this?  https://hackaday.com/2016/02/01/need-a-nano-ammeter-you-already-have-one/

Yes. I used aneg 8002 or 8008 for my measurements.

[SilverSolder]

Not the best example,

but maybe being able to use the picoammeter to show the AM3 current flow on a oscilliscope?  This is a SPICE simulation on TI TINA.  At the right it shows a virtual oscilliscope with voltages and currents, and so it provides a target of sorts for the picoammeter (if connected to an oscilliscope)  to match up against.

Edit1: If the current picoammeter can do this, then great.  But if the frequency were greater....?

The frequency looks quite low, like about 1Hz?    How high would you like to take the frequency?

[NeverDie]

I'd be happy with this frequency, as long as it can show the waveform reasonably well.  I guess I'll know soon enough because...

I finished the assembly!  Here it is with the lid and back removed so that you can have a good look:


I air-wired most of it.  The white pillars are teflon.  I purchased a 3/8" teflon rod 12 inches long and then sawed it into short stand-offs.  Loctite makes a superglue plastic adhesion promoter that allows even teflon to be glued, and that's what I used.  The battery, the offset-voltage module, and the main module sit on a 1/8" thick FR-4 using nylon standoffs.  Aside from the teflon standoff that's supporting the op-amp, the other teflon standoffs are just there to support the wiring so that the legs on the op-amp don't get strained.  At the top of the tallest teflon standoff is a reed-switch for powering everything either on or off via a magnet on the outside of the aluminum casing.



The whole thing is more or less floating and, so far, not grounded to the case:


If it turns out to be needed, I can replace the nylon stand-offs with teflon stand-offs, since teflon offers a much higher resistance than nylon.  It seems the overarching goal is suppressing leakage as much as possible, since in the picoamp realm even small leaks might look big.

[NeverDie]

Another view:


And here it is all buttoned up:


I won't be able to try it out until tomorrow, but I'm looking forward to it.   

One improvement I might make is drilling a hole in the lid above the trim pot so that I can fit in a screwdriver to zero it without having to take off the lid.  If I were to do that, though, I'm not sure what kind of removeable conductor I could seal the hole with when I'm not trimming the pot.  Thinking...  Maybe the best solution would be to tap some threads into the hole and then plug it with a metal bolt when not in use?  That way the integrity of the conductive enclosure shield would be maintained.  If I were to pursue this, I should maybe (?) also elevate the trim pot so that it's higher up and closer to the lid, so that it would be easier to fit a screwdriver into its screw slot of the trim pot.  Well, I'll see how it goes with just the hole first.

[SilverSolder]

Looks awesome!   

 - will be interesting to see the results of your tests! 

[Gyro]

Oh Wow, that's a lot of Teflon, you had some fun there!   

I see you've used a reed switch for the power too.

A few points I can see:

- You seem to have the protection resistor directly in series with the input terminal, rather than between the 1G feedback resistor and the opamp input / capacitor junction. This will raise the input resistance of the circuits to 10M ohm, rather than acting as a virtual ground (the input should try to stay at ground potential for maximum accuracy). The protection resistor should be inside the feedback loop for maximum accuracy.

- I can't see where the case is grounded to the Green ground terminal, to form a proper screen.

- Ideally, you would air-wire the junctions between the op-amp input, and the input end of the 1G feedback resistor, the 10M protection resistor and the polystyrene cap through to the input terminal. Teflon (PTFE) is good, air is better.

- The most critical leakage path (weakest link) is on the LMC662 package. This needs to be as clean as possible. I think I see some flux residues, not critical as long as the package itself is clean (IPA is best) between the pins.


The great, and much missed, Bob Pease wrote some interesting stuff on the use of Teflon in high impedance circuits, charge storage etc....

https://www.electronicdesign.com/technologies/test-measurement/article/21773611/whats-all-this-teflon-stuff-anyhow

and one of his fun videos...




It will be good to see your results. Note that it will probably take a while to settle - leave it with the lid off in a warm dry place, superglue vapours can settle on surfaces (the fingerprint trick in Beverly Hills Cop 2).


P.S. A small hole for trimming the pot won't make any difference to the screening - you can always put an adhesive label over it to keep dust out.

[NeverDie]

Thanks!  I'll make the changes you suggest.  I'm not sure how conductive superglue is, but I did superglue the opamp to the teflon mount.  Based on what you said just now about the op-amp package being the most critical leakage path, and for it needing to be clean, that may have been a fatal mistake on my part.  I have some spares, so maybe I should  replace the op-amp with a fresh one just to be sure.

I did end-up drilling a hole above the trim-pot.  I also installed a green LED near the trim-pot which lights whenever the reed switch turns on the system.  I can see it through the same hole.  The LED draws just 10 microamps, and I like having visual confirmation that the system is turned-on.  This turned out to be helpful already, because I hadn't anticipated that the magnet, when left alone, would be dragged toward the 9-volt battery (the battery case is steel).  It turns out that shift of the magnet's position was enough to turn-off the system.  For now I'm taping the magnet in position once I get the green light.  Maybe later I'll move the reed switch closer to the battery.

[Gyro]

Good question, I did some searching on the web but couldn't find a conductivity figure for superglue (Cyanoacrylate). I know that it needs moisture to set, but don't know how that is bound up. To remove superglue or its deposits you need something more aggressive like Acetone, IPA won't touch it.

If you want the lowest leakage then maybe replace the opamp and make it completely air wired (you seem to have enough supported wires around it), but you could just wait and see how it performs anyway.

To correct the circuit location of the 10M resistor, I would simply unsolder the end of the 1G feedback resistor, extend it with a bit of bare wire and solder directly to the input connector. The 10M resistor lead is sharing a pillar with the yellow PVC insulated wire, but it would probably be a bit anal to suggest lifting that off, they should be at the same potential anyway. The rest of the circuit nodes are all low impedance.

A power indicator is a good idea, as long as you make sure it isn't taking more current than the rest of the circuit (battery-life wise).


P.S. Don't forget to ground the case - it may look like a Faraday shield, but only until you make a connection to one of the terminals, then it becomes a big antenna for noise pick-up.

[NeverDie]

Good catch!  I see now that I did incorrectly wire the gigaohm resistor.  Maybe that's the reason why the picoammeter is totally not working right now.

Uh, you've twice referred to a 10megaohm resistor, but, unless I'm overlooking something, in your schematic it's 1 Megaohm.  Based on context, I'm assuming that's what you meant.

The yellow wire isn't actually sharing a standoff with the megaohm resistor.  Maybe it looks differently in the picture.   However, there is a slight, unintentional glue bridge between the black wire and the blue wire's support offset, so I'll clear that away during the repair.

In case you're interested, the green LED I'm using is a CRE, model 630-HLMP-CM1G-350DD.  Costs about 10 cents each  in quantity 10 from mouser.  I had previously tested it as being dimly visible even at just a 500na current drain, provided you can look at it dead-on (which is when it's narrow optics helps some).  Here I'm running it at 10ua because the orientation isn't so great.  I had earlier thought of having the LED do double-duty by also illuminating the trim-pot, as a navigation aid for screwdriver insertion, but it turned out the hole is big enough and the trim-pot is high enough that room light alone is enough to see where the screwdriver needs to go.

Lest I lead someone down the wrong path, I should mention that I'm not thrilled with the aluminum enclosure I'm using.  I purchased it from amazon, and in pictures it looked like plain aluminum, but in reality it has some kind of non-conductive silvery coating on it.  Therefore, as belt and suspenders, I'm going to tap each of the four pieces that constitute the shell of the enclosure and wire them together, similar to what an electrician might do, to guarantee that they really are all electrically connected.  The drill-tap for doing that should arrive tomorrow.  If I were to do the project over from scratch, I'd probably use a die cast aluminum box instead of this coated semi extruded aluminum box.