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

[Gyro]

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?

You've already got the protection resistor - it's the 1M one on the input of the opamp. As I mentioned previously, I used a 1W carbon film type for body size, it has a voltage rating of 500V (higher on withstand voltage). At 500V input it is only putting 0.5mA through the LMC662s bootstrapped input protection diodes, which are rated at 5mA max. So basically overload capability is just based on the voltage rating of this resistor. As the resistor is inside the feedback loop, it has no effect on accuracy.


@Kleinstein: The schematic is shown in reply #27:  https://www.eevblog.com/forum/beginners/static-control-requirements-for-picoamp-measurements-using-ucurrent-gold/msg3068708/#msg3068708  It's the same as the one in the Picoammeter thread in the Metrology section.

[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.

”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 

If someone happens to know Dave Jones, they might want to suggest Gyro's picoammeter as a video topic.  It's an elegant circuit and deserves to reach a wider audience, and in that sense it fits with both the theme and the appeal of Dave Jones's other videos.

[Gyro]

Nah, the circuit itself isn't that special and cerainly it isn't the first such implementation.   He could maybe do one on TIAs though - if he hasn't done one already.

[Electro Fan]

I think you guys are onto something very good and deserve a lot credit for figuring stuff out at an impressive level in terms of small signal measurement both in theory and practice, not to mention pretty cost-effectively.  Seems like it's subject worthy of Dave doing a video - he could reference your work, add his own thoughts, and discuss it from an educational standpoint.  In any event, you have covered a lot of very good ground.  Congrats on all the good work.

[NeverDie]

Nah, the circuit itself isn't that special and cerainly it isn't the first such implementation.   He could maybe do one on TIAs though - if he hasn't done one already.

Either way.

A big difference now exists between your circuit though and most other picoammeter circuits on the internet, and that difference is that yours is now independently vetted and shown to be accurate--by this very thread.   There's value in knowing that if you build a particular circuit, it will perform well, as compared to other circuits which may be similar but whose tested performance is unknown. 

[Gyro]

Thanks. 

He could probably do something on the lessons in effective screening - and also the actual measurement and pA test sources (probably the biggest areas of work you have done in this thread).

[Trader]

I found this very useful:

http://www.hep.fsu.edu/~wahl/phy3802/expinfo/labmanual/intlabdoc/instruments/keithley/Low_Cur_Meas_AN.pdf

https://download.tek.com/document/LowLevelHandbook_7Ed.pdf

[Jesse T]

I think the people here are still active on the forum or atleast i hope so. I am building this circuit to test the picoampere meter in an exhaust gas analyzer. I am still very much a beginner and have some questions on how to put the input on the analyzer.
The situation: there is an electrode that creates a small voltage that is going to the analyzers pcb with an bnc cable. In my understanding the input on the pa meter flows the currunt to ground/negative of the circuit under testing. So i cant put it in series with the positive terminal of the voltage source since this would cause a short circuit since there isnt a load/consumption present. the pa meter of gyro needs to be behind the load, correct?
now you could put it in series on the negative coming from the pcb going to the voltage source. so in series of the bnc shielding. my concern is that static discharge gets picked up on the shield and causes an inaccurate measurement. the shielding is of course to protect the inner cable.

so my idea was to create another shielding around the shielding of a regular bnc cable to shield the negative of the circuit from static noise. my question am i thinking the correct way and would this work. if not has somebody other ideas. the pcb needs to be connected since the idea is to compare the pa readings of gyros circuit to the industrial grade one in the analyzer. I dont have room for mistakes since the electrode is already several thousand euros and the analyzer way more. thats why i wanted to get my theory checked by people who know the inner workings of gyros circuit.

[Kleinstein]

The pA meters and likely also the input at the analyzer are usually as a TIA and this one end at the power supply of meter.

For high currents it is easy to just have 2 ampmeters in series. This does not work that well with the TIA type meters. One at least has the shielding issue.
At the pA level it is more a test source and 1 meter and than exchange the sources and meters. For comparing 2 meters directly one would have a large resistor between the meters inputs (center wire) and a voltage source between the grounds (shield). Here the voltage source and resistor make up a special source that can serve the 2 meters.

[Jesse T]

thanks for the swifty reply. I just fineshed the meter and it works great. okay so the safest option would be just to calculate the resistance load that the analyzer has since the voltage and pico amps is known from the analyzer. then feed the analyzers voltage source trough a resistor to gyros meter. this compares the two. to trick the analyzer i can just feed the pcb the known voltage from a different source.
thanks for the help i greatly appreciate it.