If you want to get exact, the resistor values aren't nice clean numbers because you have to account for the parallel resistances:
![Beginner level: DIY an accurate picoammeter (measure picoamps!) 1002871-0](https://www.eevblog.com/forum/index.php?action=dlattach;topic=241746.0;attach=1002872;image)
R2=9980, and R4=9891. Assuming the 9v battery is exactly 9v, then R1=7271. R1 would need to be adjusted, depending on what the true voltage on the battery actually is.
This way, with the potentiometer set to 1000 ohms, it generates 1na = 1000pa of current at the output (ammeter A5 in the schematic). i.e. 1 picoamp for every ohm on the potentiometer, making it's easy to dial-in an exact pa current if it's a 10-turn 1K potentiometer with a dial-indicator on it.
A bit of a hassle to construct, but it passes SPICE simulation.
If I'm not mistaken, attaching a current mirror to the output would effectively create a picoamp current source that you could also use in other applications.
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/cool.gif)
In my case, though, instead of 9v, I'm going to use an REF102CP precision 10-volt reference, just for convenience. REF102CP outputs exactly 10 volts to within plus or minus 0.0025 volts, so plenty good enough for this endeavor. It means not having to worry about or recheck the source voltage level, as it will always be 10v pretty much exactly. REF102CP has practically no noise: just 5uVpp at the chip, and that number will get divided by 10 million after it goes through the voltage dividers.
https://www.ti.com/lit/ds/symlink/ref102.pdfEdit1: The other advantage of making use of a 10v reference voltage source is that with it, each stage, if equiped with a trim potentiometer, can be precisely set by applying a precise 10v to it and then, measuring with a dmm, tweaking the potentiometer until the correct voltage division is achieved. This, then, gives an exact result to the accuracy of the DMM, which is more accurate measuring voltages than resistances anyway. No need to calculate the effect of parallel resistances, because those will be accounted for implicitly when tweaking the voltage dividers. In this way, only one precision resistor is needed, namely, the final 1K ohm resistor at the very final stage. All this is good, because with this approach I won't need a 4-wire resistance meter for precisely measuring resistances.
Of course, none of this would account for temperature drift on the resistances. Other than doing recalibrations, I'm not sure how I would adjust for that. Hopefully there won't be any meaningful temperature drift over the short duration of doing the measurements.
Edit2: To mitigate against temperature drift, I'll use high resistance in the voltage dividers, so that very little power will get dissipated into the resistors.