Yes, some way of logging the readings is probably the way to go. Maybe your next investment could be a bench DMM with logging?
Don't forget, an Arduino also radiates RF simply because it runs at 16MHz and does a lot of fast switching. It could play havoc if it is near a sensitive measurement. Depending on how sensitive the measurements you are trying to make are, it might be best to keep things analog inside the experiment's shield, and convert to digital outside?
Getting a stable 10's of pA measurement is an excellent achievement. A lot of things have to be "just so" for that to happen.
Getting a stable 10's of pA measurement is an excellent achievement. A lot of things have to be "just so" for that to happen.
The credit goes to Gyro for coming up with a good design, and the credit goes to you for figuring out that Gyro had a worthwhile design and then suggesting it!
Getting a stable 10's of pA measurement is an excellent achievement. A lot of things have to be "just so" for that to happen.
The credit goes to Gyro for coming up with a good design, and the credit goes to you for figuring out that Gyro had a worthwhile design and then suggesting it!
Thank you @NeverDie. Whilst the 'Gryo Picoammeter' has a nice ring to it, I can't claim any credit for transimpedance amplifiers, bias networks, air wiring, or even relying on suitably protected internal protection diodes (maybe noticing that they are bootstrapped on the 662 though). Putting them all together in a screened box basically comes down a bit of attention to detail and conservative engineering, which is always good to strive for.
P.S. Agreed, really nice scope captures! I wish I'd done some work on characterising the AC performance myself. Although you don't want to mess with unnecessarily soldering Polystyrene capacitors, there's probably a sweet spot for being able to externally resolve the wanted signal, while achieving maximum frequency response. It probably varies from case to case though.
What is the max number of picoamps that the Gyro meter can display?
What is the max number of picoamps that the Gyro meter can display?+1
Maybe @Gyro can comment?
Just wanted to say impressive results and very nice teamwork. Hope at some point when you guys think the time is right or as you go you might add some summarizing thoughts on this in way that includes a blend of high level fundamental principles for people just getting started with very low current measurements and your key learnings for more advanced measurers. fwiw, this endeavor reminds me a little of other “time nut, volt nut, and other nut” threads and endeavors in which people are pushing the boundaries of what can be cost-effectively measured with respect to pretty small and low level signals. It’s very much a testament to collaborative ingenuity.
You can probably dispense with the 1k resistor on the +Ve output and use a non-isolated BNC for the output too. I put the resistor there as a precaution but it shouldn't affect stability as long as the one on the -Ve terminal (centre pin) is still present.
In the scope shots I posted just above, it's clear that the swing of the output voltage on the picoammeter is getting reduced as frequency increases. This is not unexpected, given what Gyro said about the bandwidth of this design. Question: Is the picoammeter still faithfully measuring and reporting the amount of picoamps passing through it on the way to ground while the bandwidth limitations are especially noticeable, or does the picoammeter lose accuracy under those conditions? What I mean is, there seem to be three possible cases: (1) are the picoamps being throttled but still accurately reported, or (2) are the picoamps actually the same (unchanged) and simply not accurately reported, or (3) are the picoamps both throttled and not accurately reported? i.e. which of the 3 cases is the reality? I can't answer the question experimentally without an accurate higher bandwidth picoammeter, which I don't have.
You can probably dispense with the 1k resistor on the +Ve output and use a non-isolated BNC for the output too. I put the resistor there as a precaution but it shouldn't affect stability as long as the one on the -Ve terminal (centre pin) is still present.
Should I use a shorting cap on the input BNC when doing the zero calibration on the picoammeter?
Or should I use a 50 ohm BNC terminator instead?