Thanks for the op amp substitute ideas for the OPA140/2140. I think JFET is the right choice, particularly for low bias current stages. Bipolar is generally higher input current, and CMOS is challenging for +/-15V, generally higher drift and noise. I don't trust the linearity (CMRR) of CMOS: they need circuit tricks to get wide input voltage range that sometimes compromise linearity.
I settled on OPA145/2145 as my go-to precision amp. Similar DC and noise specs to the OPA140/2140, slower (5.5MHz vs 11MHZ), better availability, and ~1/2 the cost.
I have built 2 complete DIY-SMUs so far. I'm getting ready to build the 'final' Rev2 Main boards and Rev3 Amp boards.
Jaromir has designed a very nice take on this project and has built a couple of units. His design uses an off-the shelf enclosure, and a custom AC transformer. And a simpler LED / buttons front panel similar to the original K236 instead of a touch screen.
If there is interest, I can get the new boards machine-assembled and can sell a few. At least the main board with its 175 parts. The Amp board (75 parts) is not bad to hand-build.
There are firmware additions and fixes that I plan:
EEPROM based calibration. Currently requires recompile. A Python app does the hard part.
Control of remote sense. Currently SCPI only
Better Clamp control. Basically works, not accurate.
Basic plotting page
Thanks,
Dave Erickson
DIY-SMU:
www.djerickson.com/diy-smuJaromir's DIY-SMU:
https://www.eevblog.com/forum/projects/how-i-made-my-smu/new/#new