Well...
1/ I'd recommend a OPA192 for this - much better Vos. Or maybe an ICL7660 - near zero Vos - but if you want to go that way I'll have to redesign.
2/ re-read my earlier posts and apply the equation - R2 has to be a small value - thought you understood this.
10k gives a very small output. Try 2 ohms
3/ You show an input voltage of 400 - if you do that you'll get magic smoke! Your original requirements only implied 200v max above ground ( your diagram in post #32) . The design is good for 250v or maybe a bit more.
And I do hope your 400v is clean dc and not just rectified 400 vac
- in which case it could go to 400 x sqrt2 or 566v peak ! - please confirm.
4/ You can't power the opamp sitting at ~+200v from 12v on the arduino sitting at ground! Magic smoke big time!
5/ Note your diagram implies a dissipation of up to 160 watts - 400v across about 1k....had you noticed ? are the resistors appropriately rated? where are they ? - they're going to get HOT!
Allan
ps .... who thought this whole thing up? - if you merely want to measure the change of resistance of a bit of foil while being etched , there are much easier ways..... why does it have to sit at 200v above ground and be driven by a split current source dissipating 160 watts? A precision constant current source of a few hundred mA at low voltages is easy-peasy.
If you're a technician given a job by postgrads ( who probably know nothing of electronics) - as I guess - show them these posts, and particularly these comments, and get them to talk to me .