Hello
A while ago I made a post asking for feedback on a milliohm meter which can be found here:
https://www.eevblog.com/forum/projects/feedback-on-milliohm-meter-v2-0/msg2201538/#msg2201538I post the schematics and the PCB design again so you don't need to read that thread:



I've been having a lot of weird issues (wrong voltage outputs and oscillations) with this design which I initially blamed on the op amp in the first version of the design as switching to a different op amp (MCP6002) seemed to solve the issue (or at least make them a lot smaller of an issue). I also redesigned the shunt network switching so the Rds(on) will not affect the measurement accuracy. Here I am again, after building the circuit and I have the exact same issues! As soon as I turn on the device (none of the mosfets turned on) the "Force-" net jumps to about 4 volts which seems odd as it's not really connected to anything except the drain of mosfets

also the non inverting pin of the op amp is at some high voltage. I even inserted a shorting link between force+ and force-, as well as switching on D10 (highest resistance of the network in case there are some shorts which I have not yet found) and still got the same results!
Voltage on Force-:

Voltage on Shunt+:

I'm really starting to think this issue is caused by using a Darlington pair instead of an N channel fet as very similar constant current sink circuits which I've built in the past have not had these weird issues. They didn't use a switched resistor network, but that should not be causing this much issue, especially with those voltage levels!
I also probed the resistance values across the resistors in the network and got correct values except for the highest values: R1: 1, R4+R5: 9, R6+R7:90, R8+R9:900, R10+R11:9k but R12+R13 is a different value every time I measure it

once I got 45k,once it was 10, now it's 15k!

Still not sure if there's something inherently wrong with my circuit that causes these issues, or it's a problem with the op amp and or the darlington pair. I bought replacement parts for both of them which hopefully arrive tomorrow, so I can swap them out to see if the results change.
I got the switching shunts idea from daves video which can be found here:
https://youtu.be/xSEYPP5Xsi0How would you recommend I go about troubleshooting this?
Why am I getting voltages on the Force- net?
Why is the Shunt+ at such high voltages?
Also why the heck is the resistance across R12+R13 not 90k and also varies so much depending on when you measure it?
Edit: I tested the circuit before ordering the parts and designing the PCB using this online simulator and it doesn't show any of the weird side effects, feel free to have a look (you can move the +5V between the mosfets to change the current output and it seems to work perfectly fine):
http://tinyurl.com/yyooo9d2