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Troubleshooting a constant current sink

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OM222O:
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/#msg2201538

I 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  :o 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/xSEYPP5Xsi0

How 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

magic:
Force- gets voltage through mosfet body diodes from shunt+.
What dumps current on shunt+ is a mystery, nothing should. Probably something is damaged, wrong part, rotated 180° or your board doesn't match the schematic. Remove parts until you get 0V there.

R12 measurement is nonsense because there are other paths from shunt+ to ground in parallel with it, through protection diodes in ICs.

Darlington surely has nothing to do with any of that because it isn't connected to shunt+. As for oscillations, a resistor from the opamp to darlington base may help. Since speed isn't important, just use the largest value that will still pass enough base current to produce whatever emitter current you want (1A?).

OM222O:
Well how can I stop the current that flows through body diodes? a set of 6 schottky diodes (to minimize burden voltage)? or maybe there is a simpler method? Would it even be necessary if I find out what's wrong with the shunt+ and why it's getting current through it?

As you can see the schematic and the PCB match perfectly and it also passes DRC:


Also I checked the soldered PCB with a blank one for orientation of chips, they're all correct but something might be damaged during soldering/out of the box which is highly unusual although not impossible, I try to remove chips one by one to see what might be causing it.

As for R2 measurement: then why does it work for every other pair of resistors and these 2 specifically have some other path? I'm taking the measurment right across the two ends of the parallel combination  ??? please correct me on that if I'm mistaken.

Yes the largest current range is 1A and given beta is minimum of 1000 I guess something just under 1K would work although I have not had a single issue using fets before, I will replace it to see if there are any changes tomorrow.

OM222O:
Update:
I removed the op amp first, still had 5V on the Shunt+ and Force-  |O the only other connection was to the ADC input:


After removing the resistor to the ADC input it's just fine now  :o I have used this exact ADC (albeit with a different filter before) and was actually blown away with how decent it was, especially for the price! I think I have to sleep on it for tonight and troubleshoot it later. Thanks a lot for the help! I will try the different component options as I have already purchased them and see which ones will make the most sense for the final product. I'll try to update the thread when I found out the issue with the ADC as well as trying out the new components.

magic:

--- Quote from: OM222O on February 27, 2019, 11:44:39 pm ---Would it even be necessary if I find out what's wrong with the shunt+ and why it's getting current through it?

--- End quote ---
Exactly.

A certain amount of current may normally flow from ADC inputs but it should be nano or microamps at worst. This would produce less than 1V of erroneous voltage on R12/R13. Something is wrong with this ADC.

And check the datasheet. If the ADC has significant input current which could produce measurable voltage on R12/R13 before even any current reaches that point from force-, you will need to buffer shunt+ with some low input leakage and low offset opamp or use a different ADC or give up on the 100k range because with only 10k resistors the voltage would be 10x less.

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