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Voltage reference not working properly #SOLVED#

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StillTrying:
It looks like there are 2 unrelated voltage ref problems then. :o
The REF5040's +/- 10mA output should very easily drive the 10k pot and DAC without dropping to 3.1V.
Are you sure the +5V is always good.

Shock:
So after you have tested out of circuit the next thing to try is in circuit with the supply section disconnected from the rest of the circuit, obviously if there needs to be a minimum load take that into consideration as well.

If that checks out, isolate it from the circuit it's powering and then feed that circuit from a bench supply (or alternate source) instead of using the supply circuit.

Anyway that is how to isolate a supply problem and also discover if it's a load based problem.

FotatoPotato:
So I did some probing and the +5v is perfectly stable, 0 ripples and 0 instability. The next thing I checked was the -5v and it seems that when I adjust R29 on my schematic, not only does the 4.096v and -2.5v change but the -5v goes down to around -4.57v which isn't as severe as the other regulators but it is still enough to throw the calibration off. It seems to me that R29 is the source of the problems which is a  big issue because I need it to calibrate the load. I also tested the -5v and -2.5v on a breadboard and everything seems to be working fine. Also once I remove the LM431 and the -2.5v is removed from R29, the 4.096v ref goes back to normal  :wtf:. Still not sure what to do because it seems that I would have to remove essential parts of the circuit which would render my load useless and wildly uncalibrated...  |O

iMo:
1. is the value of R29=10k?
2. is the value of R28=4M7?
3. it could be at a specific R29 position the whole stuff starts to oscillate wildly (look at it with o'scope). You may also verify that easily - remove the R28 from your circuit, and wire the 4M7 from R29's wiper to GND, for example. Turn your R29 up/down - my bet the 4.096V and -2.5V will stay stable (assuming the R29 is not defunct per se).

StillTrying:
"Still not sure what to do because it seems that I would have to remove essential parts of the circuit which would render my load useless and wildly uncalibrated..."

When you're fault-finding on a new circuit with lots of interacting parts it's quite normal to spend nearly all of the time "not sure what to do"! You just have to eliminate details one by one by very careful checking, while keeping an overall view of how it works so that you're hopefully concentrating on the most likely.

"It seems to me that VR29 10k is the source of the problems"

I agree with imo's checks immediately above, but unless you've got it on very long leads I don't think it's the problem, it should just be a very contant 0.66 mA load between the 4.096V and -2.5V.

Are you sure the output of U2 U7 OPA277 always stays ~0V or above, especially during calibration. If its output tries to go lower than about -0.3V it will start to draw a lot more than its 0.8 mA from the -5V. It's about the only place I can see where the varying load on the -5V would be coming from.

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