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Voltage reference not working properly #SOLVED#

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iMo:

--- Quote from: StillTrying on March 02, 2019, 01:52:17 pm ---..Are you sure the output of U2 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.

--- End quote ---
Good point - when the OPA277' output goes below -0.3V the AD8630's input lower side clamping diode starts to conduct, and the current from the -5V source flows via OPA277's output to AD8630's input, then via the clamping diode to GND (assuming the V- of the AD8630 is GND). It could be many mAmps.
Double check - put a 100k (for example) resistor between the OP277 output and the AD8630 input as a test.
PS: if it helped you may leave the 100k resistor there as the input current of that AD is less than 100pA - that creates <10uV offset.

FotatoPotato:
So I just did some probing and here are my results:

OPA277 output: 0.645v ± 0.01v
LM7805 out: 5.027v ± 0.02v
LTC1046 out: -4.882v ±0.24v
LM431 out: -2.654v (not connected to anything) and -0.186v (Connected to the rest of the circuit, voltage depends on the value of R29)
REF5040 out: 3.479v ± 0.8v depending on what the value of R29 is
Value of R29: 9.952K \$\Omega\$
Value of R28: 22.85K \$\Omega\$

R28 is very weird considering it IS a 4.7M resistor, I triple checked the color bands on it and it is a 4.7M resistor but for some unexplainable reason, it is 22.8K...  :wtf:

I think we are getting close to a solution and I definitely seem to be the circuitry surrounding R29 and R28. I am going to replace R28 and see what happens but I doubt it will make a difference.

iMo:

--- Quote ---So I just did some probing and here are my results:

OPA277 output: 0.645v ± 0.01v   <<<<< Still you have to limit its output current with a resistor - see above >>>>>
LM7805 out: 5.027v ± 0.02v
LTC1046 out: -4.882v ±0.24v
LM431 out: -2.654v (not connected to anything) <<<< 2.65 not good afaik >>>> and -0.186v (Connected to the rest of the circuit, voltage depends on the value of R29)
REF5040 out: 3.479v ± 0.8v depending on what the value of R29 is <<<< it must be 4.096 always >>>>>
Value of R29: 9.952K \$\Omega\$
Value of R28: 22.85K \$\Omega\$      :palm:

R28 is very weird considering it IS a 4.7M resistor, I triple checked the color bands on it and it is a 4.7M resistor but for some unexplainable reason  <<<<< DOUBLE CHECK ALL 40 RESISTORS >>>>, it is 22.8K... 
--- End quote ---

FotatoPotato:
Just replaced R28, no difference

StillTrying:
Something else to try.

What value have you reduced R1 2k2 supplying the LM431 to, I don't think ~1k is low enough.

The -2.5V supply needs 1mA for the 431(they usually work fine @ 0.7mA), 0.8mA for the OPSA277 and 0.66 mA for the VR29 10k pot = 2.46 mA.
Assuming the -5V is only -4.8V, R1 has to be (4.8-2.55)/2.46 = 915R, 820R should do.
 :palm:
(4.8-2.55)/1.66 = 1.4k or slightly lower for R1.


Edit: While you're there add a 1u to 10u to the -2.5V supply. :)

"REF5040 out: 3.479v ± 0.8v depending on what the value of R29 is"

It's still a mystery why the REF5040's 10mA 4.096V output isn't more than enough to power the DAC, unless the DAC is oscillating, or it's output is shorted.

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