Electronics > Beginners
Voltage reference not working properly #SOLVED#
<< < (5/11) > >>
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.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod