I'm trying to set up this circuit as a sum opamp, trying to get 8,3 V out, but not matter what I do, I get 9 V out. I tried setting both inputs to 3,3 V, which also resulted in 9 V out.

Does the summing opamp setup only work in the inverting input of the opamp?

 

hi SP,
You should have the OPA inverting input with the resistor network for feedback , not the non inverting.
Do you want a sketch.?
E

EDIT:
Something like this option.

 

Thanks Eric. Not sure I quite understand the how (read some old threads, too)

 

hi PG,
That amp with a feedback and an input resistor of 10K, will give a Gain of one on the Inverting input.
ie: Gain =Rfb/Rin, G=10k/10k= 1,,, for the non inverting input the Gain is 1+ [Rfb/Rin] =2.

So I have added a resistive 10k divider on the non invert, as you have 10k input resistors.
So a 5Vin will appear as 2.5V at the NI input, the Gain of 2, will make the OPA Vout=5V
Same applies to the 3.3Vin.
I have assumed the output impedance of the 5V and 3.3V to be zero, in an actual circuit there will be some source resistance.

You may have to increase that 10k to 0v from the NI input to a higher value, I would use a 50k pot and adjust as required to to get correct Vout from the OPA.
Do you follow OK.?
E

 

Thanks a whole lot. Much better

 

For a true summing amplifier, you *need* to have the input look like a virtual ground. That means you need to use an inverting configuration. Take your original circuit and swap the inverting and non-inverting inputs. Then add an inverting stage with a gain of -1 on the output.

 

I don't have a -12V supply, hence the non-inverting setup. When I made my computer PSU, I never thought I would need -12V for anything

I will try that.

 

hi SP,
Do you have a couple more of those OPA's to use for the project.?
E

 

How accurate do you need it to be? This will be pretty close (the AD820 is probably not the best op amp. You want one with rail-rail inputs, designed for single supply)

 

hi SP,
What is the purpose of combining the 5v & 3.3v to give 8.3v.?
They maybe alternative ways of getting the required result.
E