# operational amplifer (un expected behaviour)

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#### Roff

##### Well-Known Member
I fail to see the need for the attenuator on the (+) input, other than the fact that the input range will be pushing the positive common-mode input limit. The part is guaranteed to work at Vcc-1.5V over temperature. I guess that, if you are conservative, and are building lots of units, this might be an issue.

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#### dark

##### Member
All good but complex solutions.

What you're experiencing is gain error; you just failed to account for your 2.5V source's Thevenin impedance of 3.75k. If you don't use the 2.5V elsewhere, you just have to account for it.

Just reduce Rin on the (-) side of the op amp by 3.75k and the output will be as you expect. You'll have to calibrate it with the sensor output at 2.5V.

The other solution is to increase Rin and Rf. Is Rin currently about 4.0k? With sub-nanoampere bias and offset currents you should be able to increase Rin and Rf by a factor of twenty or more and reduce their influence on the 2.5V. You can still reduce the Rin on the (-) side by 3.75kΩ which should now be a small percentage.

Hi ,

I take Rin=30K , Rf=120K .

and reduce the source impedance of the reference divider network to less than 3.7K as pointed as follows;
Code:
             OE              200E              300E
GND----/\/\/\/\-----/\/\/\/\--------/\/\/\/\/\-------10V
^
|
|_________to  rest of the ckt
As said in your post, when I reduced the Rin too low (~3K) it created problems . And the following suggestion of increasing the Rin and Rf , it works good with reduced source impedance from the 2.5V reference CMIIAW. How did you arrive to 3.7K figure? formula would be helpful in understanding further. With the above I could actually get a full output signal swing and differential voltage input to opamp as 0.01 perfect .
Thanks

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