Help!!how to design a voltage adder??

[Question]

I will design a voltage adder with amplifer, need Vo=v1+v2. The basic architecture as the image below(but use two input with 100K resistors),Rf=Rs=100K. The gain of amplifer is more than 80dB, and the bandwidth is 10KHz.

But I can not get the accurate output. How to make sure that two input terminals of amplifer are equal?

Who can give me some advice of how to design a voltage adder.

Thank a lot!!

 

[simce]

Maybe this will help:
**broken link removed**

 

[VVV]

Make one of the input resistors variable. Then apply a voltage to one input with the other shorted and measure the output. Record the output. Then swap the inputs: connect to GND the one that had the voltage applied to it and apply the same voltage to the other input. Adjust the variable resistor to obtain EXACTLY the same output as before. Repeat until it seems perfect.

Then you can short the two inputs together and apply the same voltage to both of them. The output should be exactly twice the input.