Summing Amplifier:
Vout = -Σ(Rfeedback/Ri*Vi)
If you make R1 = R2 = Rfeedback,
Vout = -(V1+V2).
Notice that the sum is negative. To get a positive sum, you can add an inverting stage, as shown in the lower circuit.
Here, three inputs are added together to yield the following output: Vout = V1 + V2 + V3.
For some real OP-Amps, an additional input bias compensation resistor placed between the noninverting input and ground may be needed to avoid offset error caused by input bias current. Its value should be equal to the parallel resistance of all the input resistors.
