As the title implies, this is an adding device. The circuit can take any number of voltage inputs, and the output voltage is simply the sum of these inputs.
The circuit in Figure has a feedback loop containing a resistor Rf and the facility to sum three input voltages.
It must again be emphasised that using negative feedback, the feedback loop forces the output to take on a value, which reduces the input difference voltage to zero.
The input voltages V1, V2 and V3 are supplied to the inverting (-) terminal via resistors R1, R2 and R3. Since the input difference voltage is zero, the potential at the inverting terminal and hence the junction of R1, R2 and R3 is the same as the non-inverting (+) terminal, which is earth. Also, since the inputs are applied to the inverting terminal, the output voltage will experience an inversion (sign change).
The output of the circuit, then, is the sum of the input voltages. Note again that inversion takes place due to the inputs being applied to the inverting terminal.