An op amp is an amplifier. But an op amp can also perform summing operation. When an op amp circuit is designed to combine numbers of input signal and to produce single output as a weighted sum of input signal, the circuit is called Summing Amplifier.
This simple inverting amplifier can easily be modified to summing amplifier, if we connect several input terminals in parallel to the existing input terminals as shown below.
Here, n numbers of input terminal are connected in parallel. Here, in the circuit, the non-inverting terminal of the op amp is grounded, hence potential at that terminal is zero. As the op amp is considered as ideal op amp, the potential of the inverting terminal is also zero. So, the electric potential at node 1, is also zero. From the circuit, it is also clear that, the current i is the sum of currents of input terminals.
Therefore, Now, in the case of ideal op amp the current at the inverting and non-inverting terminal are zero. So, as per Kirchhoff Current Law, the entire input current passes through the feedback path of resistance Rf. That means, From, equation (i) and (ii), we get, This indicates that output voltage v0 is weighted sum of numbers of input voltages.