INVERTING ADDER WITH GAIN

The output gives addition of all input voltages multiplied by Gain along with phase shift of 180 deg

The output Vo of this op amp circuit gives the sum of all input voltages V1, V2 & V3 multiplied by Gain (RF/R) along with phase shift of 180 degrees. That is Vo = -(RF/R)(V1+V2+V3)