Definition:
The circuit which produces the output as the sum(addition) of the inputs,then the circuit is called ADDER (or) SUMMER.Since ,the circuit uses the inverting terminal input of op-amp then it is called as INVERTING ADDER (or) SUMMER.
Example:
consider the circuit,the circuit has three inputs namely V1,V2,&V3.The input voltage V1 consists of 5V and the input voltage V2 consists of 2V and then the input voltage V3 consists of 3V.The circuit produces the output as (5V+2V+3V)=10V.Since it is an inverting input it produces the output as-10V.
