The Summing Amplifier is a very flexible circuit based upon the standard Inverting
Operational Amplifier configuration. In the Inverting Amplifier if we add another input
resistor equal in value to the original input resistor, Rin we end up with another
operational amplifier circuit called a Summing Amplifier, "Summing Inverter" or even
a "Voltage Adder" circuit as shown below.