Summing Amplifier:
This circuit is an inverting amplifier with different weights assigned to each input of the summing amplifier through the input resistors.
The weight of a particular input is set by the ratio of feedback Rf to the resistance of this input Rx.
The output voltage is:
Vout = - (Rf/R1 + Rf/R2 + Rf/R3 + Rf/R4 + ...)Vin
A summing amplifier can be made to produce the average of the input voltages by using equal input resistors (R1 = R2 = R3 = R4 = R) so that the ratio Rf/R is equal to the reciprocal of the number of inputs (N).
Vout = -(1/N)*(V1 + V2 + V3 + ... +Vn) = -Vavg
