Summing amplifier can be constructed using non-inverting configuration. The summing amplifier below shows V1 and V2 are connected to the non-inverting input (V+) of the op-amp. We can apply superposition theory to calculate the V+, then use standard non-inverting feedback gain equation to evaluate the output voltage, Vout.
The circuit example below shows V1 connects to a 2kΩ resistor and V2 connects to a 3kΩ resistor, which connect to the V+ of the op-amp. The 9kΩ feebdack and the 1kΩ resistors are arranged in standard inverting configurations.
Apply superposition theory, first let V1 = 0 (ground), we then get V+ = (2/5)V2, based on voltage divider rule.
Vo1 = (1 + 9kΩ/1kΩ)V+ = 4(V2)
We then let V2 = 0 (ground), we get V+ = (3/5)V1
Vo2 = (1 + 9kΩ/1kΩ)V+ = 6(V1)
Hence, Vout = Vo1 +Vo2 = 6(V1) + 4(V2)
For V1 = 3V and V2 = -4V
We get Vout = (6 X 3V) - (4 X 4V) = 2V