Electrical Engineering Dictionary
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
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An operational amplifier can easily be made into a summing circuit by using additional input resistors. An analog adder or summer can be made using either an inverting amplifier or a non-inverting amplifier. However when using an inverting amplifier the output will be inverted, although still the sum of all the inputs.

An inverting adder is shown below, which means that the output is the inverted sum of the three inputs: Vo = -(V1 + V2 + V3). Three inputs are shown in the schematic as an example, although any number of inputs over two could be used. Normally all of the resistors are selected to be the same value [R1 = R2 = R3 = R4], so Rf/Ri is equal to one [for an inverting amplifier], producing an output that is the sum of the input voltages. Each input has the same gain through the amplifier; R4/R1, R4/R2, and R4/R3. A Scaling Adder can be produced by changing the input resistors for each input. Because the sum of the individual inputs depend on the ratio of the output and input resistors, the input voltages may be scaled. Set the feedback resistor [R4] to some value. For a 1-to-1 ratio set one of the input resistors to the same value. For a 1-to-10 ratio set the input resistor to a tenth of the feedback resistor value, and so on.

Using a standard 10-to-1 ratio set R4 to 100k, R3 to 100k, R2 to 10k and R1 to 1k. This produces an output with three different scaled inputs. So Vo = 100k/100k + 100k/10k + 100k/1k.
Than: Vo = V1*1 + V2*10 + V3*100.
However those are just examples, the inputs could be scaled to any values.

The non-inverting amplifier could also be used as an adder circuit, but normally only the inverting type is shown. Refer to the circuit below 