# Connecting signal to multiple summing amplifiers

For a normal op-amp inverting summing amplifier, each signal is tied to the negative input of the op-amp. Imagine the scenario where I have three signals. In adder 1, I want to sum A and B. In adder 2, I want to sum B and C. So I connect B to both negative inputs.

Will there be any interference between the signals? Is this going to cause any problems?

simulate this circuit – Schematic created using CircuitLab

Will there be any interference between the signals? Is this going to cause any problems?

A single op-amp with negative feedback seeks to maintain the differential voltage across its inputs at zero volts by adjusting its output voltage. If the op-amp has got an input offset voltage error of +1 mV then the op-amp will have an error on the output. That error is generally gain*input_offset. So if the gain is -1 (as per an inverting op-amp) there will be a -1mV error at the output.

This is normal and accepted by design engineers however, if you have two op-amps (one with +1 mV input offset error and the other with -1 mV input offset error) they will compete to set the common inverting input at a voltage that suits them. There is no winner here; one op-amp will output against a power rail and the other may do the same but the opposite power rail.

The input resistors are a crucial part of the circuit, and sharing the R3 resistor into two summing junctions just isn't feasible. Try this:

simulate this circuit – Schematic created using CircuitLab

Signal B is driving 5k ohms load to ground, if that matters, in this revision.

Think about the way the current is going to flow. As designed, equal current from each signal flows into both summing amplifiers. Their outputs will always be the same, $\frac{A+B+C}{2}$. This is the ideal case and assumes zero op amp offsets.

You need to isolate the two summing nodes to prevent this from happening, like this. The voltage buffer is not required if the source has low source impedance. Note that the outputs are negative(A+B) and negative(B+C) because summing amplifiers invert. If you need to maintain polarity, just stick an inverting buffer in series.