Producing the Sum and Difference of two signals with two Op-Amps

Question

I have two signals, one is a baseline voltage (V1) and the other is a 'difference' voltage (V2). From these I want to produce two signals, V1 + V2 and V1 - V2. The circuit I currently have is:

^{simulate this circuit – Schematic created using CircuitLab}
Where R is the same everywhere.

Based on the diagrams found here:
http://www.electronics-tutorials.ws/category/opamp

Using This Op-Amp

I have a feeling something about this is wrong, and I can't just drive both a summing and differential amplifier in parallel. If so, can someone give me advice on how to get both of these voltage values with just the two inputs?

Answer 1

You can drive two amplifiers in parallel as long as you don't load the inputs too much (choose your resistor value in consideration of the drive strength of your signal sources).

You need a different summing amplifier circuit in order to get a non-inverting sum, however. Try a simple voltage divider connected between the two inputs, the result is the average of the input signals \$\frac{V_1 + V_2}{2}\$. Connect this to the positive terminal of an non-inverting amplifier with a gain of 2.

^{simulate this circuit – Schematic created using CircuitLab}

The differencing circuit you have now looks ok.

Answer 2

This may help:

^{simulate this circuit – Schematic created using CircuitLab}

This can be realized with 4 opamps.

Instead of using LMx58, I recommend using TL074 (single IC, 4 opamps) with dual (e.g. ±12V) supply.

Answer 3

I have a feeling something about this is wrong

Your "sum" amplifier produces an inverted output -(b+a). Ben has pointed out a possible alternative strategy that will give you (b+a)

and I can't just drive both a summing and differential amplifier in parallel. If so, can someone give me advice on how to get both of these voltage values with just the two inputs?

There are a number of questions you need to ask yourself before you can even begin on a design like this.

1. What is the impedance of your input signal sources.
2. To what level is it acceptable to load the inputs.
3. Is it ok if the loading on one input depends on the voltage currently present on the other input potentially causing input to input crosstalk?
4. How accurate do the calculations need to be.
5. What frequency does this all need to work at.

Depending on your answers to these questions the soloution may turn out to be very easy or very difficult.