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I want to add two voltages.

I have tried using the Op-Amp adder here. The resistor schematic before the summing amplifier contains resistors in parallel which would average the voltage.

My problem is that I would have one steady voltage and another variable voltage coming from a pressure sensor. It would vary.

What I want is to add 0.5 volts to the voltage from a sensor (say 1.5) so that resultant voltage is 2.0.

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Why would you add the voltages instead of either multiplying or just using a comparator/ADC? – Ignacio Vazquez-Abrams Dec 23 '13 at 16:24
Well I have to pass it to some other system that i can not modify – Lagrangian Dec 23 '13 at 16:46

If you want to add two voltages together without introducing an inversion it's very easy. Here's one where three voltages are added but remember the output is an average of the three (or two or however many sources you wish to add): -

enter image description here

Next, you need to provide a gain stage from an op-amp so that the "3" part in the denominator is converted to "1".

In the OP's question, a gain of two would be required and this is simply achieved in a non-inverting op-amp configuration with a 1k feedback resistor and a 1k resistor to 0V: -

enter image description here

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Since you want to add two voltages you can use the following differential amplifier

Diff amp

The circuit amplifies the difference of the two inputs with a gain set by the resistor ratio and adds the offset \$V_R\$

In your case, use the same value for all resistors to get a gain of 1 and then connect \$V_1\$ to ground, \$V_2\$ to the variable voltage from the pressure sensor and apply the 0.5v offset to \$V_R\$

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To help the OP in case he/she thinks these are two different methods, please note that this suggestion and my suggestion are identical – Andy aka Dec 23 '13 at 17:41
Starting from a difference amplifier is a very confusing way to describe and implement a simple summing amplifier. What did you add that was not in @Andyaka 's answer? – Joe Hass Dec 23 '13 at 18:51
@JoeHass I'm not why you are getting personal with me but everything I do today is wrong according to you. Why do you see such a problem when I name the schematic I have provided a differential amplifier ? What did you add that was not in @Andyaka 's answer? A single schematic that I think is easier to understand and the equation used to calculate the output. – alexan_e Dec 23 '13 at 19:14

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