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Summing Op-Amp Circuit Trouble

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wayfar3r

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I've made an opamp summing circuit to take an input signal in the range of 0-1.5V and scale it between 1.25V-4V (directly related). The circuit works great in LTSpice. I've been unable to realize it on a breadboard though. Now, I'm not using a linear tech opamp (AD712) on the breadboard, so maybe it would work fine if I ordered the LT part. I see a voltage around 1.8V at the output of the opamp when the reference signal (V2) is at 0V. As the voltage is increased, the output eventually jumps to 4V. One observation, the power supply shows 3/10 of a volt at V2 even when that supply is set for 0 (feedback through voltage divider). Any reason why this circuit would be realizable in a simulator but not in real life? Do I likely just need the LT part? Thanks!

circuit.PNGPlot.PNG
 

FvM

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When using AD712, you should also observe the specified supply and common mode voltage range. AD712 will only run in this application with a negative power supply, e.g. V4 = 5.
 

wayfar3r

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FvM, you are correct. I also added an input buffer before my input signal so that it could sink current as it does in the simulation. This seems to be working. Thanks!
 

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