OP-Amp analog Subtractor

[Cali-Sam]

I am trying to incorporate an Op-Amp subtractor into a project. ( Op Amp Circuit attached)

I am applying about 2.5v into V1 and V2, therefore I am expecting a Vout value close to Zero.  I know that I cant get exactly Zero due to opamp offset
instead, I'm getting about 700 millivolt or so at Vout.

Node A and Node B are not equal, as I would expect if negative feedback is working.

Thanks

[T3sl4co1l]

What op-amp are you simulating?

I don't know Digikey's schematic tools, but it doesn't look like a theoretical (three terminal) op-amp model, rather, one that has the output constrained within the power supply limits.  Check the datasheet for V_OL (output voltage low / saturation limit).

Tim

[tszaboo]

700mV, so your opamp is not rail to rail output. If you want 0V, use an opamp with dual rail. Meaning, connect it to -5V;+5V or -1V;+5V or -12V;+12V or something like that. Even with a true rail to rail opamp, you will never get 0V (unless you do some dirty analog tricks).

[natbuk]

An alternative to running dual rail would be to move the 'ground' reference from the bottom of the non-inverting input resistor and make this, say your voltage rail /2 or 2.5V. Then you would have an output whose output was V1-V2+2.5. It's usual when using single-rail circuits to assume some kind of mid-point (such as half-rail) as a reference instead of ground. You will need to make that reference 'stiff' enough, depending on your resistor values,  - can use capacitor and resistors if you need only AC subtraction, but otherwise, might need another amp, or some other way to make low-ish impedance. If source impedance is known, though, you can just choose some resistor dividers carefully.

nat

[TimFox]

With a single-ended positive supply, the output cannot go below zero, therefore cannot go to zero.
(There are extraordinary exceptions, with some op amps that include booster circuitry to develop a negative voltage, but they are very rare.)
If you need to go to zero, you should have at least some negative supply voltage--read the data sheet to see how close to the negative rail the output can swing.