Inverting op-amp unexpected voltage shift

Question

I'm trying to create an op-amp circuit that attenuates a 5 V input signal down to a 1 VRMS Vout signal (1.414 V Vout).

I need to shift the output signal by 2.5 V so that it can be fed into a unipolar ADC.

Below is my current inverting op-amp circuit in Multisim:

As can be seen above, the Vout signal amplitude is spot on: (4.6338 - 1.7895) / 2 = 1.422 V (roughly 1.414 V).

The problem is that the blue Vout signal appears to have shifted much further than expected. The midpoint should be on the purple 2.5 V line but it is actually at (4.6338 + 1.7895)/2 = 3.211V.

Does anyone have any idea what's causing the unexpected voltage shift, and how to fix it?

My circuit on Multisim

Answer 1

Your voltage divider applies 2.5V to the non-inverting input of the OpAmp. To see what's happening there, just set Vin=0 and draw the circuit again:

^{simulate this circuit – Schematic created using CircuitLab}

It should be easy to see that this is a non-inverting OpAmp amplifier circuit with a gain of about 1.286. As a result, if you apply 2.5V DC to the non-inverting input, you get around 3.2V DC at the output. That's exactly what you're seeing in your simulation.

If you want 2.5V at the output, you have to apply 2.5V / 1.286 = 1.944V at the non-inverting input instead.

Answer 2

Mid-point of vout is (4.6338 + 1.7895)/2 = 3.211V which seems expected.

2.5V is the DC applied at the opamp +ve input. At output, it is 2.5*(1+1/3.5) which is what you got