# Inverting op-amp unexpected voltage shift

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

• If you don't need to go down to DC, just use capacitative coupling with a suitable bias at the input to the ADC with a couple of resistors. No need for an opamp. Commented Mar 29, 2023 at 23:20

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.

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

• Ah thanks. How did you arrive at the "2.5*(1+1/3.5)" equation? Commented Mar 29, 2023 at 17:59
• @tpalmer345 That's the formula for calculating the gain of a non-inverting OpAmp stage. Commented Mar 29, 2023 at 18:06
• That is basically the gain of the non-inverting amplifier (1+Rfb/Rin) multiplied with the DC voltage on the +ve terminal which is 2.5V
– sai
Commented Mar 29, 2023 at 18:08
• A brilliant thank you so much guys, really appreciate it! Commented Mar 29, 2023 at 18:09