I want to know how to calculate the output of the below op amp. I know this is supposed be an inverting amplifier with offset but I have never seen the connection of the offset to the negative side of op amp.
simulate this circuit – Schematic created using CircuitLab
I have come up with this formula:
$$V_{\text{out}} = V_{\text{in}} \times \frac{R_2}{R_1} + \frac{R_2}{R_4+R_7}V_\text{p}$$
Is it correct? \$V_\text{p}\$ is the voltage out of the potentiometer.
R7 does not do anything of value, it is actually harmful (increases output noise and output offset voltage- and related drift). Ideally there is no voltage across R7 so it has no effect, but op-amps are not ideal and if you think about the resistor approaching zero it's clear there will be problems. You can calculate the effects if you model the op-amp with Vos and Vn voltage in series withe one of the inputs (offset and noise).
Vout ~= \$ -V_{IN} \cdot \frac{R_2}{R_1} -V_P \cdot \frac{R_2}{R_4}\$, ignoring op-amp gain and offset voltage.
You can find that by applying KCL to the non-inverting input node, assuming zero input current to the op-amp and then find Vout such that V- = V+ = 0V.
