I have created the following differential op-amp circuit with the aim of converting a 0-3.28V analog signal into a -10V to +10V output. I have calculated the gain required 3.28/20 = 6.1
From looking at differential circuits the gain is equal to the ratio of resistors used so I chose a 10k and 61k. However when testing the circuit I needed close to 74k to get the required output range. Can anyone explain why there is such a difference between the design values and the actual resistor values?
PS. The voltage divider in the top left is to provide the midpoint value 1.64V for the circuit.
This should really be added as a comment but need to add the link. I found a formula on the following website, maybe I misinterpreted it.
Clearly this is not the case, thank you for your explanation, it is by far the easiest to understand that I have seen so far.