# Difference between Proteus results and KCL-deduced formula

I'm designing a small circuit using an op-amp for a temperature sensor (PT100). I took the design online (see below). I tried to find out the formula for finding the value Vout as a function of the input voltage. I've used the KCL (I think it's Kirshoff-current-law) at the various junction points. I got a formula that gives values that are offsetted by 0.2Volts from the output produce by a simulation of the circuit in Proteus (original diagram in LTSpice as seen in the picture above). My question is: is it common for proteus to add an offset during the simulation, or am I necessarily making an error in my computations. Thanks in advance

• How about drawing a normal circuit with right angles at the very least? Apr 30, 2019 at 19:14
• Isn't this LTSpice, not Proteus? Apr 30, 2019 at 19:16
• Most definitely LTSpice Apr 30, 2019 at 19:42
• Do you know about net labels in LT spice? Hit the F4 key Apr 30, 2019 at 19:42
• @Nwells clean up your question, it's confusing, there is no way to answer it Apr 30, 2019 at 19:43

I've found a pretty acceptable solution. The formula for the output voltage of the diagram above is provided in another question, I've recently posted: Output Voltage Formula verification of Operational Amplifier circuit. The diagram can easily be simulated in Proteus and provide a temperature range between 0 and 400 degree celcius. Where Rx=R6=Resistance of the PT100 sensor.