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I'm designing a small circuit using an op-amp for a temperature sensor (PT100). I took the design online (see below). enter image description here

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

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    \$\begingroup\$ How about drawing a normal circuit with right angles at the very least? \$\endgroup\$
    – Eugene Sh.
    Apr 30 '19 at 19:14
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    \$\begingroup\$ Isn't this LTSpice, not Proteus? \$\endgroup\$ Apr 30 '19 at 19:16
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    \$\begingroup\$ Most definitely LTSpice \$\endgroup\$
    – Eugene Sh.
    Apr 30 '19 at 19:42
  • \$\begingroup\$ Do you know about net labels in LT spice? Hit the F4 key \$\endgroup\$
    – Voltage Spike
    Apr 30 '19 at 19:42
  • \$\begingroup\$ @Nwells clean up your question, it's confusing, there is no way to answer it \$\endgroup\$
    – Voltage Spike
    Apr 30 '19 at 19:43
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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. enter image description here Where Rx=R6=Resistance of the PT100 sensor.

The problem I was having was that I was wrongly connecting my PT100 (3-wires) RTD within the circuit (the above circuit assumes a 2-wire RTD). The other mistake was to use a 5V supply source for the Op-Amp: no, the voltage source for the op-amp needs to be at least 6V. Preferably, the op-amp should be powered by a 6V and -6V as I still get wrong results with temperatures below 50 degree Celsius. In my case, I'm mostly concerned with temperatures above 100 degree Celcius. So I'm fine with only a 6V supply for the Op-Amp.

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