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I'm trying to built an op-amp circuit for a PT100 temperature sensor. I've found the following circuit diagram online (from a polish website): enter image description here And I'm trying to find the formula for the output voltage as a function of the input voltage. I came up with the following formula (sorry, it's an image, no Latex here): enter image description here Can someone please help me verify that the last formula expressing Vout as a function of Vin is correct? Thanks in advance

EDIT1: at the beginning of the demonstration, R1 and R2's values have been switched.

EDIT2: Please, this question IS NOT a duplicate of a previous question I've posted. In this question, I'm trying to verify that my computations are OK. In the other question I was trying to ask if it was common for Proteus to present results that slightly differ from the electric model.

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    \$\begingroup\$ Possible duplicate of Difference between Proteus results and KCL-deduced formula \$\endgroup\$ – Kevin Kruse May 1 '19 at 19:43
  • \$\begingroup\$ I think there is an error in the OpAmp law: \$V_1 = V_2 = v\$ should be \$V_4 = V_2 = v\$ \$\endgroup\$ – Huisman May 1 '19 at 19:59
  • \$\begingroup\$ Rest of equations (including last steps) seem correct. Next step, expanding A and B. Good luck :) \$\endgroup\$ – Huisman May 1 '19 at 20:08
  • \$\begingroup\$ @Huisman, you're right! but doesn't change the result! Thank you!! \$\endgroup\$ – N. Wells May 1 '19 at 20:25
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    \$\begingroup\$ You can use TeX for math stuff here. \$\endgroup\$ – JRE May 1 '19 at 20:56
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The above design is an amalgamation of a Wheatstone bridge and a differential amp. It is hard to analyze and IMO an improper way to design a temperature sensor circuit. A better way is to have a wheatstone bridge followed by a diffamp as shown below:

enter image description here
Source: https://electronics360.globalspec.com/article/5424/design-notebook-linearization-of-a-wheatstone-bridge

The equations for that bridge are as follows:

enter image description here
Source: https://electronics360.globalspec.com/article/5424/design-notebook-linearization-of-a-wheatstone-bridge

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  • \$\begingroup\$ The TS circuit is using positive feedback to linearization the temp pt100 sensor. ti.com/lit/an/slyt442/slyt442.pdf \$\endgroup\$ – G36 May 1 '19 at 19:58
  • \$\begingroup\$ @laptop2d, great answer, but sorry doesn't solve my problem. My circuit works pretty well on Proteus, although with slight different results. However, how do I find out the output voltage in your diagram? \$\endgroup\$ – N. Wells May 1 '19 at 20:29
  • \$\begingroup\$ @G36, thanks for the PDF link. Do you how I can get rid of VRTD in the formula presented in the document? Thanks. \$\endgroup\$ – N. Wells May 1 '19 at 20:33
  • \$\begingroup\$ @N.Wells As you can see the Author is using superposition principle to find the voltage at the noninverting input (V"+"). So, you need to do the same and find the expression for the inverting input (V"-") next set V"+" = V"-". ti.com/lit/zip/SLYT442 \$\endgroup\$ – G36 May 1 '19 at 20:45
  • \$\begingroup\$ @G36, I read the attached files and I don't understand anything in it; yet, I'll give a try in a proteus Simulation and let you know ASAP. \$\endgroup\$ – N. Wells May 2 '19 at 9:59

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