Hi guys, I designed the attached circuit for a pt100 RTD temperature sensor, 3 wire. The top part is a current driver set up for about 3mA. The bottom part below the RTD is for amplifying and adjusting the offset and sensitivity of the output.
The problem I'm having is as follows: The circuit behaves normally up too the last (4th) op amp. Voltage does change on the output of the previous (3rd) op amp with temperature. The output of the last (4th) op amp is only about 15 mV no matter what, where it should be outputting more like 300 mV or more when adjusted correctly at room temp. Is it because I don't have a 56k pull-down resistor on the non-inverting input (+) on the last (4th) op amp?
Another problem I noticed is that the circuit actually works when I connect a test resistor in place of the RTD, but when I connect the RTD it fails to output from the final op amp. I verified this with two different RTDs. Strange, but it's what I observed. They act as described in the above paragraph.