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[on:]

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.

[Reply #1 on:]

Remove the 10K at the i/p of the 4th amp what happens?
As usual, all the component values are correct?

[Reply #2 on:]

Thanks for the reply. Removing the 10k resistor on the positive input of OA4 has little effect. Even with the output gain up so it's ouputing almost 1V, there is a 1mV difference. I'll try the pull-down and see what it does.

[Reply #3 on:]

A pull-down resistor did the trick. Please feel free to use this circuit. Just give credit where it's due. If you have any modifications to it, please post here so everyone can benefit. This is the first complete RTD pt100 input circuit I've seen in the public domain which includes a current source and output-scaling. There are others out there which do different parts better, but not all together like this.

[Reply #4 on:]

It appears to me that IC1C does nothing.  R7 and R5 inputs are tied together so there will never be a potential difference to give IC1C and output. Or am I missing something?

[Reply #5 on:]

It appears to me that IC1C does nothing.  R7 and R5 inputs are tied together so there will never be a potential difference to give IC1C and output. Or am I missing something?

Yes, it's just being used as a non-inverting buffer with a gain of 1 to isolate the impedance of the RTC circuit.
IC1A&B form a constant current source to drive the RTD. And IC1C is forming a gain stage with offset adjustment control, although I'm not sure that stage is wired correctly or not.

Lefty

[Reply #6 on:]

Pin 9 of IC1C is a virtual ground. Does it not effectively "ground" R5 at the input of the opamp? If so, the "buffer" input resistance is no higher than R5. By removing R5 you now have an "infinite" input impedance of IC1C, and not limited by R5.

Maybe I have been out of school to long.

[Reply #7 on:]

OK, I know why I was confused. The RTD is a 3-wire unit. If the schematic had shown the cable resistances of the 3 wires it would have been clearer. They could have been shown as Rw1, Rw2, and Rw3 - as an example.
With that in mind, IC1C will act not just as a buffer but also cancels the voltage drop produced by the current through the RTD wire resistance. This technique increases the RTD measuring accuracy when compared to a 2-wire RTD.

I'll go back to sleep now....

[Reply #8 on:]

A pull-down resistor did the trick. Please feel free to use this circuit. Just give credit where it's due. If you have any modifications to it, please post here so everyone can benefit. This is the first complete RTD pt100 input circuit I've seen in the public domain which includes a current source and output-scaling. There are others out there which do different parts better, but not all together like this.

I don't read schematics that well.
Where are the wires to the PT100 and where are the pins going to the Arduino?

Thank you.