I'm trying to make a circuit with Op-Amps so I can get readings from a PT100 temperature sensor with an Arduino ADC.
I have based my design on this design: RTD Temperature Sensing
- I want to read from 0°C to 500°C with the greatest resolution possible.
- I'm using a LM2902NFS-ND Op-Amp
- I'm using a single 5V supply
- The PT100 has a 4.7kΩ pull-up resistor
- The 1kΩ and 47kΩ voltage divider so 0°C will output 0V
- The gain should be 19.6 so I have an output of 3.309V at 500°C
After a lot of verification, it seems like the circuit does not behave as expected. The voltage offset at 0°C seems to works, but the gain doesn't. I have taken multiple readings between the PT100 and the output, and it seems like the gain drops after the output is 1.2V, so about 200°C.
At 490°C, I should have an output voltage of 3.25V, but I have 1.35V.
From 0°C up to 200°C, the gain is about 15 and is pretty stable.
I have tried powering the Op-Amp with 12V without success. The rest of the circuit was still powered at 5V using a LDO 5V regulator from the 12V supply.
I have the same set-up for 5 sensors and they all behave like the same.
I'm not an expert in electronics and I would be really happy to learn what is wrong with this setup!
Here are some measurements:
R_RTD - V_RTD - VOA2_out - VOA1_out - VOA3_out
100 - 0.107 - 0.107 - 0.105 - 0.002
138 - 0.147 - 0.107 - 0.145 - 0.754
175 - 0.185 - 0.121 - 0.183 - 1.194
212 - 0.221 - 0.154 - 0.219 - 1.263
247 - 0.255 - 0.186 - 0.253 - 1.306
280 - 0.287 - 0.216 - 0.285 - 1.339
As you pointed out, the VOA2_out is problematic. I will try the fixes proposed!