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!