Electrical – Op-amp gain decrease, output not linear


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!

Many thanks

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!


simulate this circuit – Schematic created using CircuitLab

Best Answer

If you are powering between 5V and ground, I suspect that the voltage on the positive input of the TL081 comning from the RTD is too close to rail to be a valid input. The other arm of your amp is probably having the same issue You are out of the allowed common mode range for that op-amp.

See table 6.6 on the relevant data sheet. \$V_{ICR}\$ is volts from the negative rail.

You should use a RRIO op-amp. The TL081 is not suitable for this use.