Op-amp gain decrease, output not linear

Question

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

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Edit: 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}

Answer 1

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.

Answer 2

To expand on Scott Seidman's answer:

Here is the datasheet in question, and references the common mode voltage limits on page 5 at the bottom. The lower limit is \$V_- + 4V\$, which means that if your signal is lower than 4V, you won't be in the linear regime of the input to your op amp. Given that you are at a quarter of a volt over ground (when at 100Ω), you are likely running into this problem.

There are a couple of different ways to fix it:

• True rail-to-rail op amps would improve things a bit: but most rail to rail op amps still aren't really meant to be linear in that regime.

• A different schema which moves the voltage to be measured to the center of your supply range:

^{simulate this circuit – Schematic created using CircuitLab}

With both of these (yours as well as mine), there will be some error from the fact that you are using resistors as a current source and trying to measure the voltage over the sensor. Since the change in the resistance of the sensor will change the total resistance, the current through the sensor will change as well as the voltage. From this, it looks like the resistance will change from 100Ω to ~400Ω over the temperature range that you are specifying. So you will need to take that into account. That change will likely mean unacceptable error over the full range of your measurement.

Answer 3

This RTD amplifier needs a precision Vref or Vcc to drive the bridge comparing the two R dividers.

The main issue is to determine the RTD (500'C) and then the Voltage across it. Then Choose a gain to generate the Vout of the ADC full scale 3.3x .

R3/R4 matches RTD/R1 ratio at 0'C. ( fix your schema). NB! Vin+ MUST be near Vcc NOT ground , so pullup RTD to Vcc and 4.7k to 0V

This common mode , CM input voltage error reduces gain. If you have noise use twisted pair to RTD thru as many turns around ferrite bead or torroid as possible with small magnet wire or buy CM choke.