Improving RTD Sensor filtering and readout

Question

For an application a temperature sensor is required. an accuracy of around +-3 degrees is acceptable.

For cost reasons I have chosen for this RTD. The sensor is sampled by a 12 bit ADC of an Microcontroller. The RTD is placed in an wheatstone bridge with a regulated 3.3v supply (looking at the supply with an scope I did not find much noise/ ripple). The Wheatstone Bridge is connected to an MCP6N11 Instrument amplifier.

^{simulate this circuit – Schematic created using CircuitLab}

The RTD is mounted on a separate board with about 50cm of wire, which runs through a noisy environment. Therefore the filter stage is placed between the bridge and amplifier.

The desired temperature range is between 50 and 200 Celsius. The gain of the amplifier is set to 15, and is supplied by a single 3.3v supply.

Unfortunately, the measurements are constantly off by 30 to 50 percent. As well as there is still some noise sneaking through the filter. The amplifier is connected directly to the 12 bit ADC.

What can i do to improve this circuit?

Answer 1

Consider getting rid of the bridge and instrumentation amplifier. Feed the RTD (via 10k) from the same reference voltage that your ADC uses - this is called ratiometric measurements and removes one of the big error sources i.e. because RTD and ADC are both ultimately fed from a common reference voltage, it doesn't matter if that reference voltage drifts.

So, from memory a 1k RTD will have a resistance of 1000 ohms at 0 degC and about 1690 ohms at 100 degC. If your ADC reference voltage is 3.3 volts and the RTD is fed from 10kohm, the voltage change between 0 degC and 100 degC will be 0.300 volts to 0.477 volts. This is a change of 177 milli volts for a temperature change of 100 degC

Your ADC is 12 bit, therefore it has a resolution of 3.3 volts/ 4096 = 0.80566 mV.

So, for a 100 degC change, the input voltage changes 0.177 volts with a resolution of 0.80566 mV - this means your ADC can resolve 219.7 steps in a range of 100 degC OR, its actual resolution is about 0.5 degrees C.

This should easily obtain the accuracy you want.

By the way, you need to calculate the full range up to 200 degC but i don't see any problems in achieving that to the accuracy you want.

In short, reduce your errors by getting rid of stuff you don't need.

When it comes to filtering, sure use an external low pass filter but also filter in software by oversampling the signal and averaging.

Answer 2

If you are using the same supply for bridge energization as the amplifier and the MCU, I don't see that much wrong with your circuit,

However, you must remove C4- it should not be present, or no more than a few hundred pF. The way you've shown it, it is highly likely to cause instability. If you can add maybe 1K between the output and C4 without causing ADC errors, it can stay.

Answer 3

You have a bug in your schematics. You have to connect the upper wire between R1 and RTD to positive output of 3.3V power supply (there shoul be a joint instead of crossing). And also disconnect the positive power supply from the R5.

And also for to have the input not floating, you should ground the negative output of 3.3V power supply to analog ground.

PS: And as Spehro Pefhany mentioned before, the C4 cant be conected directly to tth op-amp output. This will cause oscillations os op-amp. Place some resistor in between output and C4 (which i did not repair in my picture).