RTD sensor circuit design

Question

Design Criteria

For a project I'm making I want to measure temperature using a 10 bit ADC on a MCU.

I don't care for absolute accuracy but I would like precision down to around 1/10th of a degree C.

To clarify I'm fine with a bias and gain error and even weak non-linearity , as long as the random error (noise) is low and measurements are consistent between readings of the same temperature.

The sensor will be powered and read once per hour so self heat-up shouldn't be an issue.

I expect that the temperatures I will measure are around 10-40 C. However to have some design headroom I chose to use the range [-40,+100] C as my design criteria when sizing the passives.

Chosen RTD

I chose to do this with a RTD. The particular one I'm looking at has:

• 0C = 100 R
• -50C = ~70 R
• +100C = ~140 R

Circuit

The circuit I have come up with is shown below. It has four major components:

• A 1mA current source to bias the RTD.
• A level shift of the RTD sense.
• A scaling of the RTD sense.

The goal of the level shift and scaling of the sensed RTD value is to scale the range of [-50C, +100C] to [5V, 0V] to get maximal use of the 10 bit ADC on the MCU.

The TEMP_EN is connected to a GPIO pin on the MCU, driven high prior to measuring the output with the ADC. The GPIO pin can drive 20 mA max.

I have chosen 300R and 5K11 resistors for this design as they are used elsewhere in the project and actually form very close to the required ratios for the voltage dividers by a happy coincidence.

A simulation of the circuit can be found here.

I used WxMaxima to calculate the resistor values:

Where:

• Va = sensed voltage after RTD
• Vb = negative input to first op-amp
• Vc = positive input to second op-amp
• Vd = negative input to second op-amp
• A = op-amp gain (assumed ideal later)

I expect 150 C / 2^10 = 0.15 C precision on the measured value.

Question

This is my first time working with Op-Amps after my EE course in Uni which was a long time ago. And my first time building a constant current source. I would very much like input on the design of the current source, will it be good enough? Are there easy modifications I can make to make it better for this application?

Can I expect the circuit to have reasonable noise resistance? The environment is a typical home environment and TEMP_AO is connected via a 30-50cm shielded, otherwise quiet cable to the MCU.

Should I decouple the op-amps? There isn't any switching going on here so I'm unsure if they are needed.

I have the shift before the gain. Should I change the order to do gain first and then shift? Which is better?

Any other considerations I should make?

Answer 1

DIN standard 100 ohm RTDs are 100 ohms at 0'C and 138.50 ohms at 100'C.

Starting at the beginning, your switched current sink looks quite inadequate. You need it steady to within +/-0.03% for 0.1'C, and it depends on the (noisy) GPIO voltage of a micro vs. the (poorly defined and temperature-sensitive) Vgs of the MOSFET Q1 (and the matching to Q2, and some other stuff). Wrap an op-amp around the current sink for sure, and give it a proper reference voltage, not the supply voltage of the micro.

Even if you have a so-called rail-to-rail op-amp you should not work right to 0V and 5V if you actually are required to read those corresponding temperatures. Knock a bit (depending on the op-amp) off each end and live with reduced resolution.

As far as EMI susceptibility goes, you should put some low pass filtering ahead of the amplifier. 0.1'C is about 38.5uV DC which is a rather healthy signal at DC, but it's easy for noise, hum etc. to creep in.

Whether self-heating is an issue or not depends on the time constant of the RTD in the measured medium (including whatever protection tube or other materials are present) vs. your measuring time (probably you want to average a number of readings).

I would suggest not switching the current to the RTD unless you have some reason to do so. It will just cause a small offset that is relatively stable.

Answer 2

You will find it easier to use a PT1000 1k resistor - less current, higher voltage for the same heating.

I can't see your current mirror arrangement being vaguely workable - you need current to be accurate to 1/5000.

Filtering (i.e. a capacitor across R3) would reduce noise, though it will not the opamp noise that is the issue. You should always have decoupling capacitors, and you should have some RF filtering capacitors in the input circuit.

As a matter of interest, there are lots of good and proper ways to do it, and then there is this way....

My son was able to use PT1000 with a simple voltage divider with a picaxe micro, by summing 10bit ADC readings 16x. (ie. averaging)

0.1C is about 1/2500 resolution of resistance, so it may work.

Of course this requires a noisy enough ADC to fuzz the input by ~1 bit.

You plot a histogram of the residual after dividing but the N sum, you can see if the averaging is working - do you have enough noise.

Answer 3

Do decouple opamps, 1uF across the supply.

Temperature is easier to measure with just pull up. Ok, you may want to use 3 or 4-wire scheme to compensate the wires, but definitely you don't need a current mirror. For PT100 and 1mA current you would get a very linear behavior. But if this is not linear enough- take ten points look up table and interpolate.

A good filter will give you your 10 bits. Be sure to use a very low drift opamp, so circuit temperature will not hurt.

Maybe the best you can do is to run few simulations. Not for noise of course, but to see that gain and offset are all good. Use ltspice. Check how resistors of 1% behave vs 0.1%

Answer 4

Here is a tested VASTLY simplified temperature sensor circuit I have used. The Amplifier is a chopper stabilized zero offset error amp, And the sensor is 10 mV/degree completely linear output. Some very simple math will give you the necessary output to the Micro. If you are attempting to get 10 bit stability, you can't beat this. (Please guys, I am well aware there are SOME tolerances) But non the less, this is far superior in linearity and stability as you are going to find for the price. The op-amp and sensor can be purchased off Digikey, and I believe the AD8639 can be bought as a single circuit 8 pin dip.