Choosing register value for NTC with voltage divider

Question

I need some help to choose resistor for voltage divider circuit to read temperature using NTC Thermistor. I'm planning to use STM32 with his 12bit ADC.

The NTC I'm using has the following properties:

• Sensing Element: Thermistor NTC
• No-load resistance at 25°C: 10Kohms
• Tolerances at 25°C: ±1%
• Beta(25/85) Constant: 3977K ±1%
• Operating temperature range: -40C ÷ 125C

More information available here: Datasheet.

The temperatures I will be measuring are between 15C and 65C.

I'm planning to build a simple circuit like this:

What would be a good value for Rs for this conditions? I believe is better to use the 3.3v rail instead of 5v from STM32, is that correct?

I think with this circuit when temperature raises the voltage on Vout will decrease. I also saw some designs where Rs and Rt are flipped. Is there any advantage of the other (flipped) approach? With this approach I believe I can pass less current on Thermistor as ideally should be max 1mA.

Can I get reasonable good precision with circuit like this or shall I consider noise cleaning with capacitor or more complex circuits using OpAmp etc?

Answer 1

The STM32 most likely uses 3.3 V as the DAC reference so for the sensing circuit it makes sense to use 3.3 V too.

Usually the approach is to select a resistance \$R_{s}\$ which will maximize the voltage swing seen at the ADC pin for the temperature range you want, which depends on the sensor resistance at those temperatures.

So, based on min and max temperature you want, calculate the min and max thermistor resistances \$R_{min}\$ and \$R_{max}\$.

Then the optimum \$R_{s}\$ that maximizes the voltage swing is the geometric mean of \$R_{min}\$ and \$R_{max}\$, i.e.

$$ R_s=\sqrt{R_{min} \cdot R_{max}} $$

Just make sure that the \$R_{s}\$ value is sensible and does not lead to too much current flowing and affect the reading by heating up the thermistor. Also it should not make much difference which way the thermistor \$R_{t}\$ and resistor \$R_{s}\$ are in the circuit.

Answer 2

As you want to cover the range 15°C to 65°C, you should use a resistor equal to its resistance at the mid temperature of 40°C. This will give you the best (or least worst) resolution, more or less equal resolution at both extremes of your temperature range, assuming your ADC goes rail to rail. That said, ADC resolution at the extremes will be somewhat worse than your mid-range resolution.

You might want to consider changing the value of R_S as the temperature changes, perhaps with the use of three-state outputs from your MCU, or with analogue switches or FETs, to keep your resolution up. The best resolution is obtained at around half rail, where R_S ~ R_T. The bonus from this is that you'll have excellent linearity of temperature to voltage over about ±5°C from the equal resistance point.