Selecting Voltage Divider Resistor and OpAmp For Temp Sensing With NTC Thermistor

Question

I've been doing some reading on here about measuring temperature with NTC thermistors and using the Steinhart-Hart Equation.

However I am still a bit confused about the selection of a resistor to put in a voltage divider with the thermistor. Should I select a resistor value that is the same as the thermistor's resistance at the centre of my range of interest (200F) to maximise the sensitivity?

Also do I need an opamp or inamp to remove any noise common to both thermistor leads as the thermistor probe will be in an espresso machine boiler, so I expect some mains and switching noise ?

If I use a precision voltage reference for the thermistor, my current understanding is that I must use that same voltage for the ADC, is this correct?

Answer 1

Probably you should use the mid-range-interest resistance. The exception might be if you were willing to give up some actual performance to make the temperature display look better at room temperature, with a low-resolution ADC.

Noise can be mitigated with a simple RC filter - temperature is a slow variable.

Reference voltage is preferably the same as the ADC but really a thermistor typically changes around -5%/degree C so any "reasonably stable" reference will give you acceptable performance for your particular application.