I need an algorithm to find out whether temp is increasing

[temtronic]

You can get better performance using constant current to drive the NTC, esp. in a bridge config. It'll do away with any VDD variations causing 'funny' readings.

Really is doesn't matter what type of sensor you use, providing you get solid, reliable, consistant readings from it.

The good thing about temperature 'systems' is they are a slooooow process. Usually takes a long time so see any big changes. You could take 16 samples over 4 seconds, and get a very accurate result.

Jay

[Mike Walne]

I agree, using a constant current source will eliminate the effect of variation in VDD.
What then happens is you get back the NTC's very non-linear characteristic.
Adding a suitable resistor in parallel with the NTC does linearise the characteristic.

However, the o/p seems to be using VDD to supply both the NTC/resistor chain and Vref.
In this case:-

Input_Voltage/VDD = ADCreading/maxADCreading

Giving

ADCreading = maxADCreading * Input_Voltage/VDD

The NTC and the resistor are a simple resistive divider so:-

If you change VDD by x% the Input voltage will change by the same x% and the ADC reading will not be affected!

Mike

[temtronic]

Good catch Mike ! I remember using the constant current method for some Honeywell pressure transducers, 3 decades ago...sigh...doing that got a 20x better performance out of the transducers. THAT stuck in my head, though lots has left in the meantime.

Jay

[Ttelmah]

The constant current will help here.
The problem is that the self heating of the NTC element changes with
drive voltage and this will affect the reading returned.

Honestly, NTC's are 'the way to go' for reading temperatures outside
the range of semiconductor sensors. If reading 1000C, only way to go.
But for temperatures like this they are hard to make work well and give
really accurate repeatable readings.
I'm sure he 'thinks' the reading is good, but he may well be surprised
how inaccurate the reading really is between different units, and 'long
term'. However that being said, simple things like where the sensor is
placed relative to the heat source and how different numbers of eggs
in the incubator affect air flow, may well give bigger errors...