NTC Thermistor equivalent Temperature reconversion from ADC reading issue

Dayanand

Hi All,

I have a query, Can you please help to understand the thermistor calculations better...Will explain the details of thermistor in detail...Please check
Basically the calculation is done in two methods
1.Linear Interpolation( In sheet 1 of attached doc)
2.Simplified Beta (Sheet2)...

We are using NTC Thermistor NCP15WB473D0SRC(47KOhm,Beta=4050).The Thermistor is connected between ADC pin and GND. The Reference voltage is 3V.
Calculating procedure is as follows
1.Temp Equivalent resistance(from Datasheet)
2.Top Resistor value(4.64K+/- 0.1% tolerance)

3.Equivalent ADC (Rth/(Rb+Rth)* Adc max,4095 for 12 bit ADC
4.Adding ADC Error 9LSB to obtained result in Step-3.
5.AD Error Round up/down for realistic ADC value reading.
6.Reconverting Equivalent Resistance from the obtained ADC value included with ADC Error(9LSB).
7.Determining the Equivalent Temperature from Equivalent Resistance.

Our concern is when we determined the Temperature from Equivalent Resistance which is obtained from Reconversion , causes the Overlap between two temperature readings.

For Eg: -40 deg after reconversion gives values in betweeen -114.37C(Max) to -32.99C(Min)[/b] where as
-39 deg after reconversion gives -92.82C(Max) to -32.19C(Min). If we compare -40deg and -39deg readings,
-39 deg values are under -40 deg and they overlapped in these readings. In this case, it is difficult to distinguish the ADC readings between -40deg and -39deg.

Could you please check the above calculation attached and suggest us how to eliminate the Overlap between two temperature readings.....Please suggest if something incorrect in the calculation as well.. Thank you.[img][/img]

Tracy Allen

Late answering, but I think you are confusing the characteristic of the individual thermistor with the statistics of a a random selection of thermistors. You said, "If we compare -40deg and -39deg readings, -39 deg values are under -40 deg and they overlapped in these readings. In this case, it is difficult to distinguish the ADC readings between -40deg and -39deg."
Each one will have a repeatable difference for each degree of temperature change. However if you choose two 1% thermistors at random, they won't follow exactly the same curve, that is, the statistical distribution overlaps as you have documented. The secret is individual calibration, at one or more points, whatever it takes to achieve the needed accuracy, and that works best if you can narrow down the needed range. Do you really need -40°?

jmg

Dayanand wrote: »

Our concern is when we determined the Temperature from Equivalent Resistance which is obtained from Reconversion , causes the Overlap between two temperature readings.
... In this case, it is difficult to distinguish the ADC readings between -40deg and -39deg.

Did you try running this thru LTSpice ?
google finds a few examples
https://www.digikey.com/en/ptm/v/vishay-bc-components/advanced-lt-spice-modeling-features

DigitalBob

Checkout Maxim app note 1753