NTC Thermistors - I''m baffled!
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First, here are links to the datasheet and a document at Maxim that descibed
how to interface a thermistor to an ADC I'm going to reference below:
http://rocky.digikey.com/WebLib/BC%20Components/Web%20Data/2322%20640%203,4,
6%20NTC%20Thermistors.pdf
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1753/ln/en
Okay, The thermistor I have is 2322-640-63102 (R25 value of 1000 ohms). The
Maxim datasheet discusses a simple voltage divider of the thermistor and
another resistor, and includes equations that (to me) should be able to
generate a table showing the relationship of the measured value (the value
of the ADC, for me 12 bits, 0-4095) and temperature.
I've written a C++ program that generates a table using the equations listed
in the Maxim document, substituting the T(R) function from the datasheet for
equation 5 of the Maxim document. I don't get the results that I should be
getting. Now, the datasheet shows two different sets of values (A1, B1, C1,
and D1) for temperatures above and below 25 degrees C, which isn't a
problem. I can make two tables, using the appropriate numbers.
But why doesn't this work? I've searched and searched the net for examples,
application notes, etc, for using a thermistor with an ADC to determine
temperature, but I just don't get how to calculate the curves and make a
lookup table.
Please help!
Steve
how to interface a thermistor to an ADC I'm going to reference below:
http://rocky.digikey.com/WebLib/BC%20Components/Web%20Data/2322%20640%203,4,
6%20NTC%20Thermistors.pdf
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1753/ln/en
Okay, The thermistor I have is 2322-640-63102 (R25 value of 1000 ohms). The
Maxim datasheet discusses a simple voltage divider of the thermistor and
another resistor, and includes equations that (to me) should be able to
generate a table showing the relationship of the measured value (the value
of the ADC, for me 12 bits, 0-4095) and temperature.
I've written a C++ program that generates a table using the equations listed
in the Maxim document, substituting the T(R) function from the datasheet for
equation 5 of the Maxim document. I don't get the results that I should be
getting. Now, the datasheet shows two different sets of values (A1, B1, C1,
and D1) for temperatures above and below 25 degrees C, which isn't a
problem. I can make two tables, using the appropriate numbers.
But why doesn't this work? I've searched and searched the net for examples,
application notes, etc, for using a thermistor with an ADC to determine
temperature, but I just don't get how to calculate the curves and make a
lookup table.
Please help!
Steve