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TMP61: R-T approximation formula

Y.Ottey
Mastermind 6145 points
Part Number: TMP61

Dear TI Team

Our customers are currently considering TMP61.
I have received a request from a customer to provide an approximate formula for calculating the measured temperature from the resistance value of TMP61.

When checking the data sheet, the formula for calculating Rtmp61 from Vtemp measured as follows is described, but the formula for calculating temperature from Rtemp61 is not described.
Could you please tell me the approximate formula to calculate the temperature from Rtemp61?
(I know there is a Thermistor Design Tool, but I would like to know the approximate expression actually used.)

Best Regards,
Y.Ottey

  • 0
    TI__Genius 9380 points

    Y.Ottey,

    The TMP6 parts are not resistors. They are silicon. There are no ohms law formulas than can be applied. Everything that is needed, is in the design tool.

    We use a 4th order polynomial to match the resistance curve. (The resistance is calculated during validation, as it can't be measured) We then plot the assumed resistance and get a 4th order polynomial equation. Then we create the regression of that 4th order polynomial to be able to calculate the temperature based on the resistance. 

    We supply the plots, formulas, an example and the C Code for you. It's that simple.

    The same thing can be done by using the measured Vsense voltage from the middle of the RBias and the TMP6. Plot the voltage curve, extract the 4th order polynomial. Then invert them to get the regression. Use the 4th order regression formula to enter the ADC voltage to get temperature. You don't need to calculate the resistance or even use the resistance at all. This is the way this part really works. (No Resistance values used) 

    The apparent resistance values are there to help the customer. and to make the use of these parts similar to an NTC that they are familiar with.