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TMP007 Calibration Coefficients - 4th root yielding imaginary #'s?

Andrew Temple
Prodigy 95 points
Other Parts Discussed in Thread: TMP007

Trying to get calibration data for TMP007 by following the TMP007 Calibration Guide, TMP007 Datasheet, and using the EasyCAL tool, but not able to match the EasyCAL tool in my own code.

I'm following this post's suggestion  of looping over possible coefficients to minimize the estimated Tobj (compared to measured Tobj) using:

When I take the coefficients EasyCal found for me as a test for the above equation - I get an imaginary number.

Values:

Coefficients s0 a1 a2 b0 b1 b22 c2
3.29E-14 1.75E-03 -1.68E-05 0.00E+00 1.00E-05 2.50E-07 13.40

Temperature values used from example in the TMP007 Calibration Guide:

Tdie: 293.15˚K

T Obj (K) 273.15 293.15 313.15 333.15 348.15
V(uV) -87.4 –27.4 45.9 133 208

Using the above to evaluate the fraction under the 4th root:

Numerator: 1.02E05

Denominator: -1.26E-13

Add the fraction to Tdie^4 = -8.1E17.


4th root of a negative == imaginary number.

What might I be missing here?

  • 0
    TI__Mastermind 35895 points
    Tdie will never be a negative number if it is expressed in kelvin. After applying fourth power to it, it should be sufficiently large to create a positive value under the fourth root when summed with the numerator/denominator.

    Tref is always 25C. It is always used to calculate how far from room temperature the Tdie is. For this reason, it's not necessary to convert Tdie to kelvin if your Tref is expressed in C. In your denominator (S equation) you have written the sum of Tdie and Tref.

    Additionally, when I plug in your first set of values, I get:
    fVobj (numerator) = -43.624E-6
    S (denominator) = 32. 598e-15
    Tdie^4 = 7.385E9
    Tobj = 5.7 C

    Ren
  • 0 in reply to Ren Schackmann
    Prodigy 95 points

    Hi Ren,

    Thanks for the quick reply, this info was what I needed!!

    Specifically:

    • Catching the S calculation error (I took that from the post I linked to as the equation was verified as "looks OK").
    • And providing the numbers, because:
      • I was using uV instead of V - so that threw in a huge offset.
      • As the Tdie^4 requires kelvin, I wouldn't mention celsuis, as celcuis isn't required - Tref just need to be converted to K
      • I was using TREF in Kelvin (298.15), along with all other #'s

    Now my numbers line up with yours - appreciate it!