DT-670 Chebyshev implementation

Hello!

I am currently trying to implement the DT-670 diode for a colleague. I am using the Keithley 2636A device to source and measure the diode.

Software is written in python.



I have trouble with the chebyshev polynomials. I must be missing something obvious but I can't see what.



output of the code given is:

The temperature(1) is 6.309166842047422 K.

1.2618333684094845



It should result in 5.0 K (the x was chosen from the curves points data)



import numpy as np



def chebyshev(x, ZL, ZU, coeffs):

   """

   Calculate the temperature using Chebychev polynomials.

   """

   # Define the normalized variable x

   z = (((x - ZL) * (ZU - x)) / (ZU - ZL))   #(x - ZL) / (ZU - ZL)

   if z < 0:

       return 0



   # Calculate the Chebychev polynomials

   T = np.zeros(len(coeffs))

   T[0] = 1

   T[1] = z

   for i in range(2, len(coeffs)):

       T[i] = 2 * z * T[i-1] - T[i-2]



   # Calculate the temperature

   temperature = 0

   for i in range(len(coeffs)):

       temperature += coeffs[i] * T[i]

   return temperature





#table 1

ZL_1 = 1.294390

ZU_1 = 1.680000

coeffs_1 = [6.429274,-7.514262,-0.725882,-1.117846,-0.562041,

           -0.360239,-0.229751,-0.135713,-0.068203,-0.029755]

#table 2

ZL_2 = 1.11230

ZU_2 = 1.38373

coeffs_2 = [17.244846,-7.964373,0.625343,-0.105068,0.292196,

           -0.344492,0.271670,-0.151722,0.121320,-0.035566,0.045966]

#table 3

ZL_3 = 0.909416

ZU_3 = 1.122751

coeffs_3 = [82.017868,-59.064244,-1.356615,1.055396,0.837341,0.431875,

           0.440840,-0.061588,0.209414,-0.120882,0.055734,-0.035974]

#table 4

ZL_4 = 0.07

ZU_4 = 0.99799

coeffs_4 = [306.592351, -205.393808, -4.695680, -2.031603, -0.071792,

         -0.437682, 0.176352, -0.182516, 0.064687, -0.027019, 0.010019]



#x = 0.783720 # Voltage, represents exactly 200K

x = 1.551450 # Voltage, represents exactly 5.0 K



temperature1 = chebyshev(x, ZL_1, ZU_1, coeffs_1)

#temperature2 = chebyshev(x, ZL_2, ZU_2, coeffs_2)

#temperature3 = chebyshev(x, ZL_3, ZU_3, coeffs_3)

#temperature4 = chebyshev(x, ZL_4, ZU_4, coeffs_4)



print(f"The temperature(1) is {temperature1} K.")

print(temperature1/5)

#print(f"The temperature(2) is {temperature2} K.")

#print(f"The temperature(3) is {temperature3} K.")

#print(f"The temperature(4) is {temperature4} K.")

#print(temperature4/200)

Do I have to add all ranges or something of the sort? There is no additional information on the curves sheet.

Thanks in advance!