AN897 Datasheet, PDF (14/16 Pages) Microchip Technology – Thermistor Temperature Sensing with MCP6SX2 PGAs

AN897
APPENDIX B: THERMISTOR MODEL
The nominal response of the 2322 640 55103
thermistor [1] is shown in Table B-1.
TABLE B-1: NOMINAL THERMISTOR
RESPONSE.
Thermistor
Temperature (°C)
Thermistor
Resistance (Ω)
-40
332.1 k
-35
240.0 k
-30
175.2 k
-25
129.3 k
-20
96.36 k
-15
72.50 k
-10
55.05 k
-5
42.16 k
0
32.56 k
5
25.34 k
10
19.87 k
15
15.70 k
20
12.49 k
25
10.00 k
30
8.059 k
35
6.535 k
40
5.330 k
45
4.372 k
50
3.606 k
55
2.989 k
60
2.490 k
65
2.084 k
70
1.753 k
75
1.481 k
80
1.256 k
85
1.070 k
90
915.4
95
786.0
100
677.3
105
585.8
110
508.3
115
442.6
120
386.6
125
338.7
130
297.7
135
262.4
140
231.9
145
205.5
150
182.6
DS00897B-page 14
The data in Table B-1 was fit to the following sets of
equations. These equations make it possible to
accurately interpolate values between the table entries,
thus making the designs easier to evaluate.
EQUATION B-1:
Resistance-to-Temperature Equations:
X = ln(R_TH / 1Ω)
z = X_SC (X – X_CTR)
T_TH ≈
T₀
1 + z(A₁ + z(A₂ + z(A₃)))
– T₂₅
where:
182.6 Ω ≤ R_TH ≤ 332.1 kΩ
X_SC = -0.266457
X_CTR = 8.960245
T₀ = 303.960°C
A₁ = -0.291639
A₂ = 0.010993
A₃ = -0.001042
T₂₅ = 273.150°C
EQUATION B-2:
Temperature-to-Resistance Equations:
1
Y=
T_TH + T₂₅
z = Y_SC (Y – Y_CTR)
X ≈ B₀ + z(B₁ + z(B₂ + z(B₃)))
R_TH = (1 Ω) e^X
where:
-40°C ≤ T_TH ≤ +150°C
T₂₅ = 273.150°C
Y_SC = -1038.499°C
Y_CTR = 0.003326156 °C^-1
B₀ = 9.101806
B₁ = -3.756408
B₂ = -0.141435
B₃ = 0.003396
The piece-wise linear interpolation tables also need
values outside of the valid range of Table B-1. These
values are only used for convenience in setting up the
interpolation tables and firmware routine; they are not
based on actual data. While the above equations are
not valid outside of the given ranges, they are good
enough to support the interpolation algorithm.
Both sets of equations are based on a min-max
polynomial fit on the normalized variable z. It is more
difficult to achieve an accurate fit to the data using the
variables X and Y.
 2004 Microchip Technology Inc.