SI7013 Datasheet, PDF (27/43 Pages) Silicon Laboratories – I2C HUMIDITY AND TWO-ZONE TEMPERATURE SENSOR

Si7013
5.4.3. Example Thermistor Calculations
For the Si7013 evaluation board with a 10 K ohm thermistor and two 24.3 K ohm bias resistors and assuming the
A/D conversion is done using VDD as a reference with buffered inputs, the ideal input voltage versus temperature
is:
Vin = V_DD *Rthemistor/(Rthermisor+46.4 K)
Since V_DD is also the reference then the expected A/D conversion result is:
A/D counts = 32768* Rthemistor/(Rthermisor+46.4 K)
If it is desired to linearize this result for the same temperature representation as the on board temperature sensor:
Temperature °C = (Output_Code*175.72/65536 – 46.85), then the desired output code is:
Output_Code = 65536*(Temperature+46.85)/175.72
Using thermistor data sheet values of resistance versus temperature and choosing to linearize at the points –15C,
–5C, 5C, 15C, 25C, 35C, 45C, 55C, 65C and 75C results in the following. The values in gray are the table entries
for Si7013:
Table 17. Example Non-Linear Correction to Thermistor Voltage Measurements
Temperature
(Degrees C)
–15
–5
5
15
25
35
45
55
65
75
Thermistor
Resistance
71746
41813
25194
15651
10000
6556
4401
3019
2115
1509
Vin/VDD
0.596164
0.462467
0.34141
0.243592
0.170648
0.118863
0.83036
0.058486
0.041704
0.030114
A/D
Codes
19535
15154
11187
7982
5592
3895
2721
1916
1367
75
Desired
Code
11879
15608
19338
23067
26797
30527
34256
37986
41715
45445
Slope Table Entry
–218
1
–241
2
–298
3
–400
4
–563
5
–813
6
–1186
7
–1739
8
–2513
9
Rev. 0.95
27