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SI7013-A20-YM0

I2C HUMIDITY AND TWO-ZONE TEMPERATURE SENSOR

Silicon Laboratories 

Si7013-A20

5.4.3. Example Thermistor Calculations

For the Si7013 evaluation board with a 10 K ohm thermistor and two 24.3 K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 = VDD *Rthemistor/(Rthermisor+48.6 K)

Since VDD is also the reference then the expected A/D conversion result is:

s A/D counts = 32768* Rthemistor/(Rthermisor+48.6 K)

n If it is desired to linearize this result for the same temperature representation as the on board temperature sensor:

ig Temperature °C = (Output_Code*175.72/65536 – 46.85), then the desired output code is:

s Output_Code = 65536*(Temperature+46.85)/175.72

e 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

D for Si7013:

Table 18. Example Non-Linear Correction to Thermistor Voltage Measurements

ew Temperature

(Degrees C)

N –15

r –5

fo 5

15

d 25

e 35

d 45

n 55

e 65

Not Recomm 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. 1.4

29

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