AN607

34

Rev. 1.7

The table must be arranged in order of decreasing input values. The table is entered into memory addresses 0x82–

0xB7 one byte at a time.

The table itself is user-programmed, and, by default, all table values are 0xFF. It should be noted that, once the

non-linear correction data is saved to memory, it cannot be overwritten.

As an aid to calculation of the table, several tools have been developed.

section of the Sensors Documentation page at http://www.silabs.com/support/pages/document-

library.aspx?p=Sensors. This spreadsheet calculates the expected output of the A/D based on an

assumed thermistor and biasing circuit (the NCP18XH103F03RB thermistor used on Si7013 evaluation

boards with 24.3 k

 biasing resistors). Then, based on the desired output after linearization (In this case,

output = (temperature + 46.85) x 65536/175.72), the slope is calculated. Finally, the spreadsheet

calculates the hexadecimal values that should go in memory locations 0x82–0xB7 based on the input/

output and slope values.

entered in a GUI. These values can be saved to file or burned into the Si7013 memory. For example:

For the Si7013 evaluation board with a 10 k

 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 version temperature is:

Vin = VDD x Rthemistor/(Rthermisor + 46.4 k

)

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

A/D counts = 32768 x Rthemistor/(Rthermisor + 46.4 k

)

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

Temperature °C = (Output_Code x 175.72/65536 – 46.85)

Then, the desired output code is:

Output_Code = 65536 x (Temperature + 46.85)/175.72

Using thermistor data sheet values of resistance versus temperature and choosing to linearize at the points

(–15 °C, –5 °C, 5 °C, 15 °C, 25 °C, 35 °C, 45 °C, 55 °C, 65 °C, and 75 °C) results in the information in Table 7.

Table 6. Memory Location Descriptions

Memory Location

Description

0x82

MSB of in1

0x83

LSB of in1

……

0x93

LSB of in9

0x94

MSB of out1

……

0xA5

LSB 0f out9

0xA6

MSB of slope1

……

0xB7

LSB of slope9