NTC thermistor is very nonlinear and can be described

by the following approximation:

where T is absolute temperature, R is the thermistor’s

resistance, and A, B, and C are coefficients that vary

with manufacturer and material characteristics. The

general shape of the curve is shown in Figure 1.

The highly nonlinear relationship between temperature

and resistance in an NTC thermistor makes it somewhat

more difficult to use than a digital-output temperature

sensor IC, for example. However, by connecting the

thermistor in series with a properly chosen resistor and

using the MAX6682 to measure the voltage across the

resistor, a reasonably linear transfer function can be

obtained over a limited temperature range. Errors

decrease for smaller temperature ranges.

Figures 2 and 3 show typical thermistor nonlinearity

curves for a standard thermistor in conjunction with

series resistors chosen to optimize linearity over two

different temperature ranges: +10°C to +40°C and 0°C

to +70°C.

Thermistor-to-Digital Converter

_______________________________________________________________________________________

5

-2.5

-2.0

2.5

-0.5

-1.5

0

1.5

2.0

3.0

020

30

10

40

60

70

50

80

THERMISTOR NONLINEARITY

vs. TEMPERATURE

TEMPERATURE (°C)

0.5

1.0

-1.0

Figure 2. Thermistor Nonlinearity vs. Temperature for a Standard

Thermistor from 0°C to +70°C

THERMISTOR

TEMPERATURE (°C)

7680

Ω*

(1LSB = 0.125

°C)

+60.000

921.6

+55.875

001 1011 1111

+50.000

830.6

+48.625

001 1000 0101

+40.000

720.5

+40.000

001 0100 0000

+30.000

595.4

+30.125

000 1111 0001

+25.000

530.1

+25.000

000 1100 1000

+20.000

464.4

+19.875

000 1001 1111

+10.000

339.7

+10.000

000 0101 0000

0

232.3

+1.500

000 0000 1100

-0.725

225.5

+1.000

000 0000 1000

-2.000

213.6

0.125

000 0000 0001

-5.000

187.4

-2.000

111 1111 0000

Ω

0

20

40

60

80

100

120

-40

0

-20

20

40

60

80

100 120

THERMISTOR RESISTANCE

vs. TEMPERATURE

TEMPERATURE (°C)

Figure 1. Thermistor Resistance vs. Temperature