Rainbow-electronics MAX6691 Bedienungsanleitung PDF herunterladen (Seite 5)

The relationship between temperature and resistance

of an NTC thermistor is highly nonlinear. However, by

connecting the thermistors in series with a properly

chosen resistor (R

EXT

) and using the MAX6691 to mea-

sure the voltage across the resistor, a reasonably linear

transfer function can be obtained over a limited temper-

ature range. Linearity improves for smaller temperature

ranges.

Figures 3 and 4 show typical T

HIGH

LOW

curves for a

standard thermistor in conjunction with values of R

EXT

chosen to optimize linearity over two series resistors

chosen to optimize linearity over two different tempera-

ture ranges.

NTC thermistors are often described by the resistance

at +25°C. Therefore, a 10kΩ thermistor has a resis-

tance of 10kΩ at +25°C. When choosing a thermistor,

ensure that the thermistor’s minimum resistance (which

occurs at the maximum expected operating tempera-

ture) in series with R

EXT

does not cause the voltage ref-

erence output current to exceed about 1mA. Some

standard 10kΩ thermistors with similar characteristics

are listed in Table 1.

Choosing R

EXT

Choose R

EXT

to minimize nonlinearity errors from the

thermistor:

1) Decide on the temperature range of interest (for

example 0°C to +70°C).

2) Find the thermistor values at the limits of the tem-

perature range. R

MIN

is the minimum thermistor

value (at the maximum temperature) and R

MAX

the maximum thermistor value (at the minimum tem-

perature). Also find R

MID

, the thermistor resistance

in the middle of the temperature range (+35°C for

the 0°C to +70°C range).

3) Find R

EXT

using the equation below:

Power-Supply Considerations

The MAX6691 accuracy is relatively unaffected by

power-supply coupled noise. In most applications,

RR RR

RR R

MIN MAX MIN MAX

MIN MAX MID

EXT

MIN

()

−×

+−

MAX6691

Four-Channel Thermistor Temperature-to-Pulse-

Width Converter

_______________________________________________________________________________________ 5

Figure 2. Thermistor Resistance vs. Temperature

100

120

-40 0-20 20 40 60 80 100 120

THERMISTOR RESISTANCE

vs. TEMPERATURE

TEMPERATURE (°C)

THERMISTOR RESISTANCE (kΩ)

Figure 3. T

HIGH

LOW

vs. Temperature, R

EXT

= 5110Ω

HIGH

LOW

vs. TEMPERATURE FOR BETATHERM

10K3A1 THERMISTOR WITH R

EXT

= 5110Ω

TEMPERATURE (°C)

HIGH

LOW

12010080604020

0.2

0.4

0.6

0.8

1.0

1.2

0 140

Figure 4. T

HIGH

LOW

vs. Temperature, R

EXT

= 5110Ω

HIGH

LOW

vs. TEMPERATURE FOR BETATHERM

10K3A1 THERMISTOR WITH R

EXT

= 7680Ω

TEMPERATURE (°C)

HIGH

LOW

1008040 600 20-20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-40 120

1 2 3 4 5 6 7

Width Converter 1

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