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CHAPTER 9 A/D CONVERTER APPLICATION
9.2 THERMOMETER
In this example, a thermistor (6 k
/0
°
C) is used in a temperature sensor and measures temperatures
between –20
°
C and +50
°
C. Changes in resistance corresponding to the temperature of the thermistor
can be represented in the following way.
R = R
0
exp {B (1/T–1/T
0
) }
R : Resistance at some temperature T [
°
K]
T : Any temperature [
°
K]
R
0
: Resistance at the reference temperature T
0
[
°
K]
T
0
: Reference temperature [
°
K]
B : Constant determined from the reference temperature T
0
[
°
K] and T
0
[
°
K]
However, the B constant is not constant and is changed by the temperature. The B constant is
transformed in the above equation and can be determined by the following equation.
B =
1
In
R
R
0
(1/T–1/T
0
)
An example circuit is shown in Figure 9-8. This circuit is set so that 0 V is input at –20
°
C and 5 V are
input at +50
°
C.
Figure 9-8. Thermometer Circuit Example
Th
ANIn
μ
PD78044F