Rainbow Electronics MAX6616 - Manual (Page 17)

Applications Information
Thermistor Considerations
NTC thermistors are resistive temperature sensors
whose resistance decreases with increasing tempera-
ture. They are available in a wide variety of packages
that are useful in difficult applications such as measure-
ment of air or liquid temperature. Some can operate
over temperature ranges beyond that of most ICs. The
relationship between temperature and resistance in an
NTC thermistor is very nonlinear and can be described
by the following approximation:
where T is absolute temperature in Kelvin, R is the ther-
mistor’s resistance, and A, B, and C are coefficients that
vary with manufacturer and material characteristics.
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. However, by connecting the thermistor in
series with a properly chosen resistor and using the
MAX6615/MAX6616 to measure the voltage across the
resistor, a reasonably linear transfer function can be
obtained over a limited temperature range. Accuracy
increases over smaller temperature ranges.
Figures 9 and 10 show a good relationship between
temperature and data. This data was taken using a
popular thermistor model, the Betatherm 10K3A1, with
R
EXT = 1.6k
Ω. Using these values produces data with
good conformance to real temperature over a range of
about +30°C to +100°C. Different combinations of ther-
mistors and R
EXT result in different curves.
ADC Noise Filtering
The integrating ADC has inherently good noise rejec-
tion, especially at low-frequency signals such as
60Hz/120Hz power-supply hum. Lay out the PC board
carefully with proper external noise filtering for high-
accuracy thermistor measurements in electrically noisy
environments.
Filter high-frequency electromagnetic interference
(EMI) at TH_ and REF with an external 100pF capacitor
connected between the two inputs. This capacitor can
be increased to about 2000pF (max), including cable
capacitance. A capacitance higher than 2000pF intro-
duces errors due to the rise time of the switched cur-
rent source.
Chip Information
PROCESS: BiCMOS
1
3
T
A B In R C In R = + + ( ) [ ( )]
MAX6615/MAX6616
Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs
______________________________________________________________________________________ 17
MAX6615/MAX6616 ERROR
-12
-10
-6
-8
0
2
-2
-4
4
ERROR (
°C)
0 40 60 20 80 100 120 140
TEMPERATURE ( °C)
OPTIMIZED FOR +30 °C TO +100°C
Figure 9. Data Error vs. Temperature Using a Betatherm
10K3A1 Thermistor
0
40
20
80
60
100
120
-50 50 0 100 150
MEASUREMENT vs. TEMPERATURE
TEMPERATURE ( °C)
MEASUREMENT (
°C)
Figure 10. Measured Temperature vs. Actual Temperature