Datasheet
NTC thermistor is very nonlinear and can be described
by the following approximation:
1 / T = A + BlnR + C(lnR)
3
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.
MAX6682
Thermistor-to-Digital Converter
_______________________________________________________________________________________ 5
-2.5
-2.0
2.5
-0.5
-1.5
0
1.5
2.0
3.0
0203010 40 60 7050 80
THERMISTOR NONLINEARITY
vs. TEMPERATURE
TEMPERATURE (°C)
LINEARITY ERROR (°C)
0.5
1.0
-1.0
Figure 2. Thermistor Nonlinearity vs. Temperature for a Standard
Thermistor from 0°C to +70°C
*Assumes V
R+
= 1.220V.
THERMISTOR
TEMPERATURE (°C)
V
REXT
(mV) WITH STANDARD
THERMISTOR AND R
EXT
=
7680Ω*
DECIMAL VALUE OF D
OUT
(1LSB = 0.125°C)
D
OUT
+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
Table 1. Temperature vs. Digital Output for Standard Thermistor with R
EXT
= 7680Ω
0
20
40
60
80
100
120
-40 0-20 20 40 60 80 100 120
THERMISTOR RESISTANCE
vs. TEMPERATURE
TEMPERATURE (°C)
THERMISTOR RESISTANCE (kΩ)
Figure 1. Thermistor Resistance vs. Temperature