MAX6682 Maxim Integrated Products, MAX6682 Datasheet - Page 5

• Manufacturer Part Number: MAX6682
• Description: Thermistor-to-Digital Converter
• Manufacturer: Maxim Integrated Products

Table 1. Temperature vs. Digital Output for Standard Thermistor with R
*Assumes V
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
Figure 1. Thermistor Resistance vs. Temperature
TEMPERATURE (°C)
THERMISTOR
R+
+60.000
+50.000
+40.000
+30.000
+25.000
+20.000
+10.000
120
100
-0.725
-2.000
-5.000
80
60
40
20
0
= 1.220V.
-40
0
1 / T = A + BlnR + C(lnR)
-20
_______________________________________________________________________________________
THERMISTOR RESISTANCE
0
vs. TEMPERATURE
TEMPERATURE (°C)
20
40
V
60
REXT
THERMISTOR AND R
80
(mV) WITH STANDARD
100 120
3
Thermistor-to-Digital Converter
7680Ω*
921.6
830.6
720.5
595.4
530.1
464.4
339.7
232.3
225.5
213.6
187.4
EXT
=
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.
Figure 2. Thermistor Nonlinearity vs. Temperature for a Standard
Thermistor from 0°C to +70°C
DECIMAL VALUE OF D
(1LSB = 0.125°C)
-0.5
-1.0
-1.5
-2.0
-2.5
3.0
2.5
2.0
1.5
1.0
0.5
+55.875
+48.625
+40.000
+30.125
+25.000
+19.875
+10.000
0
+1.500
+1.000
-2.000
0.125
0
10
THERMISTOR NONLINEARITY
20
vs. TEMPERATURE
TEMPERATURE (°C)
30
OUT
40
50
EXT
60
001 1011 1111
001 1000 0101
001 0100 0000
000 1111 0001
000 1100 1000
000 1001 1111
000 0101 0000
000 0000 1100
000 0000 1000
000 0000 0001
111 1111 0000
= 7680Ω
70
D
OUT
80
5