Datasheet

MAX6682
Detailed Description
The MAX6682 is a sophisticated interface circuit that
energizes a low-cost thermistor and converts its tem-
perature-dependent resistance to 10-bit digital data.
The MAX6682 powers the thermistor only when a mea-
surement is being made; the power dissipated in the
thermistor is minimized. This virtually eliminates self-
heating, a major component of thermistor error. The
simple serial interface is compatible with common
microcontrollers.
Temperature Conversion
The MAX6682 converts the voltage drop across the
resistor R
EXT
to a digital output using an internal 10-bit
ADC. By measuring the voltage across R
EXT
, the output
code is directly related to temperature when using an
NTC thermistor.
Although the relationship between a thermistor’s resis-
tance and its temperature is very nonlinear, the voltage
across R
EXT
is reasonably linear over a limited temper-
ature range, provided that R
EXT
is chosen properly. For
example, over a +10°C to +40°C range, the relationship
between the voltage across R
EXT
and temperature is
linear to within approximately 0.2°C. Wider temperature
ranges result in larger errors.
The digital output is available as a 10-bit + sign word.
The relationship between the 11-bit digital word and the
voltage across R
EXT
(normalized to V
R+
) is given by:
where V
REXT
/V
R+
is the voltage across R
EXT
normal-
ized to the value of V
R+
.
Table 1 shows the relationship between the voltage
across R
EXT
and the MAX6682’s digital output code. It
also shows the temperature that would produce the list-
ed value of V
REXT
when a standard thermistor is used
in conjunction with R
EXT
= 7680Ω. The MAX6682 pro-
duces output codes scaled to the actual temperature
when used with the standard thermistor and R
EXT
=
7680Ω over the +10°C to +40°C temperature range.
Under these conditions, the nominal accuracy is about
0.2°C between +10° and +40°C, and about 1.5°C from
0°C to +50°C. In Table 1, the 3LSBs of the output code
represent fractional temperatures. The LSB has a value
of 0.125°C.
All table entries assume no errors in the values of R
EXT
or the thermistor resistance. Table 1 also assumes the
use of one of the following standard thermistors:
Betatherm 10K3A1, Dale 1M1002, or Thermometrics
C100Y103J. These thermistors have a nominal resis-
tance of 10kΩ at +25°C and very similar temperature-
to-resistance functions. They give the results shown in
Table 1.
Different temperature ranges can be accommodated as
well using different values of R
EXT
(see Choosing the
External Resistor). The MAX6682 works with thermistors
other than the ones listed above, but the transfer func-
tions vary somewhat.
Applications Information
Thermistors and Thermistor Selection
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
D
V
V
OUT
REXT
R
=
−






×
+
0 174387 8
0 010404
.
.
Thermistor-to-Digital Converter
4 _______________________________________________________________________________________
PIN NAME FUNCTION
1 I.C. Internally Connected. Connect to GND or leave unconnected.
2 R+ Reference Voltage Output. External resistor positive input.
3R-
External Resistor Negative Input. Connect R- to the junction of the external resistor and the
thermistor.
4 GND Ground. Ground connection for MAX6682 and ground return for external thermistor.
5 CS Chip Select. Drive CS low to enable the serial interface.
6 SO Serial Data Output
7 SCK Serial Clock Input
8V
CC
Positive Supply. Bypass V
CC
to GND with a 0.1µF capacitor.
Pin Description