General Description
The MAX6682 converts an external thermistor’s temperature-dependent resistance directly into digital form.
The thermistor and an external fixed resistor form a voltage-divider that is driven by the MAX6682’s internal
voltage reference. The MAX6682 measures the voltage
across the external resistor and produces a 10-bit +
sign output code dependent on that voltage.
The MAX6682 does not linearize the highly nonlinear
transfer function of a typical negative temperature coefficient (NTC) thermistor, but it does provide linear output data over limited temperature ranges when used
with an external resistor of the correct value. Over the
0° to +50°C temperature range, the MAX6682 produces
output data that is scaled to 8LSBs/°C (for 0.125°C resolution), provided that the correct thermistor and external resistor values are used. Other temperature ranges
can be easily accommodated, but do not necessarily
yield data scaled to an even number of LSBs per
degree.
The 3-wire SPI™-compatible interface can be readily
connected to a variety of microcontrollers.
The MAX6682 is a read-only device, simplifying use in
systems where only temperature data is required.
Power-management circuitry reduces the average thermistor current, minimizing self-heating. Between conversions, supply current is reduced to 21µA (typ). The
internal voltage reference is shut down between measurements.
The MAX6682 is available in a small, 8-pin µMAX package and is specified over the -55°C to +125°C temperature range.
Applications
HVAC
Medical Devices
Battery Packs/Chargers
Home Appliances
Features
♦ Converts Thermistor Temperature to Digital Data
♦ Low Average Thermistor Current Minimizes Self-
Heating Errors
♦ Low Supply Current, 21µA (typ) Including 10kΩ
Thermistor Current
♦ Internal Voltage Reference Isolates Thermistor
from Power-Supply Noise
♦ 10-Bit Resolution
♦ Accommodates Any Thermistor Temperature
Range
♦ Output Data Scaled for Direct Temperature
Readings from 0°C to +50°C
♦ Simple SPI-Compatible Interface
♦ Small, 8-Pin µMAX Package
MAX6682
Thermistor-to-Digital Converter
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
19-2219; Rev 0; 2/02
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
SPI is a trademark of Motorola, Inc.
Pin Configuration appears at end of data sheet.
3.3V
0.1µF
MC68HCXX
I/O
SCLK
MISO
CS
SCLK
SO
V
CC
R+
R-
GND
MAX6682
R
EXT
THERMISTOR
Typical Operating Circuit
PART TEMP RANGE PIN-PACKAGE
MAX6682MUA -55°C to +125°C 8 µMAX
MAX6682
Thermistor-to-Digital Converter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC= 3V to 5.5V, TA= -55°C to +125°C, unless otherwise noted. Typical values are specified at VCC= 3.3V and TA= +25°C.) (Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage (VCCto GND) .................................-0.3V to +6V
SO, SCK, CS, R-, R+ to GND ....................-0.3V to (V
CC
+ 0.3V)
R+ Current ........................................................................±20mA
R- Current ...........................................................................±1mA
SCK, CS, SO Current .........................................-1mA to +50mA
ESD Protection (Human Body Model) .............................±2000V
Continuous Power Dissipation (T
A
= +70°C)
8-Pin µMAX (derate 4.1mW/°C above +70°C) ............ 328mW
Operating Temperature Range
(T
MIN
to T
MAX
) ...............................................-55°C to +125°C
Storage Temperature Range .............................-65°C to +150°C
Junction Temperature .....................................................+150°C
Supply Voltage V
ADC Total Unadjusted Error TUE
ADC Conversion Time t
R- Input Impedance Z
R- Leakage Current 150nA
Conversion Rate 0.5 Hz
Reference Voltage Output V
Reference Load Regulation 0 < I
Reference Supply Regulation 0.7 mV/V
Conversion Supply Current I
Average Supply Current I
Standby Current I
Idle Current I
SERIAL INTERFACE
Input Low Voltage V
Input High Voltage V
Input Leakage Current I
Output High Voltage V
Output Low Voltage V
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CC
CONV
IN
REF
C
A
S
ID
IL
IH
LEAK
OH
OL
DOUT = 768.935 x (V
> 0.1V
V
IN
I
LOAD
REF
= 1mA 1.10 1.22 1.40 V
< 2mA 0 0.1 %/mA
LOAD
REXT/VR+
During conversion, no load 220 300 µA
0.5 conversions/s, no load 17 29 µA
CS low, SCK inactive 3 7 µA
CS high, analog circuits off 10 17 µA
VIN = GND or V
I
SOURCE
I
= 1.6mA 0.4 V
SINK
CC
= 1.6mA
) - 134.0923;
3.0 5.5 V
-3 +3 LSB
64 80 ms
1MΩ
0.2 x
V
CC
0.8 x
V
CC
1µA
V
-
CC
0.4
V
V
V
MAX6682
Thermistor-to-Digital Converter
_______________________________________________________________________________________ 3
TIMING CHARACTERISTICS
(VCC= 3V to 5.5V, TA= T
MIN
to T
MAX
, unless otherwise noted. Typical values are specified at VCC= 3.3V and TA= +25°C.) (Note 2)
Note 1: All specifications are 100% tested at T
A
= +25°C. Specification limits over temperature are guaranteed by design,
not production tested.
Note 2: Guaranteed by design.
Typical Operating Characteristics
(VCC= 5V, thermistor = 10k nominal, R
EXT
= 7680Ω, TA = +25°C, unless otherwise noted.)
0
0.3
0.2
0.1
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0105152025
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6682 toc01
FREQUENCY (MHz)
TEMPERATURE ERROR (°C)
VIN = SQUARE WAVE
APPLIED TO V
CC
WITH
NO V
CC
BYPASS CAPACITOR
VIN = 250mV
P-P
40
60
50
80
70
90
100
1k 100k10k 1M 10M
AVERAGE SUPPLY CURRENT
vs. CLOCK FREQUENCY
MAX6682 toc02
SCK FREQUENCY (Hz)
SUPPLY CURRENT (µA)
SCK IS DRIVEN
RAIL-TO-RAIL
®
30
50
40
70
60
90
80
100
3.0 4.03.5 4.5 5.0 5.5
AVERAGE SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6682 toc03
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (µA)
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
SERIAL INTERFACE TIMING (Figures 5 and 6)
Serial Clock Frequency f
SCK Pulse High Width t
SCK Pulse Low Width t
CS Fall to SCK Rise t
CS Fall to Output Data Valid t
SCK Fall to Output Data Valid t
CS Rise to Output High-Z t
SCK Fall to Output High-Z t
CS Pulse Width t
SCL
CH
CL
CSS
DV
DO
TR
HIZ
CSW
5 MHz
50 ns
50 ns
35 ns
CL = 10pF 35 ns
CL = 10pF 35 ns
CL = 10pF 25 ns
CL = 10pF 35 ns
75 ns
MAX6682
Detailed Description
The MAX6682 is a sophisticated interface circuit that
energizes a low-cost thermistor and converts its temperature-dependent resistance to 10-bit digital data.
The MAX6682 powers the thermistor only when a measurement is being made; the power dissipated in the
thermistor is minimized. This virtually eliminates selfheating, 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 resistance 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 VR+) is given by:
where V
REXT/VR+
is the voltage across R
EXT
normal-
ized to the value of VR+.
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 listed 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 resistance of 10kΩ at +25°C and very similar temperatureto-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 functions vary somewhat.
Applications Information
Thermistors and Thermistor Selection
NTC thermistors are resistive temperature sensors
whose resistance decreases with increasing temperature. They are available in a wide variety of packages
that are useful in difficult applications such as measurement of air or liquid temperature. Some can operate
over temperature ranges beyond that of most ICs. The
relationship between temperature and resistance in an
Thermistor-to-Digital Converter
4 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 I.C. Internally Connected. Connect to GND or leave unconnected.
2 R+ Reference Voltage Output. External resistor positive input.
3R-
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
8VCCPositive Supply. Bypass VCC to GND with a 0.1µF capacitor.
External Resistor Negative Input. Connect R- to the junction of the external resistor and the
thermistor.
D
OUT
V
REXT
V
R
+
=
0 174387 8
.
−
0 010404
.
×
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
Figure 2. Thermistor Nonlinearity vs. Temperature for a Standard
Thermistor from 0°C to +70°C
*Assumes VR+= 1.220V.
Table 1. Temperature vs. Digital Output for Standard Thermistor with R
EXT
= 7680Ω
Figure 1. Thermistor Resistance vs. Temperature
THERMISTOR
TEMPERATURE (°C)
+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
(mV) WITH STANDARD
V
REXT
THERMISTOR AND R
7680Ω*
EXT
=
DECIMAL VALUE OF D
(1LSB = 0.125°C)
OUT
D
OUT
THERMISTOR RESISTANCE
vs. TEMPERATURE
120
100
80
60
40
THERMISTOR RESISTANCE (kΩ)
20
0
-40 0-20 20 40 60 80 100 120
TEMPERATURE (°C)
THERMISTOR NONLINEARITY
3.0
2.5
2.0
1.5
1.0
0.5
0
-0.5
LINEARITY ERROR (°C)
-1.0
-1.5
-2.0
-2.5
0203010 40 60 7050 80
vs. TEMPERATURE
TEMPERATURE (°C)
MAX6682
NTC thermistors are often described by the resistance
at +25°C. Therefore, a 10kΩ thermistor has a resistance
of 10kΩ at +25°C. When choosing a thermistor, ensure
that the thermistor’s minimum resistance (which occurs
at the maximum expected operating temperature) in
series with R
EXT
does not cause the voltage reference
output current to exceed about 1mA. Some standard
10kΩ thermistors with similar characteristics are listed
in Table 2. When used with one of these thermistors
and the recommended series resistor, the MAX6682
provides output data scaled in °C over the +10°C to
+40°C temperature range.
Choosing the External Resistor
Choose R
EXT
to minimize nonlinearity errors from the
thermistor:
1) Decide on the temperature range of interest (for
example 0°C to +70°C).
2) Find the thermistor values at the limits of the temperature range. R
MIN
is the minimum thermistor
value (at the maximum temperature) and R
MAX
is
the maximum thermistor value (at the minimum temperature). Also find R
MID
, the thermistor resistance
in the middle of the temperature range (+35°C for
the 0°C to +70°C range).
3) Find R
EXT
using the equation below:
Table 3 shows nominal output data for several temperatures when R
EXT
has been chosen according to the
equation above for a temperature range of 0°C to
+70°C. The output data is not conveniently scaled to
the actual temperature over this range, but the linearity
is better than 2.4°C over the 0°C to +70°C range
(Figure 2). The temperature weighting over this range is
0.14925°C/LSB.
Serial Interface
The Typical Application Circuit shows the MAX6682
interfaced with a microcontroller. In this example, the
MAX6682 processes the reading from R
EXT
and trans-
mits the data through an SPI-compatible interface.
Force CS low and apply a clock signal at SCK to read
the results at SO. Forcing CS low immediately stops
any conversion in process. Initiate a new conversion by
forcing CS high.
Force CS low to output the first bit on the SO pin. A
complete read requires 11 clock cycles. Read the 11
output bits on the rising edge of the clock, if the first bit
D10 is the sign bit. Bits D10–D0 contain the converted
temperature in the order of MSB to LSB.
After the 11th clock cycle, SO goes to a high-impedance state. SO remains high impedance until CS is
pulsed high and brought back low. Figure 4 is the SO
output.
Power-Supply Considerations
The MAX6682 accuracy is relatively unaffected by
power-supply coupled noise. In most applications,
bypass VCCto GND by placing a 0.1µF ceramic
bypass capacitor close to the supply pin of the
devices.
Thermal Considerations
Self-heating degrades the temperature measurement
accuracy of thermistors. The amount of self-heating
depends on the power dissipated in the thermistor and
the dissipation constant of the thermistor. Dissipation
constants depend on the thermistor’s package and can
vary considerably.
A typical thermistor might have a dissipation constant
equal to 1mW/°C. For every mW the thermistor dissipates, its temperature rises by 1°C. For example, con-
Thermistor-to-Digital Converter
6 _______________________________________________________________________________________
Figure 3. Thermistor Nonlinearity vs. Temperature for a Standard
Thermistor from +10°C to +40°C
Figure 4. SO Output
THERMISTOR NONLINEARITY
0.25
0.20
0.15
0.10
0.05
0
Bit 10 987654321 0
-0.05
-0.10
LINEARITY ERROR (°C)
-0.15
-0.20
-0.25
0 1015205 2530354045
10-BIT TEMPERATURE READING
MSB
(Sign)
vs. TEMPERATURE
TEMPERATURE (°C)
LSB
R
RR R RR
MID MIN MAX MIN MAX
=
EXT
+
()
+−
RR R
MIN MAX MID
−
2
2
sider a 10kΩ (at +25°C) NTC thermistor in series with a
5110Ω resistor operating at +40°C with a constant 5V
bias. If it is one of the standard thermistors in Table 2,
its resistance is 5325Ω at this temperature. The power
dissipated in the thermistor is:
(5)2(5325) / (5325 + 5110)2= 1.22mW
This thermistor would therefore have a self-heating
error at +40°C of 1.22°C. Because the MAX6682 uses a
small reference voltage and energizes the thermistor
less than 2% of the time, the self-heating of the thermistor under the same conditions when used with the
MAX6682 is only:
(1.22)2(5325)(0.02) / (5325 + 5110)
2
=1.46µW, or only about 0.0015° (self-heating
error)
MAX6682
Thermistor-to-Digital Converter
_______________________________________________________________________________________ 7
Figure 5. Serial Interface Timing
Table 3. Temperature vs. Digital Output for Standard Thermistor with R
EXT
= 5110Ω
Table 2. Standard Thermistors
*Assumes VR+= 1.220V.
MANUFACTURER PART WEBSITE
Betatherm 10K3A1 www.betatherm.com
Dale 1M1002
Thermometrics C100Y103J www.thermometrics.com
THERMISTOR
TEMPERATURE
(°C)
+75.000 946.0 57.75 001 1100 1110
+70.000 908.6 54.875 001 1011 0111
+60.000 820.6 47.875 001 0111 1111
+50.000 715.7 39.625 001 0011 1101
+40.000 597.4 30.25 000 1111 0010
+30.000 473.5 20.5 000 1010 0100
+25.000 412.6 15.750 000 0111 1110
+20.000 354.1 11.125 000 0101 1001
+10.000 249.2 2.875 000 0001 0111
0 165.1 -3.750 111 1110 0010
-5.000 131.5 -6.375 111 1100 1101
V
(mV) WITH
REXT
STANDARD THERMISTOR
AND R
= 5110Ω*
EXT
DECIMAL VALUE OF D
(USING 1LSB = 0.125°C)
OUT
www.vishay.com/brands/
dale/main.html
D
OUT
t
CS
SCK
t
DV
SO
CSS
1
B10
MSB
B9 B1B2B8 B7 B6 B4B5 B3
t
CH
t
t
DO
CL
t
TR
B0
LSB
MAX6682
Thermistor-to-Digital Converter
8 _______________________________________________________________________________________
Figure 6. Serial Interface Timing 2
Chip Information
TRANSISTOR COUNT: 4909
PROCESS: BiCMOS
Pin Configuration
Functional Diagram
CS
t
CSW
SCK
t
DV
SO
V
CC
R+
R-
12
B10
MSB
B9 B1B2
BANDGAP
DIGITAL
CONTROL
ADC
CS
SCK
SO
11
t
HIZ
B0
LSB
TOP VIEW
1
I.C.
2
3
R-
4
12
B10
MSB
B9
87V
CC
SCKR+
MAX6682
SO
6
CSGND
5
µMAX
MAX6682
Thermistor-to-Digital Converter
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 9
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
8LUMAXD.EPS
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