General Description
The MAX6675 performs cold-junction compensation and
digitizes the signal from a type-K thermocouple. The
data is output in a 12-bit resolution, SPI™-compatible,
read-only format.
This converter resolves temperatures to 0.25°C, allows
readings as high as +1024°C, and exhibits thermocouple
accuracy of 8LSBs for temperatures ranging from 0°C to
+700°C.
The MAX6675 is available in a small, 8-pin SO package.
Applications
●Industrial
●Appliances
●HVAC
Features
●Direct Digital Conversion of Type -K Thermocouple
Output
●Cold-Junction Compensation
●Simple SPI-Compatible Serial Interface
●12-Bit, 0.25°C Resolution
●Open Thermocouple Detection
PART TEMP RANGE PIN-PACKAGE
MAX6675ISA -20°C to +85°C 8 SO
Vcc
GND
T+
T-
SO
SCK
CS
MICROCONTROLLER
68HC11A8
MISO
SCK
SSB
0.1µF
MAX6675
CS
SCKVCC
1
2
8
7
N.C.
SOT-
T+
GND
SO
TOP VIEW
3
4
6
5
MAX6675
MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
19-2235; Rev 2; 4/14
Typical Application Circuit
Pin Conguration
Ordering Information
SPI is a trademark of Motorola, Inc.
EVALUATION KIT AVAILABLE
Supply Voltage (VCC to GND) ............................... -0.3V to +6V
SO, SCK, CS, T-, T+ to GND .....................-0.3V to VCC + 0.3V
SO Current ....................................................................... 50mA
ESD Protection (Human Body Model) .......................... ±2000V
Continuous Power Dissipation (TA = +70°C)
8-Pin SO (derate 5.88mW/°C above +70°C) ............. 471mW
Operating Temperature Range .......................... -20°C to +85°C
Storage Temperature Range ........................... -65°C to +150°C
Junction Temperature ................................................... . +150°C
SO Package
Vapor Phase (60s) . .....................................................+215°C
Infrared (15s) .............................................................. +220°C
Lead Temperature (soldering, 10s) ............................... +300°C
(VCC = +3.0V to +5.5V, TA = -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Temperature Error
TTHERMOCOUPLE = +700°C,
TA = +25°C (Note 2)
VCC = +3.3V -5 +5
LSB
VCC = +5V -6 +6
TTHERMOCOUPLE = 0°C to
+700°C, TA = +25°C (Note 2)
VCC = +3.3V -8 +8
VCC = +5V -9 +9
TTHERMOCOUPLE = +700°C
to +1000°C, TA = +25°C (Note 2)
VCC = +3.3V -17 +17
VCC = +5V -19 +19
Thermocouple Conversion
Constant 10.25 µV/LSB
Cold-Junction
Compensation Error
TA = -20°C to
+85°C (Note 2)
VCC = +3.3V -3.0 +3.0 °C
VCC = +5V -3.0 +3.0
Resolution 0.25 °C
Thermocouple Input
Impedance 60 kW
Supply Voltage VCC 3.0 5.5 V
Supply Current ICC 0.7 1.5 mA
Power-On Reset Threshold VCC rising 1 2 2.5 V
Power-On Reset Hysteresis 50 mV
Conversion Time (Note 2) 0.17 0.22 s
SERIAL INTERFACE
Input Low Voltage VIL 0.3 x
VCC V
Input High Voltage VIH 0.7 x
VCC V
Input Leakage Current ILEAK VIN = GND or VCC ±5 µA
Input Capacitance CIN 5 pF
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Electrical Characteristics
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.
Absolute Maximum Ratings
Note 1: All specifications are 100% tested at TA = +25°C. Specification limits over temperature (TA = TMIN to TMAX) are guaranteed
by design and characterization, not production tested.
Note 2: Guaranteed by design. Not production tested.
(VCC = +3.3V, TA = +25°C, unless otherwise noted.)
(VCC = +3.0V to +5.5V, TA = -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Output High Voltage VOH ISOURCE = 1.6mA VCC -
0.4 V
Output Low Voltage VOL ISINK = 1.6mA 0.4 V
TIMING
Serial Clock Frequency fSCL 4.3 MHz
SCK Pulse High Width tCH 100 ns
SCK Pulse Low Width tCL100 ns
CSB Fall to SCK Rise tCSS CL = 10pF 100 ns
CSB Fall to Output Enable tDV CL = 10pF 100 ns
CSB Rise to Output Disable tTR CL = 10pF 100 ns
SCK Fall to Output Data Valid tDO CL = 10pF 100 ns
-5
0
5
10
-10 0 30 50
OUTPUT CODE ERROR
vs. VOLTAGE DIFFERENTIAL
MAX6675 toc02
VOLTAGE DIFFERENTIAL (mV)
OUTPUT CODE ERROR (LSB)
10 20 40
10
8
6
4
2
0
0 4515 30 60 75 90
OUTPUT CODE ERROR
vs. AMBIENT TEMPERATURE
MAX6675 toc01
TEMPERATURE (°C)
OUTPUT CODE ERROR (LSB)
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Typical Operating Characteristics
Electrical Characteristics (continued)
Detailed Description
The MAX6675 is a sophisticated thermocouple-to-digi-
tal converter with a built-in 12-bit analog-to-digital con-
verter (ADC). The MAX6675 also contains cold-junction
compensation sensing and correction, a digital con-
troller, an SPI-compatible interface, and associated
control logic.
The MAX6675 is designed to work in conjunction with an
external microcontroller (µC) or other intelligence in ther-
mostatic, process-control, or monitoring applications.
Temperature Conversion
The MAX6675 includes signal-conditioning hardware to
convert the thermocouple’s signal into a voltage compat-
ible with the input channels of the ADC. The T+and T-
inputs connect to internal circuitry that reduces the intro-
duction of noise errors from the thermocouple wires.
Before converting the thermoelectric voltages into
equivalent temperature values, it is necessary to com-
pensate for the difference between the thermocouple
cold-junction side (MAX6675 ambient temperature) and
a 0°C virtual reference. For a type-K thermocouple, the
voltage changes by 41µV/°C, which approximates the
thermocouple characteristic with the following linear
equation:
VOUT = (41µV / °C) 5 (TR - TAMB)
Where:
VOUT is the thermocouple output voltage (µV).
TR is the temperature of the remote thermocouple junc-
tion (°C).
TAMB is the ambient temperature (°C).
Cold-Junction Compensation
The function of the thermocouple is to sense a differ-
ence in temperature between two ends of the thermo-
couple wires. The thermocouple’s hot junction can be
read from 0°C to +1023.75°C. The cold end (ambi-
ent temperature of the board on which the MAX6675
is mounted) can only range from -20°C to +85°C.
While the temperature at the cold end fluctuates, the
MAX6675 continues to accurately sense the tempera-
ture difference at the opposite end.
The MAX6675 senses and corrects for the changes in
the ambient temperature with cold-junction compen-
sation. The device converts the ambient temperature
reading into a voltage using a temperature-sensing
diode. To make the actual thermocouple temperature
measurement, the MAX6675 measures the voltage from
the thermocouple’s output and from the sensing diode.
The device’s internal circuitry passes the diode’s volt-
age (sensing ambient temperature) and thermocouple
voltage (sensing remote temperature minus ambient
temperature) to the conversion function stored in the
ADC to calculate the thermocouple’s hot-junction tem-
perature.
Optimal performance from the MAX6675 is achieved
when the thermocouple cold junction and the MAX6675
are at the same temperature. Avoid placing heat-gen-
erating devices or components near the MAX6675
because this may produce cold-junction-related errors.
Digitization
The ADC adds the cold-junction diode measurement
with the amplified thermocouple voltage and reads out
the 12-bit result onto the SO pin. A sequence of all
zeros means the thermocouple reading is 0°C. A
sequence of all ones means the thermocouple reading
is +1023.75°C.
PIN NAME FUNCTION
1 GND Ground
2 T- Alumel Lead of Type-K Thermocouple.
Should be connected to ground externally.
3 T+ Chromel Lead of Type-K Thermocouple
4 VCC Positive Supply. Bypass with a 0.1µF
capacitor to GND.
5 SCK Serial Clock Input
6CS Chip Select. Set CS low to enable the serial
interface.
7 SO Serial Data Output
8 N.C. No Connection
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Pin Description
Applications Information
Serial Interface
The Typical Application Circuit shows the MAX6675
interfaced with a microcontroller. In this example, the
MAX6675 processes the reading from the thermocou-
ple and transmits the data through a serial interface.
Force CS low and apply a clock signal at SCK to read
the results at SO. Forcing CS low immediately stops
any conversion process. Initiate a new conversion
process by forcing CS high.
Force CS low to output the first bit on the SO pin. A
complete serial interface read requires 16 clock cycles.
Read the 16 output bits on the falling edge of the clock.
The first bit, D15, is a dummy sign bit and is always
zero. Bits D14–D3 contain the converted temperature
in the order of MSB to LSB. Bit D2 is normally low and
goes high when the thermocouple input is open. D1 is
low to provide a device ID for the MAX6675 and bit D0
is three-state.
Figure 1a is the serial interface protocol and Figure 1b
shows the serial interface timing. Figure 2 is the SO out-
put.
Open Thermocouple
Bit D2 is normally low and goes high if the thermocou-
ple input is open. In order to allow the operation of the
open thermocouple detector, T- must be grounded.
Make the ground connection as close to the GND pin
as possible.
Noise Considerations
The accuracy of the MAX6675 is susceptible to power-
supply coupled noise. The effects of power-supply
noise can be minimized by placing a 0.1µF ceramic
bypass capacitor close to the supply pin of the device.
Thermal Considerations
Self-heating degrades the temperature measurement
accuracy of the MAX6675 in some applications. The
magnitude of the temperature errors depends on the
thermal conductivity of the MAX6675 package, the
mounting technique, and the effects of airflow. Use a
large ground plane to improve the temperature mea-
surement accuracy of the MAX6675.
The accuracy of a thermocouple system can also be
improved by following these precautions:
●Use the largest wire possible that does not shunt
heat away from the measurement area.
●If small wire is required, use it only in the region of
the measurement and use extension wire for the
region with no temperature gradient.
●Avoid mechanical stress and vibration, which could
strain the wires.
●When using long thermocouple wires, use a twisted-
pair extension wire.
●Avoid steep temperature gradients.
●Try to use the thermocouple wire well within its tem-
perature rating.
●Use the proper sheathing material in hostile environ-
ments to protect the thermocouple wire.
●Use extension wire only at low temperatures and
only in regions of small gradients.
●Keep an event log and a continuous record of ther-
mocouple resistance.
Reducing Effects of Pick-Up Noise
The input amplifier (A1) is a low-noise amplifier
designed to enable high-precision input sensing. Keep
the thermocouple and connecting wires away from elec-
trical noise sources.
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Chip Information
TRANSISTOR COUNT: 6720
PROCESS: BiCMOS
Figure 2. SO Output
Figure 1b. Serial Interface Timing
Figure 1a. Serial Interface Protocol
BIT DUMMY
SIGN BIT
12-BIT
TEMPERATURE READING
THERMOCOUPLE
INPUT
DEVICE
ID STATE
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 MSB LSB 0 Three-
state
D15 D0D1D2D3
SCK
SO
tDV
tCSS
tDO
CS
tTR
tCH tCL
CS
SCK
SO D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1
D0
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
8 SO S8+2 21-0041 90-0096
S1
S3
S2
T-
REFERENCE
VOLTAGE
1
GND
ADC
300kΩ
300kΩ
30kΩ
1MΩ
20pF
COLD-JUNCTION
COMPENSATION
DIODE
S5
S4
DIGITAL
CONTROLLER
SCK
SO
7
5
4
VCC
A1 A2
6CS
T+
3
2
0.1µF
30kΩ
MAX6675
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Block Diagram
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
2 4/14 Removed automotive reference 1
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. © 2014 Maxim Integrated Products, Inc.
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MAX6675 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Revision History
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.