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MAX6675ISA
Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C)
General DescriptionThe MAX6675 performs cold-junction compensation
and digitizes the signal from a type-K thermocouple.
The data is output in a 12-bit resolution, SPI™-compati-
ble, read-only format.
This converter resolves temperatures to 0.25°C, allows
readings as high as +1024°C, and exhibits thermocou-
ple accuracy of 8LSBs for temperatures ranging from
0°C to +700°C.
The MAX6675 is available in a small, 8-pin SO package.
ApplicationsIndustrial
Appliances
HVAC
Automotive
FeaturesDirect Digital Conversion of Type -K
Thermocouple OutputCold-Junction CompensationSimple SPI-Compatible Serial Interface12-Bit, 0.25°C ResolutionOpen Thermocouple Detection
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Pin Configuration
Typical Application Circuit19-2235; Rev 1; 3/02
Ordering InformationSPI is a trademark of Motorola, Inc.
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= +3.0V to +5.5V, TA= -20°C to +85°C, unless otherwise noted. Typical values specified at +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, 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
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Typical Operating Characteristics(VCC= +3.3V, TA= +25°C, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)(VCC
Note 1:
Note 2:Guaranteed by design. Not production tested.
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Detailed DescriptionThe 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 ConversionThe 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) ✕(TR- TAMB)
Where:
VOUTis the thermocouple output voltage (µV).is the temperature of the remote thermocouple junc-
tion (°C).
TAMBis the ambient temperature (°C).
Cold-Junction CompensationThe 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 (ambient
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 compensa-
tion. 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-gener-
ating devices or components near the MAX6675
because this may produce cold-junction-related errors.
DigitizationThe 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.
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Applications Information
Serial InterfaceThe Typical Application Circuitshows 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 CSlow and apply a clock signal at SCK to read
the results at SO. Forcing CSlow immediately stops
any conversion process. Initiate a new conversion
process by forcing CShigh.
Force CSlow 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 ThermocoupleBit 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 ConsiderationsThe 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 ConsiderationsSelf-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 NoiseThe input amplifier (A1) is a low-noise amplifier
designed to enable high-precision input sensing. Keep
the thermocouple and connecting wires away from
electrical noise sources.
Chip InformationTRANSISTOR COUNT: 6720
PROCESS: BiCMOS
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)Figure 1a. Serial Interface Protocol
Figure 1b. Serial Interface Timing
