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MAX6675ISA+T |MAX6675ISATMAXN/a2500avaiCold-Junction-Compensated K-Thermocouple-to-Digital Converter (0°C to +1024°C)


MAX6675ISA+T ,Cold-Junction-Compensated K-Thermocouple-to-Digital Converter (0°C to +1024°C)Electrical Characteristics(V = +3.0V to +5.5V, T = -20°C to +85°C, unless otherwise noted. Typical ..
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MAX6675ISA+T
Cold-Junction-Compensated K-Thermocouple-to-Digital Converter (0°C to +1024°C)
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
PARTTEMP RANGEPIN-PACKAGE

MAX6675ISA-20°C to +85°C8 SO
Vcc
GND
SCK
MICROCONTROLLER
68HC11A8
MISO
SCK
SSB
0.1µF
MAX6675
SCKVCC
N.C.T-
GND
TOP VIEW
MAX6675
MAX6675Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Typical Application Circuit
Pin Coniguration
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 PackageVapor Phase (60s) . .....................................................+215°CInfrared (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)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS

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
Constant10.25µV/LSB
Cold-Junction
Compensation ErrorA = -20°C to +85°C (Note 2)
VCC = +3.3V-3.0+3.0°CVCC = +5V-3.0+3.0
Resolution0.25°C
Thermocouple Input
Impedance60kΩ
Supply VoltageVCC3.05.5V
Supply CurrentICC0.71.5mA
Power-On Reset ThresholdVCC rising122.5V
Power-On Reset Hysteresis50mV
Conversion Time(Note 2)0.170.22s
SERIAL INTERFACE

Input Low VoltageVIL0.3 x
VCCV
Input High VoltageVIH0.7 x
VCCV
Input Leakage CurrentILEAKVIN = GND or VCC±5µA
Input CapacitanceCIN5pF
MAX6675Cold-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)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS

Output High VoltageVOHISOURCE = 1.6mAVCC -
0.4V
Output Low VoltageVOLISINK = 1.6mA0.4V
TIMING

Serial Clock FrequencyfSCL4.3MHz
SCK Pulse High WidthtCH100ns
SCK Pulse Low WidthtCL100ns
CSB Fall to SCK RisetCSSCL = 10pF100ns
CSB Fall to Output EnabletDVCL = 10pF100ns
CSB Rise to Output DisabletTRCL = 10pF100ns
SCK Fall to Output Data ValidtDOCL = 10pF100ns
OUTPUT CODE ERROR
vs. VOLTAGE DIFFERENTIAL
MAX6675 toc02
OUTPUT CODE ERROR (LSB)2040451530607590
OUTPUT CODE ERROR
vs. AMBIENT TEMPERATURE

MAX6675 toc01
OUTPUT CODE ERROR (LSB)
MAX6675Cold-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.
PINNAMEFUNCTION
GNDGroundT-Alumel Lead of Type-K Thermocouple.
Should be connected to ground externally.T+Chromel Lead of Type-K ThermocoupleVCCPositive Supply. Bypass with a 0.1µF
capacitor to GND.SCKSerial Clock InputCSChip Select. Set CS low to enable the serial
interface.SOSerial Data OutputN.C.No Connection
MAX6675Cold-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.
MAX6675Cold-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
BITDUMMY
SIGN BIT
12-BIT
TEMPERATURE READING
THERMOCOUPLE
INPUT
DEVICESTATE

Bit1514131211109876543210MSBLSB0Three-
state
D15D0D1D2D3
SCK
tDV
tCSS
tDO
tTR
tCHtCL
SCKD15D14D13D12D11D10D9D8D7D6D5D4D3D2D1
MAX6675Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
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