Lead Temperature (soldering, 10s) ............................... +300°C
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.
Electrical Characteristics
(V
CC
= +3.0V to +5.5V, T
A
= -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
T
THERMOCOUPLE
= +700°C,
T
A
= +25°C (Note 2)
Temperature Error
T
THERMOCOUPLE
= 0°C to
+700°C, T
A
= +25°C (Note 2)
T
THERMOCOUPLE
= +700°C
to +1000°C, T
A
= +25°C (Note 2)
Thermocouple Conversion
Constant
Cold-Junction
Compensation Error
Resolution
Thermocouple Input
Impedance
Supply Voltage
Supply Current
Power-On Reset Threshold
Power-On Reset Hysteresis
Conversion Time
SERIAL INTERFACE
Input Low Voltage
Input High Voltage
Input Leakage Current
Input Capacitance
V
IL
V
IH
I
LEAK
C
IN
V
IN
= GND or V
CC
5
0.7 x
V
CC
±5
0.3 x
V
CC
V
V
µA
pF
(Note 2)
V
CC
I
CC
V
CC
rising
1
3.0
0.7
2
50
0.17
0.22
T
A
= -20°C
+85°C (Note 2)
t o V
CC
= +3.3V
V
CC
= +5V
-3.0
-3.0
0.25
60
5.5
1.5
2.5
V
CC
= +3.3V
V
CC
= +5V
V
CC
= +3.3V
V
CC
= +5V
V
CC
= +3.3V
V
CC
= +5V
MIN
-5
-6
-8
-9
-17
-19
10.25
+3.0
+3.0
TYP
MAX
+5
+6
+8
+9
+17
+19
µV/LSB
°C
°C
kW
V
mA
V
mV
s
LSB
UNITS
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Maxim Integrated
│
2
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Electrical Characteristics (continued)
(V
CC
= +3.0V to +5.5V, T
A
= -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1)
PARAMETER
Output High Voltage
Output Low Voltage
TIMING
Serial Clock Frequency
SCK Pulse High Width
SCK Pulse Low Width
CSB Fall to SCK Rise
CSB Fall to Output Enable
CSB Rise to Output Disable
SCK Fall to Output Data Valid
f
SCL
t
CH
t
C
L
t
CSS
t
DV
t
TR
t
DO
C
L
= 10pF
C
L
= 10pF
C
L
= 10pF
C
L
= 10pF
100
100
100
100
100
100
4.3
MHz
ns
ns
ns
ns
ns
ns
SYMBOL
V
OH
V
OL
CONDITIONS
I
SOURCE
= 1.6mA
I
SINK
= 1.6mA
MIN
V
CC
-
0.4
0.4
TYP
MAX
UNITS
V
V
Note 1:
All specifications are 100% tested at T
A
= +25°C. Specification limits over temperature (T
A
= T
MIN
to T
MAX
) are guaranteed
by design and characterization, not production tested.
Note 2:
Guaranteed by design. Not production tested.
Typical Operating Characteristics
(V
CC
= +3.3V, T
A
= +25°C, unless otherwise noted.)
OUTPUT CODE ERROR
vs. AMBIENT TEMPERATURE
MAX6675 toc01
OUTPUT CODE ERROR
vs. VOLTAGE DIFFERENTIAL
MAX6675 toc02
10
OUTPUT CODE ERROR (LSB)
8
6
4
2
0
10
OUTPUT CODE ERROR (LSB)
5
0
0
15
30
45
60
75
90
-5
-10
0
10
20
30
40
50
TEMPERATURE (°C)
VOLTAGE DIFFERENTIAL (mV)
www.maximintegrated.com
Maxim Integrated
│
3
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Where:
FUNCTION
Ground
Alumel Lead of Type-K Thermocouple.
Should be connected to ground externally.
Chromel Lead of Type-K Thermocouple
Positive Supply. Bypass with a 0.1µF
capacitor to GND.
Serial Clock Input
Chip Select. Set
CS
low to enable the serial
interface.
Serial Data Output
No Connection
Pin Description
PIN
1
2
3
4
5
6
7
8
NAME
GND
T-
T+
V
CC
SCK
CS
SO
N.C.
V
OUT
is the thermocouple output voltage (µV).
T
R
is the temperature of the remote thermocouple junc-
tion (°C).
T
AMB
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.
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:
V
OUT
= (41µV / °C) 5 (T
R
- T
AMB
)
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.
www.maximintegrated.com
Maxim Integrated
│
4
MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
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.
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.
Reducing Effects of Pick-Up Noise
Noise Considerations
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.
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.
Chip Information
Thermal Considerations
TRANSISTOR COUNT: 6720
PROCESS: BiCMOS
Self-heating degrades the temperature measurement
accuracy of the MAX6675 in some applications. The
magnitude of the temperature errors depends on the
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