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
=
+100°C, T
A
= +25°C
(Note 2)
T
THERMOCOUPLE
=
0°C to +125°C, T
A
=
+25°C (Note 2)
V
CC
= +3.3V
V
CC
= +5V
V
CC
= +3.3V
V
CC
= +5V
MIN
-1
-1.5
-2
-3
5.125
T
A
= +25°C
Cold-Junction Compensation
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
0.7 x
V
CC
-5
5
5
0.3 x
V
CC
V
V
µA
pF
(Note 2)
V
CC
I
CC
V
CC
rising
1
3.0
1
2
50
0.15
0.18
T
A
= -20°C to +85°C
(Note 2)
V
CC
= +3.3V
V
CC
= +3.3V and
+5V
-1
-3
0.125
20
5.5
2
2.5
+1
+3
°C
°C
kW
V
mA
V
mV
s
TYP
MAX
+1
+1.5
+2
+3
µv/LSB
°C
UNITS
Temperature Error
Temperature
Constant
Conversion
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Maxim Integrated
│
2
MAX6674
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +128°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
CL
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
= -20°C to +85°C) are guaran-
teed 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. TEMPERATURE
MAX6674 toc01
OUTPUT CODE ERROR
vs. VOLTAGE DIFFERENTIAL
MAX6674 toc02
2
OUTPUT CODE ERROR (
°
C)
2
OUTPUT CODE ERROR (
°
C)
1
1
0
0
-1
-1
-2
0
15
30
45
60
75
90
-2
-1200
0
1200
2400
3600
4800
TEMPERATURE (°C)
VOLTAGE DIFFERENTIAL (µV)
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Maxim Integrated
│
3
MAX6674
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +128°C)
For a type-K thermocouple, the voltage changes by
41µV/°C, which approximates the thermocouple charac-
teristic with the following linear equation:
V
OUT
= (41µV/°C) 5 (T
R
- T
AMB
)
where:
V
OUT
is the thermocouple output voltage (µV).
T
R
is the temperature of the remote point (°C).
T
AMB
is the ambient temperature (°C).
Pin Description
PIN
1
2
3
4
5
6
7
8
NAME
GND
T-
T+
V
CC
SCK
CS
S0
N.C.
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
FUNCTION
Cold-Junction Compensation
Detailed Description
The function of the thermocouple is to sense a difference
in temperature between two ends. The thermocouple’s
hot junction can be read from 0°C to +127.875°C. The
cold end (ambient temperature of the board on which
the MAX6674 is mounted) can only range from -20°C to
+85°C. While the temperature at the cold end fluctuates,
the MAX6674 continues to accurately sense the tempera-
ture difference at the opposite end.
The MAX6674 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 MAX6674 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 MAX6674 is achieved
when the thermocouple cold junction and the device
are at the same temperature. Avoid placing heat-gener-
ating devices or components near the MAX6674
because this may produce cold-junction-related errors.
The MAX6674 is a sophisticated thermocouple-to-digi-
tal converter with a built-in 10-bit analog-to-digital con-
verter (ADC). The device also contains cold-junction
compensation sensing and correction, a digital con-
troller, an SPI-compatible interface, and associated
control logic.
The MAX6674 is designed to work in conjunction with
an external microcontroller (µC) or other intelligence
in thermostatic, process-control, or monitoring applica-
tions. The µC is typically a power-management or key-
board controller, generating SPI serial commands by
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