Data Sheet

MAX6675
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
4 _______________________________________________________________________________________
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)
✕
(T
R
- T
AMB
)
Where:
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 (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.
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 Description
PIN
NAME
FUNCTION
1
GND
Ground
2T-
Alumel Lead of Type-K Thermocouple.
Should be connected to ground
externally.
3T+C hr om el Lead of Typ e- K Ther m ocoup l e
4
V
CC
Positive Supply. Bypass with a 0.1µF
capacitor to GND.
5
SCK
Serial Clock Input
6 CS
Chip Select. Set CS low to enable the
serial interface.
7 SO Serial Data Output
8
N.C.
No Connection