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MAX6675ISA+

P1-P3P4-P6P7-P8
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
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
MAX6675
4
Maxim Integrated
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
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
electrical noise sources.
Chip Information
TRANSISTOR COUNT: 6720
PROCESS: BiCMOS
MAX6675
Maxim Integrated
5
Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +1024°C)
Figure 1a. Serial Interface Protocol
Figure 1b. Serial Interface Timing
BIT
SIGN BIT
THERMOCOUPLE
STATE
14131211109876543
MSB LSB
state
Figure 2. SO Output
MAX6675
6
Maxim Integrated
P1-P3P4-P6P7-P8

MAX6675ISA+

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Buy MAX6675ISA+
Manufacturer:
Maxim Integrated
Description:
Sensor Interface K Thermocouple to Digital Converter
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