MAX6675ISA Datasheet, PDF (5/8 Pages) Maxim Integrated Products – Cold-Junction-Compensated K-Thermocouple to-digital converter

MAX6675
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
Maxim Integrated
5