Datasheet

Applications Information
Serial Interface
The Typical Application Circuit shows the MAX6674
interfaced with a microcontroller. In this example, the
MAX6674 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 S0. 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 S0 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 always zero.
Bits D14–D5 contain the converted temperature in the
order of MSB to LSB. Bit D4 reads a high value when
any of the thermocouple inputs are open. Bit D3 is
always low to provide a device ID for the MAX6674.
Bits D2–D0 are in three-state when CS is high.
Figure 1a is the serial interface protocol and Figure 1b
shows the serial interface timing. Figure 2 is the S0 output.
Open Thermocouple
Bit D4 is normally low and goes high if the thermocou-
ple input is open. The open thermocouple detection cir-
cuit is implemented completely into the MAX6674. In
order to allow the operation of the open thermocouple
detector, T- must be grounded. Make the ground con-
nection as close to the GND pin as possible.
Noise Considerations
The accuracy of the MAX6674 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 MAX6674 in some applications. The
magnitude of the temperature errors depends on the ther-
mal conductivity of the MAX6674 package, the mounting
technique, and the effects of airflow. Use a large ground
plane to improve the temperature measurement accuracy.
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 that could
strain the wires.
● When using long thermocouple wires, use a twist-
ed-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.
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Maxim Integrated │ 5
MAX6674 Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +128°C)