MAX6674ISA+T Maxim Integrated Products, MAX6674ISA+T Datasheet - Page 4

• Manufacturer Part Number: MAX6674ISA+T
• Description: IC THERMOCOUP TO DGTL 8-SOIC
• Manufacturer: Maxim Integrated Products
• Type: Temperature Sensorr

Specifications of MAX6674ISA+T

• Input Type: Logic
• Output Type: Logic
• Interface: 3-Wire SPI Serial
• Current - Supply: 2mA
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Full Temp Accuracy: +/- 3 C
• Digital Output - Bus Interface: Serial (SPI)
• Digital Output - Number Of Bits: 10 bit
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 20 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Cold-Junction-Compensated K-Thermocouple-
to-Digital Converter (0°C to +128°C)
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
“bit-banging” general-purpose input-output (GPIO) pins
or through a dedicated SPI interface block.
The MAX6674 includes signal conditioning hardware to
convert the thermocouple’s signal into a voltage that is
compatible with the input channels of the ADC. The T+
and T-inputs connect to internal circuitry that reduces the
introduction 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 (MAX6674 ambient temperature) and
a 0°C virtual reference.
4
PIN
1
2
3
4
5
6
7
8
_______________________________________________________________________________________
NAME
GND
SCK
N.C.
V
CS
T+
S0
T-
CC
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
Detailed Description
Temperature Conversion
Pin Description
FUNCTION
For a type-K thermocouple, the voltage changes by
41µV/°C, which approximates the thermocouple char-
acteristic with the following linear equation:
where:
V
T
T
The function of the thermocouple is to sense a differ-
ence in temperature between two ends. The thermo-
couple’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 temperature 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 ADC adds the cold-junction diode measurement
with the amplified thermocouple voltage and reads out
the 10-bit sequence onto the S0 pin. A sequence of all
zeros means the thermocouple reading is 0°C. A
sequence of all ones means the thermocouple reading
is +127.875°C.
R
AMB
OUT
is the temperature of the remote point (°C).
is the thermocouple output voltage (µV).
is the ambient temperature (°C).
V
OUT
= (41µV/°C)
Cold-Junction Compensation
✕
(T
R
- T
AMB
Digitization
)