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19-2241; Rev 1; 8/02
Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C)
General Description
The MAX6674 cold-junction-compensation thermocouple-to-digital converter performs cold-junction compensation and digitizes the signal from a type-K thermocouple. The data is output in a 10-bit resolution, SPITM-compatible, read-only format. This converter resolves temperatures to 0.125C, allows readings as high as +128C, and exhibits thermocouple accuracy of 2C for temperatures ranging from 0C to +125C. The MAX6674 is available in a small, 8-pin SO package. o Cold-Junction Compensation o Simple SPI-Compatible Serial Interface o 10 Bit, 0.125C o Open Thermocouple Detection
Features
MAX6674
Ordering Information
PART MAX6674ISA TEMP RANGE -20C to +85C PIN-PACKAGE 8 SO
Applications
Industrial Appliances HVAC Automotive
GND TT+ 1 2
Pin Configuration
TOP VIEW
8 7
N.C. SO CS SCK
MAX6674
3 6 5 VCC 4
SPI is a trademark of Motorola, Inc.
SO
Typical Application Circuit
Vcc 0.1F MICROCONTROLLER 68HC11A8 MAX6674 GND SO T+ TSCK CS MISO SCK SSB
________________________________________________________________ Maxim Integrated Products
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C) MAX6674
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC to GND) ................................. -0.3V to +6V SO, SCK, CS, T-, T+ to GND .......................-0.3V to VCC + 0.3V SO Current .........................................................................50mA ESD Protection (Human Body Model) ........................... 2000V Continuous Power Dissipation (TA = +70C) 8-Pin SO (derate 5.88mW/C above +70C)................471mW Operating Temperature Range ...........................-20C to +85C Storage Temperature Range .............................-65C to +150C Junction Temperature ......................................................+150C SO Package Vapor Phase (60s) ......................................................+215C Infrared (15s) ..............................................................+220C Lead Temperature (soldering, 10s) ................................ +300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VCC = +3.0V to +5.5V, TA = -20C to +85C, unless otherwise noted. Typical values specified at +25C.) (Note 1)
PARAMETER SYMBOL CONDITIONS TTHERMOCOUPLE = +100C, TA = +25C (Note 2) Temperature Error TTHERMOCOUPLE = 0C to +125C, TA = +25C (Note 2) Temperature Conversion Constant TA = +25C Cold-Junction Compensation Resolution Thermocouple Input Impedance Supply Voltage Supply Current Power-On Reset Threshold Power-On Reset Hysteresis Conversion Time SERIAL INTERFACE Input Low Voltage Input High Voltage Input Leakage Current Input Capacitance VIL VIH ILEAK CIN VIN = GND or VCC 0.7 x VCC -5 5 5 0.3 x VCC V V A pF (Note 2) VCC ICC VCC rising 1 3.0 1 2 50 0.15 0.18 TA = -20C to +85C (Note 2) VCC = +3.3V VCC = +3.3V and +5V -1 -3 0.125 20 5.5 2 2.5 VCC = +3.3V VCC = +5V -2 -3 5.125 +1 +3 C C k V mA V mV s +2 +3 v/LSB VCC = +3.3V VCC = +5V MIN -1 -1.5 TYP MAX +1 +1.5 C UNITS
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C) MAX6674
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +3.0V to +5.5V, TA = -20C to +85C, unless otherwise noted. Typical values specified at +25C.) (Note 1)
PARAMETER Output High Voltage Output Low Voltage TIMING Serial Clock Frequency SCK Pulse High Width SCK Pulse Low Width CSB Fall to SCK Rise CSB Fall to Output Enable CSB Rise to Output Disable SCK Fall to Output Data Valid fSCL tCH tCL tCSS tDV tTR tDO CL = 10pF CL = 10pF CL = 10pF CL = 10pF 100 100 100 100 100 100 4.3 MHz ns ns ns ns ns ns SYMBOL VOH VOL CONDITIONS ISOURCE = 1.6mA ISINK = 1.6mA MIN VCC 0.4 0.4 TYP MAX UNITS V V
Note 1: All specifications are 100% tested at TA = +25C. Specification limits over temperature (TA = -20C to +85C) are guaranteed by design and characterization, not production tested. Note 2: Guaranteed by design. Not production tested.
Typical Operating Characteristics
(VCC = +3.3V, TA = +25C, unless otherwise noted.)
OUTPUT CODE ERROR vs. TEMPERATURE
MAX6674 toc01
OUTPUT CODE ERROR vs. VOLTAGE DIFFERENTIAL
MAX6674 toc02
2
2
OUTPUT CODE ERROR (C)
1
OUTPUT CODE ERROR (C) 0 15 30 45 60 75 90
1
0
0
-1
-1
-2 TEMPERATURE (C)
-2 -1200
0
1200
2400
3600
4800
VOLTAGE DIFFERENTIAL (V)
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C) MAX6674
Pin Description
PIN 1 NAME GND Ground Alumel Lead of Type-K Thermocouple. Should be connected to ground externally. Chromel Lead of Type-K Thermocouple Positive Supply. Bypass with a 0.1F capacitor to GND. Serial Clock Input Chip Select. Set CS low to enable the serial interface. Serial Data Output No Connection FUNCTION
For a type-K thermocouple, the voltage changes by 41V/C, which approximates the thermocouple characteristic with the following linear equation: VOUT = (41V/C) (TR - TAMB) where: VOUT is the thermocouple output voltage (V). TR is the temperature of the remote point (C). TAMB is the ambient temperature (C).
2
T-
3 4 5 6 7 8
T+ VCC SCK CS S0 N.C.
Cold-Junction Compensation
The function of the thermocouple is to sense a difference in temperature between two ends. The thermocouple's hot junction can be read from 0C to +127.875C. The cold end (ambient temperature of the board on which the MAX6674 is mounted) can only range from -20C to +85C. 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 compensation. 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 voltage (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 temperature. Optimal performance from the MAX6674 is achieved when the thermocouple cold junction and the device are at the same temperature. Avoid placing heat-generating devices or components near the MAX6674 because this may produce cold-junction-related errors.
Detailed Description
The MAX6674 is a sophisticated thermocouple-to-digital converter with a built-in 10-bit analog-to-digital converter (ADC). The device also contains cold-junction compensation sensing and correction, a digital controller, an SPI-compatible interface, and associated control logic. The MAX6674 is designed to work in conjunction with an external microcontroller (C) or other intelligence in thermostatic, process-control, or monitoring applications. The C is typically a power-management or keyboard controller, generating SPI serial commands by "bit-banging" general-purpose input-output (GPIO) pins or through a dedicated SPI interface block.
Temperature Conversion
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 compensate for the difference between the thermocouple cold-junction side (MAX6674 ambient temperature) and a 0C virtual reference.
Digitization
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 0C. A sequence of all ones means the thermocouple reading is +127.875C.
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C)
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 thermocouple 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.
Thermal Considerations
Self-heating degrades the temperature measurement accuracy of the MAX6674 in some applications. The magnitude of the temperature errors depends on the thermal 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 twisted-pair extension wire. Avoid steep temperature gradients. Try to use the thermocouple wire well within its temperature rating. Use the proper sheathing material in hostile environments 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 thermocouple resistance.
MAX6674
Open Thermocouple
Bit D4 is normally low and goes high if the thermocouple input is open. The open thermocouple detection circuit is implemented completely into the MAX6674. 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 MAX6674 is susceptible to powersupply coupled noise. The effects of power-supply noise can be minimized by placing a 0.1F ceramic bypass capacitor close to the supply pin of the device.
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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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C) MAX6674
CS
SCK
SO D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3
D2
D1
D0
Figure 1a. Serial Interface Protocol
tCSS CS tCH SCK tDV SO D15 D3 D2 D1 D0 tDO tTR tCL
Figure 1b. Serial Interface Timing
BIT Bit
DUMMY SIGN BIT 15 0 14 MSB 13 12
10-BIT TEMPERATURE READING 11 10 9 8 7 6 5 LSB
THERMOCOUPLE DEVICE INPUT ID 4 3 0 2
STATE 1 0
Three-state
Figure 2. S0 Output
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C)
Block Diagram
VCC
MAX6674
0.1F
4 DIGITAL CONTROLLER COLD-JUNCTION COMPENSATION DIODE
S5 5 SCK
300k 10k S2 10k 2 S1 20pF 6 CS 1M A1 A2 S4 ADC 7 SO
T+ 3 T-
S3
MAX6674
300k REFERENCE VOLTAGE
1 GND
Chip Information
TRANSISTOR COUNT: 6460 PROCESS: BiCMOS
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +128C) MAX6674
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
SOICN.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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