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19-2235; Rev 1; 3/02
Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C) MAX6675
General Description
The MAX6675 performs cold-junction compensation and digitizes the signal from a type-K thermocouple. The data is output in a 12-bit resolution, SPITM-compatible, read-only format. This converter resolves temperatures to 0.25C, allows readings as high as +1024C, and exhibits thermocouple accuracy of 8LSBs for temperatures ranging from 0C to +700C. The MAX6675 is available in a small, 8-pin SO package. o Direct Digital Conversion of Type -K Thermocouple Output o Cold-Junction Compensation o Simple SPI-Compatible Serial Interface o 12-Bit, 0.25C Resolution o Open Thermocouple Detection
Features
Ordering Information
PART MAX6675ISA 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
MAX6675
3 6 5 VCC 4
SO SPI is a trademark of Motorola, Inc.
Typical Application Circuit
Vcc 0.1F
MAX6675
GND SO T+ TSCK CS MISO SCK SSB
MICROCONTROLLER 68HC11A8
________________________________________________________________ Maxim Integrated Products
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C) MAX6675
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 = +700C, TA = +25C (Note 2) Temperature Error TTHERMOCOUPLE = 0C to +700C, TA = +25C (Note 2) TTHERMOCOUPLE = +700C to +1000C, TA = +25C (Note 2) Thermocouple Conversion Constant Cold-Junction Compensation Error 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 5 0.7 x VCC 5 0.3 x VCC V V A pF (Note 2) VCC ICC VCC rising 1 3.0 0.7 2 50 0.17 0.22 TA = -20C to +85C (Note 2) VCC = +3.3V VCC = +5V -3.0 -3.0 0.25 60 5.5 1.5 2.5 VCC = +3.3V VCC = +5V VCC = +3.3V VCC = +5V VCC = +3.3V VCC = +5V MIN -5 -6 -8 -9 -17 -19 10.25 +3.0 +3.0 TYP MAX +5 +6 +8 +9 +17 +19 V/LSB C C k V mA V mV s LSB UNITS
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C)
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
MAX6675
Note 1: All specifications are 100% tested at TA = +25C. Specification limits over temperature (TA = TMIN to TMAX) 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. AMBIENT TEMPERATURE
MAX6675 toc01
OUTPUT CODE ERROR vs. VOLTAGE DIFFERENTIAL
MAX6675 toc02
10
10
OUTPUT CODE ERROR (LSB)
OUTPUT CODE ERROR (LSB)
8
5
6
4
0
2
0 0 15 30 45 60 75 90 TEMPERATURE (C)
-5 -10 0 10 20 30 40 50 VOLTAGE DIFFERENTIAL (mV)
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C) MAX6675
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
Where: VOUT is the thermocouple output voltage (V). TR is the temperature of the remote thermocouple junction (C). TAMB is the ambient temperature (C).
2 3 4 5 6 7 8
TT+ VCC SCK CS SO N.C.
Cold-Junction Compensation
The function of the thermocouple is to sense a difference in temperature between two ends of the thermocouple wires. The thermocouple`s hot junction can be read from 0C to +1023.75C. The cold end (ambient temperature of the board on which the MAX6675 is mounted) can only range from -20C to +85C. While the temperature at the cold end fluctuates, the MAX6675 continues to accurately sense the temperature difference at the opposite end. The MAX6675 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 MAX6675 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 MAX6675 is achieved when the thermocouple cold junction and the MAX6675 are at the same temperature. Avoid placing heat-generating devices or components near the MAX6675 because this may produce cold-junction-related errors.
Detailed Description
The MAX6675 is a sophisticated thermocouple-to-digital converter with a built-in 12-bit analog-to-digital converter (ADC). The MAX6675 also contains cold-junction compensation sensing and correction, a digital controller, an SPI-compatible interface, and associated control logic. The MAX6675 is designed to work in conjunction with an external microcontroller (C) or other intelligence in thermostatic, process-control, or monitoring applications.
Temperature Conversion
The MAX6675 includes signal-conditioning hardware to convert the thermocouple's signal into a voltage compatible with the input channels of the ADC. The T+ and Tinputs 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 (MAX6675 ambient temperature) and a 0C virtual reference. 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)
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 0C. A sequence of all ones means the thermocouple reading is +1023.75C.
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C)
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 thermocouple 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 output. mounting technique, and the effects of airflow. Use a large ground plane to improve the temperature measurement 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 twistedpair 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.
MAX6675
Open Thermocouple
Bit D2 is normally low and goes high if the thermocouple 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.
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.
Noise Considerations
The accuracy of the MAX6675 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.
Chip Information
TRANSISTOR COUNT: 6720 PROCESS: BiCMOS
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
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C) MAX6675
CS
SCK
SO
D15
D14
D13
D12
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D0 D1
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 11
12-BIT TEMPERATURE READING 10 9 8 7 6 5 4 3 LSB
THERMOCOUPLE DEVICE STATE INPUT ID 2 1 0 0 Threestate
Figure 2. SO Output
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C) MAX6675
Block Diagram
VCC
0.1F
4 DIGITAL CONTROLLER COLD-JUNCTION COMPENSATION DIODE
S5 5 SCK
300k 30k S2 30k 2 S1 20pF 6 CS 1M A1 A2 S4 ADC 7 SO
T+ 3 T-
S3
MAX6675
300k REFERENCE VOLTAGE
1 GND
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Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0C to +1024C) MAX6675
Package Information
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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