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MAX6674 19-2241; Rev 0; 11/01 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) 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, SPI™-compatible, read-only format. This converter resolves temperatures to 0.125°C, allows readings as high as +128°C, and exhibits thermocouple accuracy of ±2°C for temperatures ranging from 0°C to +125°C. The MAX6674 is available in a small, 8-pin SO package. o Cold-Junction Compensation Features o Simple SPI-Compatible Serial Interface o 10 Bit, 0.125°C o Open Thermocouple Detection PART MAX6674ISA Ordering Information TEMP. RANGE -20°C to +85°C PIN-PACKAGE 8 SO Industrial Appliances HVAC Automotive Applications SPI is a trademark of Motorola, Inc. TOP VIEW Pin Configuration GND 1 T- 2 T+ 3 VCC 4 MAX6674 8 N.C. 7 SO 6 CS 5 SCK SO Vcc 0.1µF MAX6674 GND SO T+ SCK T- CS Typical Application Circuit MICROCONTROLLER 68HC11A8 MISO SCK SSB ________________________________________________________________ Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. MAX6674 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) 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 = +70°C) 8-Pin SO (derate 5.88mW/°C above +70°C)................471mW Operating Temperature Range ...........................-20°C to +85°C Storage Temperature Range .............................-65°C to +150°C Junction Temperature ......................................................+150°C SO Package Vapor Phase (60s) ......................................................+215°C Infrared (15s) ..............................................................+220°C Lead Temperature (soldering, 10s) ................................ +300°C 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 = -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP Temperature Error TTHERMOCOUPLE = VCC = +3.3V -1 +100°C, TA = +25°C (Note 2) VCC = +5V -1.5 TTHERMOCOUPLE = VCC = +3.3V -2 0°C to +125°C, TA = +25°C (Note 2) VCC = +5V -3 MAX +1 +1.5 +2 +3 UNITS °C Temperature Conversion Constant VCC = +3.3V 5.125 µV/LSB Cold-Junction Compensation TA = +25°C VCC = +3.3V -1 TA = -20°C to +85°C (Note 2) VCC = +3.3V and +5V -3 +1 °C +3 Resolution Thermocouple Input Impedance Supply Voltage Supply Current Power-On Reset Threshold Power-On Reset Hysteresis Conversion Time SERIAL INTERFACE VCC ICC VCC rising (Note 2) 0.125 °C 20 kΩ 3.0 5.5 V 1 2 mA 1 2 2.5 V 50 mV 0.15 0.18 s Input Low Voltage VIL 0.3 x VCC V Input High Voltage Input Leakage Current Input Capacitance VIH ILEAK CIN VIN = GND or VCC 0.7 x VCC -5 5 V 5 µA pF 2 _______________________________________________________________________________________ MAX6674 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) ELECTRICAL CHARACTERISTICS (continued) (VCC = +3.0V to +5.5V, TA = -20°C to +85°C, unless otherwise noted. Typical values specified at +25°C.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Output High Voltage VOH ISOURCE = 1.6mA VCC 0.4 V Output Low Voltage VOL ISINK = 1.6mA 0.4 V TIMING Serial Clock Frequency fSCL 4.3 MHz SCK Pulse High Width tCH 100 ns SCK Pulse Low Width tCL 100 ns CSB Fall to SCK Rise tCSS CL = 10pF 100 ns CSB Fall to Output Enable tDV CL = 10pF 100 ns CSB Rise to Output Disable tTR CL = 10pF 100 ns SCK Fall to Output Data Valid tDO CL = 10pF 100 ns Note 1: All specifications are 100% tested at TA = +25°C. 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. OUTPUT CODE ERROR (°C) MAX6674 toc01 OUTPUT CODE ERROR (°C) MAX6674 toc02 (VCC = +3.3V, TA = +25°C, unless otherwise noted.) OUTPUT CODE ERROR vs. TEMPERATURE 2 1 0 -1 -2 0 15 30 45 60 75 90 TEMPERATURE (°C) Typical Operating Characteristics OUTPUT CODE ERROR vs. VOLTAGE DIFFERENTIAL 2 1 0 -1 -2 -1200 0 1200 2400 3600 4800 VOLTAGE DIFFERENTIAL (µV) _______________________________________________________________________________________ 3 MAX6674 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) Pin Description PIN NAME FUNCTION 1 GND Ground Iron Lead of Type-K Thermocouple. 2 T- Should be connected to ground externally. 3 T+ Constantan Lead of Type-K Thermocouple 4 VCC Positive Supply. Bypass with a 0.1µF capacitor to GND. 5 SCK Serial Clock Input 6 CS Chip Select. Set CS low to enable the serial interface. 7 SO Serial Data Output 8 N.C. No Connection 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 (µC) or other intelligence in thermostatic, process-control, or monitoring applications. The µC 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 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 (MAX6674 ambient temperature) and a 0°C virtual reference. For a type-K thermocouple, the voltage changes by 41µV/°C, which approximates the thermocouple characteristic with the following linear equation: VOUT = (41µV/°C) ✕ (TR - TAMB) Where: VOUT is the thermocouple output voltage (µV). TR is the temperature of the remote point (°C). TAMB is the ambient temperature (°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 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 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. Digitization The ADC adds the cold-junction diode measurement with the amplified thermocouple voltage and reads out the 10-bit sequence onto the SO 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. 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 SO. Forcing CS low immediately stops 4 _______________________________________________________________________________________ MAX6674 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) 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 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 SO output. 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 0.1µF 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 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. 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. _______________________________________________________________________________________ 5 MAX6674 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) CS SCK SO D2 D1 D0 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 Figure 1a. Serial Interface Protocol CS SCK tDV SO tCSS tCH D15 Figure 1b. Serial Interface Timing tCL tDO tTR D3 D2 D1 D0 BIT DUMMY SIGN BIT Bit 15 14 13 0 MSB Figure 2. SO Output 10-BIT TEMPERATURE READING 12 11 10 9 8 7 THERMOCOUPLE DEVICE INPUT ID STATE 65 4 3 210 LSB 0 Three-state 6 _______________________________________________________________________________________ MAX6674 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) Block Diagram VCC 0.1µF 4 DIGITAL CONTROLLER COLD-JUNCTION COMPENSATION S5 DIODE 5 SCK T+ S3 3 T2 10kΩ S2 10kΩ S1 300kΩ A1 1MΩ A2 S4 20pF 300kΩ MAX6674 REFERENCE VOLTAGE ADC 7 SO 6 CS 1 GND _______________________________________________________________________________________ 7 Cold-Junction-Compensated K-Thermocoupleto-Digital Converter (0°C to +128°C) Package Information MAX6674 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 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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