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, spi -compati- ble, read-only format. this converter resolves temperatures to 0.25?, allows readings as high as +1024?, and exhibits thermocou- ple accuracy of 8lsbs for temperatures ranging from 0? to +700?. the max6675 is available in a small, 8-pin so package. applications industrial appliances hvac automotive features direct digital conversion of type -k thermocouple output cold-junction compensation simple spi-compatible serial interface 12-bit, 0.25? resolution open thermocouple detection cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) cs sck v cc 1 2 8 7 n.c. so t- t+ gnd so top view 3 4 6 5 max6675 pin configuration vcc gnd t+ t- so sck cs microcontroller 68hc11a8 miso sck ssb 0.1f max6675 typical application circuit ordering information spi is a trademark of motorola, inc. part temp range pin-package MAX6675ISA -20 c to +85 c 8 so 19-2235; rev 1; 3/02 functional diagrams pin configurations appear at end of data sheet. functional diagrams continued at end of data sheet. ucsp is a trademark of maxim integrated products, inc. available for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim?s website at www.maxim integrated.com. max6675
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) absolute maximum ratings electrical characteristics (v cc = +3.0v to +5.5v, t a = -20 c to +85 c, unless otherwise noted. typical values specified at +25 c.) (note 1) stresses beyond those listed under ?bsolute 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. supply voltage (v cc to gnd) ................................ -0.3v to +6v so, sck, cs , t-, t+ to gnd .......................-0.3v to v cc + 0.3v so current ........................................................................ 50ma esd protection (human body model) ........................... 2000v continuous power dissipation (t a = +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 parameter symbol conditions min typ max units v cc = +3.3v -5 +5 t thermocouple = +700 c, t a = +25 c (note 2) v cc = +5v -6 +6 v cc = +3.3v -8 +8 t thermocouple = 0 c to +700 c, t a = +25 c (note 2) v cc = +5v -9 +9 v cc = +3.3v -17 +17 temperature error t thermocouple = +700 c to +1000 c, t a = +25 c (note 2) v cc = +5v -19 +19 lsb thermocouple conversion constant 10.25 v/lsb v cc = +3.3v -3.0 +3.0 cold-junction compensation error t a = -20 c to +85 c (note 2) v cc = +5v -3.0 +3.0 c resolution 0.25 c thermocouple input impedance 60 k ? supply voltage v cc 3.0 5.5 v supply current i cc 0.7 1.5 ma power-on reset threshold v cc rising 1 2 2.5 v power-on reset hysteresis 50 mv conversion time (note 2) 0.17 0.22 s serial interface input low voltage v il 0.3 x v cc v input high voltage v ih 0.7 x v cc v input leakage current i leak v in = gnd or v cc 5a input capacitance c in 5p f max6675 2 maxim integrated
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) 10 8 6 4 2 0 045 15 30 60 75 90 output code error vs. ambient temperature max6675 toc01 temperature (c) output code error (lsb) -5 0 5 10 -10 0 30 50 output code error vs. voltage differential max6675 toc02 voltage differential (mv) output code error (lsb) 10 20 40 typical operating characteristics (v cc = +3.3v, t a = +25 c, unless otherwise noted.) electrical characteristics (continued) (v cc = +3.0v to +5.5v, t a = -20 c to +85 c, unless otherwise noted. typical values specified at +25 c.) (note 1) note 1: all specifications are 100% tested at t a = +25 c. specification limits over temperature (t a = t min to t max ) are guaranteed by design and characterization, not production tested. note 2: guaranteed by design. not production tested. parameter symbol conditions min typ max units output high voltage v oh i source = 1.6ma v cc - 0.4 v output low voltage v ol i sink = 1.6ma 0.4 v timing serial clock frequency f scl 4.3 mhz sck pulse high width t ch 100 ns sck pulse low width t cl 100 ns csb fall to sck rise t css c l = 10pf 100 ns csb fall to output enable t dv c l = 10pf 100 ns csb rise to output disable t tr c l = 10pf 100 ns sck fall to output data valid t do c l = 10pf 100 ns max6675 maxim integrated 3
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) detailed description the max6675 is a sophisticated thermocouple-to-digi- tal converter with a built-in 12-bit analog-to-digital con- verter (adc). the max6675 also contains cold-junction compensation sensing and correction, a digital con- troller, 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 ther- mostatic, process-control, or monitoring applications. temperature conversion the max6675 includes signal-conditioning hardware to convert the thermocouple s signal into a voltage compat- ible with the input channels of the adc. the t+ and t- inputs connect to internal circuitry that reduces the intro- duction 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 (max6675 ambient temperature) and a 0 c virtual reference. for a type-k thermocouple, the voltage changes by 41v/ c, which approximates the thermocouple characteristic with the following linear equation: v out = (41v / c) ? (t r - t amb ) where: v out is the thermocouple output voltage (v). t r is the temperature of the remote thermocouple junc- tion ( c). t amb is the ambient temperature ( c). cold-junction compensation the function of the thermocouple is to sense a differ- ence in temperature between two ends of the thermo- couple wires. the thermocouple s hot junction can be read from 0 c to +1023.75 c. the cold end (ambient temperature of the board on which the max6675 is mounted) can only range from -20 c to +85 c. while the temperature at the cold end fluctuates, the max6675 continues to accurately sense the tempera- ture difference at the opposite end. the max6675 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 max6675 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 max6675 is achieved when the thermocouple cold junction and the max6675 are at the same temperature. avoid placing heat-gener- ating devices or components near the max6675 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 12-bit result 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 +1023.75 c. pin description pin name function 1 gnd ground 2t- alumel lead of type-k thermocouple. should be connected to ground externally. 3t+c hr om el lead of typ e- k ther m ocoup l e 4 v cc positive supply. bypass with a 0.1f 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 max6675 4 maxim integrated
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) 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 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 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 out- put. open thermocouple bit d2 is normally low and goes high if the thermocou- ple 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. noise considerations the accuracy of the max6675 is susceptible to power- supply 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. 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 mounting technique, and the effects of airflow. use a large ground plane to improve the temperature mea- surement 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 twisted- 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. chip information transistor count: 6720 process: bicmos max6675 maxim integrated 5
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) cs sck so d15 d14 d13 d12 d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0 figure 1a. serial interface protocol d15 d0d1d2d3 sck so t dv t css t do cs t tr t ch t cl figure 1b. serial interface timing bit dummy sign bit 12-bit temperature reading thermocouple input device id state bit 15 14131211109876543 210 0 msb lsb 0 three- state figure 2. so output max6675 6 maxim integrated
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) s1 s3 s2 t- reference voltage 1 gnd adc 300k? 300k? 30k? 1m? 20pf cold-junction compensation diode s5 s4 digital controller sck so 7 5 4 v cc a1 a2 6 cs t+ 3 2 0.1f 30k? max6675 block diagram max6675 maxim integrated 7
cold-junction-compensated k-thermocouple- to-digital converter (0? to +1024?) package information soicn.eps max6675 8 maxim integrated 160 rio robles, san jose, ca 95134 usa 1-408-601-1000 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. the parametric values (min and max limits) shown in the electrical characteristics table are guaranteed. other parametric values quoted in this data sheet are provided for guidance. ? 2002 maxim integrated the maxim logo and maxim integrated are trademarks of maxim integrated products, inc.
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