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precision thermocouple amplifier s with cold junction compensation ad8494/ad8495/ad8496/ad8497 rev. b information furnished by analog devices is believed to be accurate and reliable. ho wever, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2010 C 2011 analog devices, inc. all rights reserved. features low cost and easy to use pretrimmed for j or k type thermocouples internal c old j unction c ompensation high i mpedance d ifferential i nput standalone 5 mv/c thermometer reference pin allows offset adjust ment thermocouple break detection laser wafer trimmed to 1c initial accuracy and 0.025c/c ambient temperature rejection low power: <1 mw at v s = 5 v wide p ower supply r ange single s upply: 2.7 v to 36 v dual s upply: 2.7 v to 1 8 v small , 8 - lead msop applications j or k type t hermocouple temperatur e mea s urement setpoint controller celsius thermometer universal cold junction compensator white goods (oven, stove top) temperature measurements exhaust gas temperature sensing catalytic converter temperature sensing functional block dia gram ?in +in out ref 08529-001 a1 a2 ad8494/ad8495/ ad8496/ad8497 a3 sense esd and ovp esd and ovp cold junction compensation 1m ? thermo- couple figure 1. table 1 . device temperature ranges part no. thermo - c ouple type optimized temperature range ambient temperature (reference junction) measurement junction ad8494 j 0c to 50c full j type range ad8495 k 0c to 50c full k type range ad8496 j 25c to 100c full j type range ad8497 k 25c to 100c full k type range general description the ad8494/ad8495/ad8496/ad8497 are precision instrumen tation amplifiers with thermocouple cold junction compensators o n an integrated circuit. they produce a high level (5 mv/c) output directly from a thermocouple signal by combining an ice point reference with a precalibrated amplifier. they can be used as standalone thermometers or as switched output setpoint controlle rs using either a fixed or remote setpoint control . the ad8494/ad8495/ad8496/ad8497 can be powered from a single - ended supply (less than 3 v) and can measure temperatures below 0c by offsetting the reference input. to minimize self - heating, an unloaded ad 849x typically operates with a total supply current of 180 a, but it is also capable of delivering in excess of 5 ma to a load. the ad8494 and ad8496 are precalibrated by laser wafer trimming to match the characteristic s of j type (iron - constantan) therm ocouples; the ad8495 and ad8497 are laser trimmed to match the characteristics of k type (chromel - alumel) thermo - couples . see table 1 for the optimized ambient temperature range of each part. the ad8494/ad8495/ad8496/ad8497 al low a wide variety of supply voltages. with a 5 v single supply, the 5 mv/c output allows the devices to cover nearly 1000 degrees of a thermo - couples temperature range. the ad8494/ad8495/ad8496/ad8497 work with 3 v supplies, allowing them to interface d irectly to lower supply adcs. they can also work with supplies as large as 36 v in industrial systems that require a wide common - mode input range. product highlights 1. complete, p recision laser wafer trimmed thermocouple signal conditioning system in a singl e ic package. 2. flexible pinout provides for operation as a setpoint controller or as a standalone celsius thermometer . 3. rugged inputs withstand 4 kv esd and provide over - voltage protection (ovp) up to v s 2 5 v . 4. differential inputs reject common - mode noise o n the thermocouple leads. 5. reference pin voltage can be offset to measure 0c on single supplies. 6. available in a small, 8 - lead msop that is fully rohs compliant.
ad8494/ad8495/ad8496/ad8497 rev. b | page 2 of 16 table of contents features .............................................................................................. 1 applications ....................................................................................... 1 functional block diagram .............................................................. 1 general description ......................................................................... 1 product highlights ........................................................................... 1 revision history ............................................................................... 2 specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 5 thermal resistance ...................................................................... 5 esd caution .................................................................................. 5 pin configuration and function descriptions ............................. 6 typical performance characteristics ............................................. 7 theory of operation ...................................................................... 11 thermocouples ........................................................................... 11 thermocouple signal conditioner .......................................... 11 ad8494/ad8495/ad8496/ad8497 architecture .................. 11 maximum error calculation .................................................... 12 recommendations for best circuit perfor mance .................. 13 applications information .............................................................. 14 basic connection ....................................................................... 14 ambient temperature sensor ................................................... 14 setpoint controller .................................................................... 15 measuring negative temperatures .......................................... 15 reference pin allows offset adjustment ................................ 15 outline dimensions ....................................................................... 16 ordering guide .......................................................................... 16 revision history 4/11 rev. a to rev. b changes to figure 1 .......................................................................... 1 changes to figure 33 and figure 34 ............................................. 14 changes to figure 35 and figure 36 ............................................. 15 changes to ordering guide .......................................................... 16 10/10 rev. 0 to rev. a changes to linearity error of the thermocouple section ........ 12 cha nges to ambient temperature sensor section .................... 14 changes to ordering guide .......................................................... 16 7 / 10 revision 0: initial version
ad8494/ad8495/ad8496/ad8497 rev. b | page 3 of 16 specifications +v s = 5 v, ?v s = 0 v , v + in = v ? in = 0 v , v ref = 0 v , t a = t r j = 25 c, r l = 1 0 0 k , unless otherwise noted. specifications do not include gain and offset errors of the thermocouple itself. t a is the ambient temperature at the ad849x ; t rj is the thermocouple refere nce junction temperature; t mj is the thermocouple measurement junction temperature. table 2 . a grade c grade parameter test conditions/comments min typ max min typ max unit temperature accuracy initial accuracy ad8494/ad8495 t a = t rj = t mj = 25c 3 1 c ad8496/ad8497 t a = t rj = 60c, t mj = 175c 3 1.5 c ambient temperature rejection 1 ad8494/ad8495 t a = t rj = 0c to 50c 0.05 0.025 c/c ad8496/ad8497 t a = t rj = 25c to 100c 0.05 0. 025 c/c gain error 2 , 3 v out = 0.125 v to 4.125 v ad8494/ad8495 0.3 0.1 % ad8496/ad8497 0.3 0.1 % transfer function 5 5 mv/c inputs input voltage range ?v s C 0.2 +v s C 1.6 ?v s C 0.2 +v s C 1.6 v overvoltage range + v s C 25 ?v s + 25 +v s C 25 ?v s + 25 v input bias current 4 25 50 25 50 na input offset current 1.5 0.5 na common - mode rejection v cm = 0 v to 3 v 1 0.3 c/v power supply rejection +v s = 2.7 v to 5 v 0.5 0.5 c/v noise voltage n oise f = 0.1 hz to 10 hz, t a = 25c 0.8 0.8 v p -p voltage noise density f = 1 khz, t a = 25c 32 32 nv/ hz current noise density f = 1 khz, t a = 25c 100 100 fa/ hz reference input input resistance 60 60 k input current 25 25 a voltage range ?v s +v s ?v s +v s v gain to output 1 1 v/v output output voltage range ?v s + 0.025 +v s C 0.1 ?v s + 0.025 +v s C 0.1 v short - circuit current 5 7 7 ma dynamic response ?3 db bandwidth ad8494 30 30 khz ad8495/ad8497 25 25 khz ad8496 31 31 khz settling time to 0.1% 4 v output step ad8494 36 36 s ad8495/ad8497 40 40 s ad8496 32 32 s power supply operating voltage range 6 single suppl y 2.7 36 2.7 36 v dual supply 2.7 18 2.7 18 v quiescent current 180 250 180 250 a
ad8494/ad8495/ad8496/ad8497 rev. b | page 4 of 16 a grade c grade parameter test conditions/comments min typ max min typ max unit temperature range (t a ) specified performance ad8494/ad8495 0 50 0 50 c ad8496/ad8497 25 100 25 100 c operational ?40 +125 ?40 +125 c 1 ambient temperature rejection specifies the change in the output measurement (in c) for a given change in temperature of the cold junction . f or the ad8494 and ad8495 , ambient temperature rejection is defined as the slope of the line connecting errors calculated at 0c and 50c ambient temperat ure. f or the ad849 6 and ad849 7, ambient temperature rejection is defined as the slope of the line connecting errors calculated at 25c and 100c ambient temperature . 2 error does not include thermocouple gain error or thermocouple nonlinearity. 3 with a 10 0 k load, measurement junction temperatures beyond approximately 880c for the ad8494 and ad8496 and beyond approximately 96 0c for the ad8495 and ad8497 require supply voltages larger than 5 v or a negative voltage applied to the reference pin. measureme nt junction temperatures below 5c require either a positive offset voltage applied to the reference pin or a negative supply. 4 input stage uses pnp transistors, so bias current always flows out of the part. 5 large output currents can increase the intern al temperature rise of the part and contribute to cold junction compensation (cjc) error. 6 unbalanced supplies can also be used. care should be taken that the common - mode voltage of the thermocouple stays within the input voltage range of the part.
ad8494/ad8495/ad8496/ad8497 rev. b | page 5 of 16 absolute maximum rat ings table 3 . parameter rating supply voltage 18 v maximum voltage at ?in or +in +v s C 25 v minimum voltage at ?in or +in C v s + 25 v ref voltage v s output short - circuit current duration indefinite storage temperature range ?65c to +150c operating temperature range ?4 0 c to +125c maximum ic junction temperature 140c esd human body model 4.5 kv field - induced charged device model 1.5 kv stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. expo sure to absolute maximum rating conditions for extended periods may affect device reliability. thermal resistance ja is specified for a device on a 4 - layer jedec pcb in free air. table 4 . package ja unit 8 - lead msop (rm -8) 135 c/w esd caution
ad8494/ad8495/ad8496/ad8497 rev. b | page 6 of 16 pin configuration an d function descripti ons 08529-002 ?in 1 ref 2 ?v s 3 nc 4 +in 8 +v s 7 out 6 sense 5 nc = no connect + ? ad849x t op view (not to scale) figure 2 . pin configuration table 5 . pin function descriptions pin no. mnemonic description 1 ?in negative input. 2 ref reference. this pin must be driven by low impedance. 3 ?v s negative supply. 4 nc no connect . 5 sense sense p in. i n meas urement mode , connect to output; in setpoint mode , connect to setpoint voltage . 6 out output. 7 +v s positive supply. 8 +in positive input .
ad8494/ad8495/ad8496/ad8497 rev. b | page 7 of 16 typical performance characteristics t a = 25c, + v s = 5 v, r l = , unless otherwise noted. 100 10 1 0.1 0.01 0.1 1 10 100 1k 10k 100k frequency (hz) cmrr (c/v) 08529-035 ad8495/ad8497 ad8494 ad8496 figure 3 . cmrr vs. frequency 1000 100 10 1 0 1 10 100 1k 10k 100k frequency (hz) psrr (c/v) 08529-036 ad8495/ad8497 ad8494 ad8496 figure 4 . psrr vs. frequency 50 40 30 20 10 0 ?10 ?20 100 1k 10k 100k 1m frequency (hz) gain (db) 08529-018 ad8494 ad8496 ad8495/ad8497 figure 5 . frequency response 1200 1000 800 600 400 200 0 ?200 time (50s/div) temperature reading (c) 08529-019 connected thermocouple open thermocouple thermocouple connection ad849x output figure 6 . output response to open thermocouple, ?in connected to ground t hrough a 1 m ? resistor 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 ?0.5 ?1.0 ?0.5 0.5 1.5 2.5 3.5 4.5 5.5 output voltage (v) input common-mode voltage (v) 08529-017 +0.05, +3.45 +0.05, +3.21 +4.91, +2.95 +4.91, +2.71 +0.05, ?0.36 +0.05, ?0.39 +4.91, ?0.37 +4.91, ?0.39 v ref = 0v v ref = 2.5v figure 7 . input common - mode voltage range vs. output voltage, +v s = 5 v, v ref = 0 v, and v ref = 2.5 v 35 30 25 20 15 10 5 40 0 1.75 1.50 1.25 1.00 0.75 0.50 0.25 2.00 0 ?40 ?20 0 20 40 60 80 100 120 temperature (c) input bias current (na) input offset current (na) 08529-042 i bias i os figure 8 . input bias current and input offset current vs. temperature
ad8494/ad8495/ad8496/ad8497 rev. b | page 8 of 16 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 2.00 1.50 1.00 0.50 0 ?0.50 ?1.00 ?30 ?10 ?5 0 5 15 20 25 ?25 ?20 ?15 10 30 input voltage (v) output voltage (v) input current (ma) 08529-021 v out i in figure 9 . ad8494 input overvoltage performance, + v s = 2.7 v (gain = 96.7) 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 2.00 1.50 1.00 0.50 0 ?0.50 ?1.00 ?30 ?10 ?5 0 5 15 20 25 ?25 ?20 ?15 10 30 input voltage (v) output voltage (v) input current (ma) 08529-022 v out i in figure 10 . ad8495/ad8497 input overvoltage performance, + v s = 2.7 v (gain = 122.4) 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 2.00 1.50 1.00 0.50 0 ?0.50 ?1.00 ?30 ?10 ?5 0 5 15 20 25 ?25 ?20 ?15 10 30 input voltage (v) output voltage (v) input current (ma) 08529-023 v out i in figure 11 . ad 8496 input overvoltage performance, + v s = 2.7 v gain = 90.35) 16 12 8 4 0 ?4 ?8 ?12 ?16 3.0 2.5 2.0 1.5 1.0 ?0.5 0 0.5 ?1.0 ?30 ?10 ?5 0 5 15 20 25 ?25 ?20 ?15 10 30 input voltage (v) output voltage (v) input current (ma) 08529-024 v out i in figure 12 . ad8494 input overvoltage performance, v s = 15 v (gain = 96.7) 16 12 8 4 0 ?4 ?8 ?12 ?16 3.0 2.5 2.0 1.5 1.0 ?0.5 0 0.5 ?1.0 ?30 ?10 ?5 0 5 15 20 25 ?25 ?20 ?15 10 30 input voltage (v) output voltage (v) input current (ma) 08529-025 v out i in figure 13 . ad8495/ad8497 input overvoltage performance, v s = 15 v (gain = 122.4) 16 12 8 4 0 ?4 ?8 ?12 ?16 3.0 2.5 2.0 1.5 1.0 ?0.5 0 0.5 ?1.0 ?30 ?10 ?5 0 5 15 20 25 ?25 ?20 ?15 10 30 input voltage (v) output voltage (v) input current (ma) 08529-026 v out i in figure 14 . ad8496 input overvoltage performance, v s = 15 v (gain = 90.35)
ad8494/ad8495/ad8496/ad8497 rev. b | page 9 of 16 08529-028 120s/div 20mv/div c l = 0pf c l = 1000pf c l = 4700pf c l = 10000pf figure 15 . ad8494/ad8496 small - signal response with various capacitive loads 08529-027 120s/div 20mv/div ad8494/ad8496 ad8495/ad8497 figure 16 . small - signal response, r l = 100 k ? , c l = 1 nf 08529-040 100s/div 2v/div 0.02%/div settling to 0.1% in 40s figure 17 . ad8495/ad8497 large - signal step response and settling time 08529-029 120s/div 20mv/div c l = 0pf c l = 1000pf c l = 4700pf c l = 10000pf figure 18 . ad8495/ad8497 small - signal response with various capacitive loads 08529-039 100s/div 2v/div 0.02%/div settling to 0.1% in 36s figure 19 . ad8494 large - signal step response and settling time 08529-041 100s/div 2v/div 0.02%/div settling to 0.1% in 32s figure 20 . ad8496 large - signal step response and settling time
ad8494/ad8495/ad8496/ad8497 rev. b | page 10 of 16 08529-030 1s/div 200nv/div figure 21 . 0.1 hz to 10 hz rti voltage noise 5 4 3 2 1 0 ?1 ?2 ?3 ?4 ?5 1k 10k 100k load resistance ( ?) output voltage swing (v) 08529-033 (+) ?40c (+) +25c (+) +85c (+) +125c (?) ?40c (?) +25c (?) +85c (?) +125c figure 22 . output voltage swing vs. load resistance, v s = 5 v 100 90 80 70 60 50 40 30 20 10 1 10 100 1k 10k 100k frequency (hz) noise (nv/ hz) 08529-031 figure 23 . voltage noise spectral density vs. frequency 08529-032 time (1.5ms/div) output voltage (50mv/div) supply voltage (1.25v/div) output voltage 5v power-up figure 24 . output voltage start - up +v s ?0.4 ?0.8 ?1.2 +1.2 +0.8 +0.4 ?v s 10 100 1m 5m output current (a) output voltage swing (v) referred to supply voltages (v s = 5v) 08529-034 (+) ?40c (+) +25c (+) +85c (+) +125c (?) ?40c (?) +25c (?) +85c (?) +125c figure 25 . out put voltage swing vs. output current, v s = 5 v
ad8494/ad8495/ad8496/ad8497 rev. b | page 11 of 16 theory of operation thermocouple s a thermocouple is a rugged, low cost temperature transducer whose output is proportional to the temperature difference between a measurement junction and a reference jun ction. it has a very wide temperature range. its low level output (typically tens of microvolts per c ) requires amplification. variation in the reference junction temperature results in measurement error unless the thermocouple signal is properly compensa ted. a thermocouple consists of two dissimilar metals. these metals are connected at one end to form the measurement junction, also called the hot junction. the other end of the thermocouple is connected to the metal lines that lead to the measurement elec tronics. this connection forms a second junction: the reference junction, also called the cold junction. 08529-004 ad849x pcb traces reference junction measurement junction thermocouple wires figure 26 . thermocouple junctions t o derive the temperature at the measurement junction (t mj ) , the user must know the differ ential voltage created by the thermo - couple . the user must also know the error voltage generated by the temperature at the reference junction (t rj ) . compensating for the reference junction error voltage is typically called cold junction compensation . t he e lectronics must compensate for any changes in temperature at the reference (cold) junction so that the output voltage is an accurate representation of the hot junction measurement . thermocouple signal conditioner the ad8494/ad8495/ad8496/ad8497 thermocoupl e amplifiers provide a simple, low cost solution for measuring thermocouple temperature s. the se amplifiers simplify many of the difficulties of measuring thermocouples. an integrated temperature sensor performs cold junction compensation. a fixed - gain inst rumen tation amplifier amplifies the small thermocouple voltage to provide a 5 mv/c output. the high common - mode rejection of the amplifier blocks common - mode noise that the long thermocouple leads can pick up. for additional protection, the high impedance inputs of the amplifier make it easy to add extra filtering. table 6 shows an example of a j type thermocouple voltage for various combinations of 0c and 50c on the reference and measurement junctions. table 6 also shows the performance of the ad849 4 amplifying the thermocouple voltage and compensating for the reference junction temperature changes , thus eliminating the error . table 6 . j type thermocouple voltages and ad84 9 4 readings measurement junction temperature (t mj ) reference junction temperature (t rj ) thermocouple voltage ad849 4 reading 50c 0c + 2. 585 mv 250 mv 50c 50c 0 mv 250 mv 0c 0c 0 mv 0 mv 0c 50c ? 2. 585 mv 0 mv ad849 4/ad8495/ad8496/ad84 97 architec ture figure 27 shows a block diagram of the ad849x circuitry. the ad849x consists of a low offset , fixed - gain instrumentation amplifier and a temperature sensor. ?in +in out ref 08529-020 a1 a2 ad8494/ad8495/ ad8496/ad8497 a3 sense esd and ovp esd and ovp cold junction compensation 1m ? thermo- couple figure 27 . block diagram the ad849x outpu t is a voltage that is proportional to the tem - perature at the measurement junction of the thermocouple (t mj ). to derive the measured temperature from the ad849x output voltage, use the following transfer function: t mj = ( v out ? v ref )/ ( 5 mv/c ) an ideal ad849x achieves this output with an error of less than 2c, within the specified operating ranges listed in table 7 . instrumentation amplifier a thermocouple signal is so small that considerable gain is required before it can be sampled properly by most adcs. the ad849x has an instrumentation amplifier with a fixed gain that generates an output voltage of 5 mv/c for j type and k type thermocouples. v out = ( t mj 5 mv/c) + v ref to accommodate the nonli near behavior of the thermocouple, each amplifier has a different gain so that the 5 mv/c is accu - rately maintained for a given temperature measurement range. ? the ad8494 and ad8496 (j type) have an instrumentation amplifier with a gain of 96.7 and 90.35, respectively. ? the ad8495 and ad8497 (k type) have an instrumentation amplifier with a gain of 122.4.
ad8494/ad8495/ad8496/ad8497 rev. b | page 12 of 16 the small thermocouple voltages mean that signals are quite vulnerable to interference, especially when measured with single - ended amplifiers. the ad849x addresses this issue in several ways. low input bias currents and high input impedance allow for easy filtering at the inputs. the excellent common - mode rejection of the ad849x prevents variations in ground potential and other common - mode noise from affect ing the measurement. temperature sensor ( cold junction compensation ) the ad849x also includes a temperature sensor for cold junc - tion compensation. this temperature sensor is used to measure the reference junction temperature of the thermocouple and to can cel its effect. ? the ad8494/ad8495 cold junction compensation is optimized for operation in a lab environment, where the ambient temperature is around 25c . the ad8494/ad8495 are specified for an ambient range of 0c to 50c . ? the ad8496/ad8497 cold junction compensation is optimized for operation in a less controlled environment , where the temperature is around 60c . the ad8496/ad8497 are specified for an ambient range of 25c to 100c. application examples for the ad8496/ad8497 include automotive applicatio ns, autoclave, and ovens. thermocouple break detection the ad849x offers open thermocouple detection. the inputs of the ad849x are pnp type transistors, which means that the bias current always flows out of the inputs. t herefore, t he input bias current dr ives any unconnected input high , which rails the output. connecting the negative input to ground through a 1 m ? resistor causes the ad849x output to rail high in an open thermocouple condition (see figure 6 , figure 28 , and the ground connection section). 08529-008 1m ? figure 28 . ground the negative input through a 1 m ? resistor for open thermocouple detection input voltage protection the ad849x has very robust inputs. input voltages can be up to 25 v from the opposite supply rail. for example, with a +5 v positive supply and a ?3 v negative supply, the part can safely withstand voltages at the inputs from ?20 v to +22 v. voltage s at the reference and sense pins should not go beyond 0.3 v of the supply rails. maximum error calcul ation as is normally the case, the ad849x outputs are subject to calibration, gain, and temperature sensitivity errors. the user can calculate the maximum error from the ad849x using the fol lowing information. the five primary sources of ad849x error are described in this section. ad849x i nitial calibration accuracy error at the initial calibration point can be easily calibrated out with a one - point temperature calibration. see table 2 for the specifications. ad849x ambient temperature rejection the specified ambient temperature rejection represents the ability of the ad849x to reject errors caused by changes in the ambient temperature/reference junction. for exam ple, with 0.025c/c ambient temperature rejection, a 20c change in the reference junction temperature adds less than 0.5c error to the measurement. see table 2 for the specifications. ad849x gain error gain error is the amo unt of additional error when measuring away from the measurement junction calibration point. for example, if the part is calibrated at 25c and the measurement junction is 100c with a gain error of 0.1%, the gain error contribution is ( 100c ? 25c ) (0. 1%) = 0 .075 c. this error can be calibrated out with a two - point calibration if needed, but it is usually small enough to ignore . see table 2 for the specifications. manufacturing tolerances of the thermocouple consult the dat a sheet for your thermocouple to find the specified tolerance of the thermocouple. linearity error of the thermocouple each part in the ad849x family is precision trimmed to optimize a linear operating range for a specific thermocouple type and for the wid est possible measurement and ambient temperature ranges. the ad849x achieves a linearity error of less than 2c, within the specified operating ranges listed in table 7 . this error is due only to the nonlinearity of the therm ocouple. table 7 . ad849x 2c accuracy temperature ranges part thermo - couple type max error ambient temperature range measurement temperature range ad8494 j 2c 0c to 50c ?35c to + 95c ad8495 k 2c 0c to 50c ?25c to +400c ad8496 j 2c 25c to 100c +55c to +565c ad8497 k 2c 25c to 100c ? 25 c to +295c for temperature ranges outside those listed in table 7 or for instructions on how to correct for thermocouple nonlinearity error with software, see the an - 1087 application note for additional details .
ad8494/ad8495/ad8496/ad8497 rev. b | page 13 of 16 recommendations for best circuit performance input filter a low - pass filter before the input of the ad849x is strongly recommended ( see figure 29 ), especially when operating in an electrically noisy environment. long thermocouple leads can function as an excellent antenna and pick up many unwanted signals. the filt er should be set to a low corner frequency that still allows the input signal to pass through undiminished. the primary purpose of the filter is to remove rf signals, which, if allowed to reach the ad849x, can be rectified and appear as temperature fluct uations. 08529-011 r r ad849x c d c c c c 1m ? connect when thermocouple ti p type is unknown filter frequency diff = 1 2r(2c d + c c ) filter frequency cm = where c d 10c c 1 2rc c figure 29 . filter for any thermocouple style to prevent input offset currents from affect ing the measurement accu racy, the filter resistor value s should be less than 50 k . ground connection it is always recommended that the thermocouple be connected to ground through a 100 k to 1 m resistor placed at the negative (inverting) input of the amplifier on the pcb (see fi gure 30 ). this solution works well regardless of the therm o - couple tip style. 08529-038 1m ? figure 30 . ground the thermocouple with a 1 m ? resistor if there is no electrical connection at the measurement junction (insulated tip), the resistor value is small enough that no mean - ingful common - mode voltage is generated. if there is an electrical connection through a grounded or exposed tip, the resistor value is large enough that any current from the measurement tip to ground is very small, preve nting measurement errors. the ad849x inputs require only one ground connection or source of common - mode voltage. any additional ground connection is detrimental to performance because ground loops can form through the thermocouple, easily swamping the smal l thermocouple signal. grounding the thermocouple through a resistor as recommended prevents such problems. keeping the ad849x at the same temperature as the reference junction the ad849x compensates for thermocouple reference junction temperature by usi ng an internal temperature sensor. it is critical to keep the reference junction (thermocouple - to - pcb connection) as close to the ad849x as possible. any differ ence in temperature between the ad849x and the reference junction appears directly as temperatur e error. temperature difference between the device and the reference junction may occur if the ad849x is not physically close to the reference junction or if the ad849x is required to supply large amounts of output power . 08529-010 ad849x pcb traces keep traces short keep junction and ad849x at same temperature measurement junction reference junction thermocouple wires figure 31 . compensating for thermocouple reference junction temperature driving the reference pin the ad849 x comes with a reference pin , which can be used to offset the output voltage. this is particularly useful when reading a negative temperature in a single - s upply system. incorrect v correct ad849x ad8613 + ? v 08529-006 ref ad849x ref figure 32 . driving the reference pin for best performance, the reference pin should be driven with a low output impedance source, not a resistor divider. the ad8613 and the op777 are good choices for the buffer amplifier. debugging tip if the ad849x is not providing the expected performance, a useful debugging step is to implement the ambient temperature configuration in figure 34 . if the ambient temperature sensor does not work as expected, the problem is likely with the ad849x or with the downstream circuitry. if the ambient temperature sensor configuration is working correctly, the problem typically lies with how the thermocoup le is connected to the ad849x. c ommon error s include an incorrect grounding configuration or lack of filtering.
ad8494/ad8495/ad8496/ad8497 rev. b | page 14 of 16 applications informa tion basic c onnection figure 33 shows an example of a basic connect ion for the ad849 x , with a j type or k type thermocouple input . ad849x out sense ref ?v s +v s 0.1f 10f 5v 08529-012 cold junction compens a tion 2 3 7 5 +in thermo- couple ?in 1m ? 8 1 0.1f 10f 6 in-am p figure 33 . basic connection for the ad849x to measure negative temperatures, apply a voltage at the refer - ence pin to offset the output voltage at 0c. t he output vo ltage of the ad849x is v out = ( t mj 5 mv/c) + v ref a filter at the input is recommended to remove high frequency noise . the 1 m resistor to ground enables open thermocouple detection and proper grounding of the thermocouple. the sense pin should be connected to the output pin of the ad849x. decoupling capacitors should be used to ensure clean power supply voltages on +v s and, if using dual supplies, on ?v s , also . a 0.1 f capacitor should be placed as close as possible to each ad849x supply pin. a 10 f tantalum capacitor can be used farther away from the part and can be shared . ambient temperature sensor the ad849 x can be confi gured as a standalone celsius thermo - meter with a 5 mv/c output , as shown in figure 34. the thermocouple sensing functionality is disabled by shorting both ad849 x inputs to ground ; the ad849 x simpl y outputs the value from the on - board temperature sensor. as a tempe rature sensor, the ad849 4 has a measurement temp - erature range of ?40c to +125c with a precision output of v out = t a 5 mv/c in-amp ad849x +in ?in out sense ref ?v s +v s 5v 08529-013 cold junction compensation 8 2 3 5 7 1 6 figure 34 . ambient temperature sensor the ad8494 is the best choice for use as an ambient temper - ature sensor. the ad8495, ad8496, and ad8497 can also be configured a s ambient temperature sensors, but their output transfer functions are not precisely 5 mv/c. for information about the exact transfer functions of the ad8494/ad8495/ ad8496/ad8497, see the an - 1087 application n ote for additional details. the thermometer mode can be particularly useful for debugging a misbehaving circuit. if the basic connection is not working, disconnect the thermocouple and short both inputs to ground. if the system reads the ambient temperatur e correctly, the problem is related to the thermocouple. if the system does not read the ambient temperature correctly, the problem is with the ad849x or with the downstream circuitry.
ad8494/ad8495/ad8496/ad8497 rev. b | page 15 of 16 setpoint controller the ad84 9x can be used as a temperatur e setpoint controller, with a thermocouple input from a remote location or with the ad849x itself being used as a temperature sensor. when the measured temperature is below the setpoint temperature, the output volt age goes to ?v s . when the measured temperature is above the setpoint temperature, the output voltage goes to +v s . for best accuracy and cmrr performance, the setpoint voltage should be created with a low impedance source. if the setpoint voltage is generat ed with a voltage divider, a buffer is recommended. ad849x out sense setpoint volt age ref ?v s +v s 5v 08529-014 cold junction compens a tion 2 3 5 7 6 +in thermo- couple ?in 1m ? 8 1 in-am p figure 35 . setpoint controller hysteresis can be added to the setpoint contro ller by using a resistor divider from the output to the reference pin, as shown in figure 36 . the hysteresis in c is c mv/ 5 ) /( + = r2 r1 r1 v t s hyst ad849x out sense setpoint voltage r2 100k? r1 1k? r1 1k? ref ?v s +v s 5v 08529-015 cold junction compensation 2 3 7 6 5 +in thermo- couple ?in 1m ? 8 1 in-am p figure 36 . adding 10 degrees of hysteresis a resistor equivalent to the output resistance of the divider should be connected to the sense pin to ensure good cmrr. measuring negative t emperatures the ad849x can measure negative temperatures on dual supplies and on a single supply. when operating on dual supplies with the reference pin grounded, a negative output voltage indicates a negative temperature at the thermocouple measurement junction. v out = ( t mj 5 mv/c) + v ref when operating the ad849x on a single supply, level - shift the output by applying a positive voltage (less than +v s ) on the reference pin. an output voltage less than v ref indicates a negat ive temperature at the thermocouple measurement junction. reference pin allows offset adjustment the reference pin can be used to level - shift the ad849x output voltage. this is useful for measuring negative temperatures on a single supply and to match the ad849x output voltage range to the input voltage range of the subsequent electronics in the signal chain. the reference pin can also be used to offset any initial calibra - tion errors. apply a small reference voltage proportional to the error to nullify the effect of the calibration error on the output.
ad8494/ad8495/ad8496/ad8497 rev. b | page 16 of 16 outline dimensions complian t to jedec standa rds mo-187-aa 6 0 0.80 0.55 0.40 4 8 1 5 0.65 bsc 0. 40 0.25 1.10 max 3.20 3.00 2.80 copla narity 0. 10 0.23 0.09 3.20 3.00 2.80 5.15 4.90 4.65 pin 1 ident ifier 15 max 0.95 0.85 0.75 0.15 0.05 10-07- 2009-b figure 37 . 8 - lead mini small outline package [msop] (rm - 8) dimensions shown in millimeters ordering guide model 1 temperature range package description packag e option branding ad8494armz ?4 0c to +12 5c 8 - lead msop rm - 8 y36 ad 8494 armz -r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y36 AD8494CRMZ ?4 0c to +12 5c 8 - lead msop rm - 8 y37 ad 8494 crmz - r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y37 ad8495armz ?4 0c to +12 5c 8 - lead msop rm - 8 y33 ad 8495 armz -r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y33 ad8495crmz ?4 0c to +12 5c 8 - lead msop rm - 8 y34 ad 8495 crmz - r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y34 ad8496armz ?4 0c to +12 5c 8 - lead msop rm - 8 y3c ad 8496 armz -r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y3c ad8496crmz ?4 0c to +12 5c 8 - lead msop rm - 8 y3d ad 8496 crmz - r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y3d ad8497armz ?4 0c to +12 5c 8 - lead m sop rm - 8 y39 ad 8497 armz -r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y39 ad8497crmz ?4 0c to +12 5c 8 - lead msop rm - 8 y3a ad 8497 crmz - r7 ?4 0c to +12 5c 8 - lead msop, 7 tape and reel rm - 8 y3a 1 z = rohs compliant part. ? 2010 C 2011 an alog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. d08529 - 0- 4/11(b)

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