APPLICATIO S BLOCK DIAGRA TYPICAL APPLICATIO. LT1025 Micropower Thermocouple Cold Junction Compensator FEATURES DESCRIPTIO

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1 Micropower Thermocouple Cold Junction Compensator FEATRES 8µA Supply Current V to 36V Operation.5 C Initial Accuracy (A Version) Compatible with Standard Thermocouples (E, J, K, R, S, T) Auxiliary Output Available in 8-Lead PDIP and SO Packages APPLICATIO S Thermocouple Cold Junction Compensator Centigrade Thermometer Temperature Compensation Network BLOCK DIAGRA W DESCRIPTIO The LT 15 is a micropower thermocouple cold junction compensator for use with type E, J, K, R, S, and T thermocouples. It utilizes wafer level and post-package trimming to achieve.5 C initial accuracy. Special curvature correction circuitry is used to match the bow found in all thermocouples so that accurate cold junction compensation is maintained over a wider temperature range. The will operate with a supply voltage from V to 36V. Typical supply current is 8µA, resulting in less than.1 C internal temperature rise for supply voltages under 1V. A output is available at low impedance, in addition to the direct thermocouple voltages of 6.9µV/ C (E), 51.7µV/ C (J),.3µV/ C (K, T) and 5.95µV/ C (R, S). All outputs are essentially independent of power supply voltage. A special kit is available (LTK1) which contains an and a custom tailored thermocouple amplifier. The amplifier and compensator are matched to allow a much tighter specification of temperature error than would be obtained by adding the compensator and amplifier errors on a worst-case basis. The amplifier from this kit is available separately as LTKAx. The is available in either an 8-pin PDIP or 8-pin SO package for temperatures between C and 7 C., LTC and LT are registered trademarks of Linear Technology Corporation. TYPICAL APPLICATIO Type K Thermometer BOW* CORRECTION VOLTAGE TEMPERATRE SENSOR *CORRECTS FOR BOW IN COLD JNCTION, NOT IN PROBE (HOT JNCTION) BFFER GND E 6.9µV/ C V R COMMON J 51.7µV/ C K,T.6µV/ C R, S 6µV/ C BD1 V K R 1Ω FLL-SCALE TRIM 1k V C.1µF LTKAx R3** 55k V OT V V *R 3µA, R IS NOT REQIRED TYPE K GND R V (OPEN) FOR TEMPERATRES C O C1 R*.1µF **SELECTED FOR C TO 1 C RANGE OR EQIVALENT. SEE V AMPLIFIER CONSIDERATIONS TA1 1

2 ABSOLTE AXI RATI GS W W W (Note 1) Input Supply Voltage... 36V Output Voltage (Forced)... 5V Output Short-Circuit Duration... Indefinite Operating Temperature Range AC, C... C to 7 C AM, M C to 15 C Storage Temperature Range C to 15 C W PACKAGE/ORDER I FOR ATIO E 6.9µV/ C V O GND 1 3 N8 PACKAGE 8-LEAD PDIP TOP VIEW S8 PACKAGE 8-LEAD PLASTIC SO T JMAX = 15 C, θ JA = 13 C/W (N8) T JMAX = 15 C, θ JA = 19 C/W (S8) J8 PACKAGE 8-LEAD CERDIP T JMAX = 15 C, θ JA = 1 C/W J 51.7µV/ C K, T.6µV/ C R, S 6µV/ C R COMMON ORDER PART NMBER ACN8 CN8 CS8 S8 PART MARKING 15 AMJ8 MJ8 OBSOLETE PACKAGE Consider the N8 Package for Alternate Source Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The denotes the specifications which apply over the full operating temperature range, otherwise specificatons are at T A = 5 C. V S = 5V, Pin 5 tied to Pin, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX NITS Temperature Error at T J = 5 C Output (Notes, 5) A.3.5 C.5. C Full Temperature Span See Curve Resistor Divider Accuracy V OT = (Notes, ) A E µv/ C J µv/ C K, T µv/ C R, S µv/ C E µv/ C J µv/ C K, T µv/ C R, S µv/ C Supply Current V 36V µa AC, C 5 15 µa AM, M µa Line Regulation (Note 3) V 36V.3. C/V Load Regulation (Note 3) I O 1mA.. C Divider Impedance E.5 kω J.1 kω K, T. kω R, S 3.8 kω Change in Supply Current V 36V.1.5 µa/v Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note : Divider accuracy is measured by applying a 1.V signal to the output divider and measuring the individual outputs. Note 3: Regulation does not include the effects of self-heating. See Internal Temperature Rise in Application Guide. Load regulation is 3µA I O 1mA for T A C. Note : To calculate total temperature error at individual thermocouple outputs, add output error to the resistor divider error. Total error for type K output at 5 C with an A is.5 C plus (.µv/ C)(5 C)/ (.6µV/ C) =.5 C.5 C =.75 C. Note 5: Temperature error is defined as the deviation from the following formula: V OT = 1mV(T) (1mV)( )(T 5 C). The second term is a built-in nonlinearity designed to help compensate the nonlinearity of the cold junction. This bow is.3 C for a 5 C temperature change.

3 TYPICAL PERFOR A CE CHARACTERISTICS W TEMPERATRE ERROR ( C) Output Temperature Error GARANTEED LIMITS* JNCTION TEMPERATRE ( C) TEMPERATRE ERROR ( C) Output Temperature Error A 5 5 GARANTEED LIMITS* A JNCTION TEMPERATRE ( C) CRRENT (µa) Supply Current DOES NOT INCLDE 3µA PLL-DOWN CRRENT REQIRED FOR TEMPERATRES BELOW C T J = 15 C T J = 5 C T J = 55 C PIN TIED TO PIN SPPLY VOLTAGE (V) G1 G G3 *ERROR CRVE FACTORS IN THE NONLINEARITY TERM BILT IN TO THE. SEE THEORY OF OPERATION IN APPLICATION GIDE SECTION *ERROR CRVE FACTORS IN THE NONLINEARITY TERM BILT IN TO THE. SEE THEORY OF OPERATION IN APPLICATION GIDE SECTION APPLICATIO S I FOR ATIO W The was designed to be extremely easy to use, but the following ideas and suggestions should be helpful in obtaining the best possible performance and versatility from this new cold junction compensator. Theory of Operation A thermocouple consists of two dissimilar metals joined together. A voltage (Seebeck EMF) will be generated if the two ends of the thermocouple are at different temperatures. In Figure 1, iron and constantan are joined at the temperature measuring point T1. Two additional thermocouple junctions are formed where the iron and constantan connect to ordinary copper wire. For the purposes of this discussion it is assumed that these two junctions are at the same temperature, T. The Seebeck voltage, V S, is the product of the Seebeck coefficient α, and the temperature difference, T1 T; V S = α (T1 T). The junctions at T are commonly called the cold junction because a common practice is to immerse the T junction in C ice/water slurry to make T independent of room temperature variations. Thermocouple tables are based on a cold-junction temperature of C. To date, IC manufacturers efforts to make microminiature thermos bottles have not been totally successful. Therefore, an electronically simulated cold-junction is required for most applications. The idea is basically to add a temperature dependent voltage to V S such that the voltage sum is the same as if the T junction were at a constant C instead of at room temperature. This voltage source is called a cold junction compensator. Its output is designed to be V at C and have a slope equal to the Seebeck coefficient over the expected range of T temperatures. TEMPERATRE T1 TO BE MEASRED Fe CONSTANTAN T Figure 1 Cu } V S Cu MST BE LOCATED NEXT TO COLD JNCTION FOR TEMPERATRE TRACKING AG1 To operate properly, a cold junction compensator must be at exactly the same temperature as the cold junction of the thermocouple (T). Therefore, it is important to locate the physically close to the cold junction with local temperature gradients minimized. If this is not possible, 3

4 APPLICATIO S I FOR ATIO an extender made of matching thermocouple wire can be used. This shifts the cold junction from the user termination to the end of the extender so that the can be located remotely from the user termination as shown in Figure. The four thermocouple outputs on the are 6.9µV/ C (E), 51.7µV/ C (J),.6µV/ C (K and T), and 6µV/ C (R and S). These particular coefficients are chosen to match the room temperature (5 C) slope of the thermocouples. Over wide temperature ranges, however, the slope of thermocouples changes, yielding a quasiparabolic error compared to a constant slope. The outputs have a deliberate parabolic bow to help compensate for this effect. The outputs can be mathematically described as the sum of a linear term equal to room temperature slope plus a quadratic term proportional to temperature deviation from 5 C squared. The coefficient (ß) of the quadratic term is a compromise value chosen to offer improvement in all the outputs. V OT = αt αß(t 5 ) HOT JNCTION ß Fe CN FRONT PANEL CONNECTOR W Fe CN EXTENDER Figure The actual ß term which would be required to best compensate each thermocouple type in the temperature range of C to 5 C is: E, ; J,.8 1 ; K,.3 1 ; R, , S, ; T, The temperature error specification for the output (shown as a graph) assumes a ß of For example, an is considered perfect if its output fits the equation V O = 1mV(T) (1mV)(5.5 1 )(T 5 C). Operating at Negative Temperatures The is designed to operate with a single positive supply. It therefore cannot deliver proper outputs for Cu AMPLIFIER Cu NEW COLD JNCTION AG temperatures below zero unless an external pull-down resistor is added to the V O output. This resistor can be connected to any convenient negative supply. It should be selected to sink at least 3µA of current. Suggested value for a 5V supply is 15kΩ, and for a 15V supply, 7kΩ. Smaller resistors must be used if an external load is connected to the output. The can source up to 1mA of current, but there is a trade-off with internal temperature rise. Internal Temperature Rise The is specified for temperature accuracy assuming no internal temperature rise. At low supply voltages this rise is usually negligible (.5 C at 5V), but at higher supply voltages or with external loads or pull-down current, internal rise could become significant. This effect can be calculated from a simple thermal formula, T = (θ JA ) (V )(I Q I L ), where θ JA is thermal resistance from junction to ambient, ( 13 C/W), V is the supply voltage, I Q is the supply current ( 8µA) and I L is the total load current including actual load to ground and any pulldown current needed to generate negative outputs. A sample calculation with a 15V supply and 5µA pull-down current would yield, (13 C/W) (15V) (8µA 5µA) =.3 C. This is a significant rise in some applications. It can be reduced by lowering supply voltage (a simple fix is to insert a 1V zener in the lead) or the system can be calibrated and specified after an initial warm-up period of several minutes. Driving External Capacitance The direct thermocouple drive pins on the (J, K, etc.) can be loaded with as much capacitance as desired, but the output should not be loaded with more than 5pF unless external pull-down current is added, or a compensation network is used. Thermocouple Effects in Leads Thermocouple voltages are generated whenever dissimilar materials are joined. This includes the leads of IC packages, which may be kovar in TO-5 cans, alloy or copper in dual-in-line packages, and a variety of other materials in plating finishes and solders. The net effect of these thermocouples is zero if all are at exactly the

5 APPLICATIO S I FOR ATIO same temperature, but temperature gradients exist within IC packages and across PC boards whenever power is dissipated. For this reason, extreme care must be used to ensure that no temperature gradients exist in the vicinity of the thermocouple terminations, the, or the thermocouple amplifier. If a gradient cannot be eliminated, leads should be positioned isothermally, especially the R and appropriate output pins, the amplifier input pins, and the gain setting resistor leads. An effect to watch for is amplifier offset voltage warm-up drift caused by mismatched thermocouple materials in the wire-bond/ lead system of the IC package. This effect can be as high as tens of microvolts in TO-5 cans with kovar leads. It has nothing to do with the actual offset drift specification of the amplifier and can occur in amplifiers with measured zero drift. Warm-up drift is directly proportional to amplifier power dissipation. It can be minimized by avoiding TO-5 cans, using low supply current amplifiers, and by using the lowest possible supply voltages. Finally, it can be accommodated by calibrating and specifying the system after a five minute warm-up period. Reversing the Polarity of the Output The can be made to stand on its head to achieve a minus output point. This is done as shown in Figure 3. The normal output (V O ) is grounded and feedback is established between the ground pin and the positive supply pin by feeding both of them with currents while coupling them with a 6V zener. The ground pin will D1 V Z 6V W I I GND V (15V) 7k V (15V) R 15k I L V O Figure 3 R L V OT AG3 now be forced by feedback to generate as long as the grounded output is supplying a net source current into ground. This condition is satisfied by selecting such that the current through (I ) is more than the sum of the supply current, the maximum load current (I L ), and the minimum zener current ( 5µA). R is then selected to supply more current than I. V V VZ ( 6V) =, R = 3 µ A I V / 8µ A L For ±15V supplies, with I L = µa maximum, = 7k and R = 15k. Amplifier Considerations Thermocouple amplifiers need very low offset voltage and drift, and fairly low bias current if an input filter is used. The best precision bipolar amplifiers should be used for type J, K, E, and T thermocouples which have Seebeck coefficients of µv/ C to 6µV/ C. In particularly critical applications or for R and S thermocouples (6µV/ C to 15µV/ C), a chopper-stabilized amplifier is required. Linear Technology offers three amplifiers specifically tailored for thermocouple applications. The LTKAx is a bipolar design with extremely low offset (< 35µV), low drift (<1.5µV/ C), very low bias current (<1nA), and almost negligible warm-up drift (supply current is µa). It is very cost effective even when compared with jellybean op amps with vastly inferior specifications. For the most demanding applications, the LTC15 and LTC15 CMOS chopper-stabilized amplifiers offer 5µV offset and.5µv/ C drift (even over the full military temperature range). Input bias current is 3pA, and gain is typically 3 million. These amplifiers should be used for R and S thermocouples, especially if no offset adjustments can be tolerated, or a large ambient temperature swing is expected. Regardless of amplifier type, it is suggested that for best possible performance, dual-in-line (DIP) packages be used to avoid thermocouple effects in the kovar leads of TO-5 metal can packages if amplifier supply current exceeds 5µA. These leads can generate both DC and AC offset terms in the presence of thermal gradients in the package and/or external air motion. 5

6 APPLICATIO S I FOR ATIO In many situations, thermocouples are used in high noise environments, and some sort of input filter is required. (See discussion of input filters). To reject 6Hz pick-up with reasonable capacitor values, input resistors in the 1k-1k range are needed. nder these conditions, bias current for the amplifier needs to be less than 1nA to avoid offset and drift effects. To avoid gain error, high open loop gain is necessary for single-stage thermocouple amplifiers with or higher outputs. A type K amplifier, for instance, with 1mV/ C output, needs a closed loop gain of,5. An ordinary op amp with a minimum open loop of 5, would have an initial gain error of (,5)/(5,) = 5%! Although closed loop gain is commonly trimmed, temperature drift of open loop gain will have a very deleterious effect on output accuracy. Minimum suggested open loop gain for type E, J, K, and T thermocouples is 5,. This gain is adequate for type R and S if output scaling is or less. Suggested Amplifier Types SPPLY VOLTAGE THERMOCOPLE ±15V ±5V SINGLE SPPLY E, J, K, T LTKAx LTKAx LTC15 LT11 LT11 LTC15 LT11 LT11 LT16 LTC15 LTC15 LT16 R, S LTKAx LTC15 LTC15 LT11 LTC15 LTC15 LTKAx LT16 Thermocouple Nonlinearities Thermocouples are linear over relatively limited temperature spans if accuracies of better than C are needed. The graph in Figure shows thermocouple nonlinearity for the temperature range of C to C. Nonlinearities can be dealt with in hardware by using offsets, breakpoints, or power series generators. Software solutions include look-up tables, power series expansions, and piece-wise approximations. For tables and power series coefficients, the reader is referred to the ASTM Publication 7A. Hardware correction for nonlinearity can be as simple as an offset term. This is shown in Figure 5. The thermocouple shown in the figure has an increasing slope (α) with 6 W ERROR TYPE E AND T ( C) TEMPERATRE ( C) Figure. Thermocouple Nonlinearity, C to C OTPT (V) V H V L J SCALE K SCALE SCALE E SCALE T ERROR BEFORE OFFSETTING ERROR AFTER OFFSETTING OFFSET AMPLIFIER SIMPLE AMPLIFIER Figure 5. Offset Curve Fitting G THERMOCOPLE T L T1/6 T M T5/6 T H TEMPERATRE ( C) G5 temperature. The temperature range of interest is between T L and T H, with a calibration point at T M. If a simple amplifier is used and calibrated at T M, the output will be very high at T L and very low at T H. Adding the proper offset term and calibrating at T1/6 or T5/6 can significantly reduce errors. The technique is as follows: 1. Calculate amplifier gain: G = (SF) (T H T L )/(V H V L ) SF = Output scale factor, e.g., V H = Thermocouple output at T H V L = Thermocouple output at T L. se precision resistors to set gain or calibrate gain by introducing a precision delta input voltage and trimming for proper delta output ERROR TYPE J AND K ( C)

7 APPLICATIO S I FOR ATIO W 3. Calibrate output by adding in a true offset term which does not affect gain (by summing, etc.). Calibration may be done at any temperature either by immersing the thermocouple in a calibrated bath or by substituting a precision input voltage. The method which tends to minimize worst-case error over the whole T L to T H range is to calibrate at 1/6 or 5/6 of span. This may be modified if best accuracy is desired at one particular point. Breakpoint correction for nonlinearity is more complicated than a simple offset, but a single breakpoint combined with offset will reduce errors typically by :1 over a simple offset technique. TYPICAL APPLICATIO S Eliminating Amplifier Feedback Resistors (Output Goes Negative with Increasing Temperature) 15V LTKAx 3k 3 C 1µF 15V TYPE K C1.1µF 15V 7 K V O GND R 3k 15V 6 V OT OR EQIVALENT. SEE AMPLIFIER CONSIDERATIONS Type K Thermometer with Grounded Thermocouple R 1Ω R3 55k V 1k TYPE K R5 1k C 1µF R6 9.1k.1µF V LTKAx V V O GND R V OT R* 7k V 15V V *R IF OTPT MST SINK CRRENT, R 3µA MST BE DECREASED APPROPRIATELY. R IS NOT REQIRED (OPEN) FOR TEMPERATRES C WHEN SORCING CRRENT ONLY OR EQIVALENT. SEE AMPLIFIER CONSIDERATIONS 7

8 TYPICAL APPLICATIO S Differential Thermocouple Amplifier C1* TYPE K V CM GND 1k. R 1k. R 1M 5V**. V O R5 3k V 15V R9 1k R3 1M. V LTKAx C* V = (V )(1k) V OT (MAX) R6 7.5k R8 5k V OT R7 5 FLL-SCALE TRIM *C1 AND C FILTER RIPPLE AND NOISE, BT WILL LIMIT AC COMMON-MODE REJECTION IF NOT MATCHED. SGGESTED VALES ARE.1µF TO.1µF **SE LOWEST POSSIBLE SPPLY VOLTAGE TO MINIMIZE INTERNAL TEMPERATRE RISE FOR BEST ACCRACY, THERMOCOPLE RESISTANCE SHOLD BE LESS THAN 1Ω OR EQIVALENT. SEE AMPLIFIER CONSIDERATIONS tilizing Negative Drive to Accommodate Grounded Thermocouple* 15V R7 15k 6V R5 7k V O GND R 11k 15V R6 1k TYPE E 1µF 15V LTKAx 15V 1k C1.1µF R 1Ω R3 11k V OT C TO 5 C *SEE REVERSING THE POLARITY OF THE OTPT OR EQIVALENT. SEE AMPLIFIER CONSIDERATIONS 8

9 TYPICAL APPLICATIO S Type S Thermocouple Amplifier with ltralow Offset and Drift R** 1Ω FLL-SCALE TRIM R3 99k V V R,S V O GND R TYPE S R7 75k 1µF R6 1k 1k R 1.37M.1µF 5 3 R5* 1k OFFSET TRIM R.7k.1µF 15V 7 LT15 V µF V OT 8 C TO 1 C 15V LT19.5V TA6 * TRIM R5 FOR V OT = 1.669V AT =.mv (INPT OF AMPLIFIER GRONDED) ** TRIM R FOR V OT = 9.998V AT T = 1 C, OR FOR AT INPT OF AMPLIFIER = 9.585mV THIS AMPLIFIER HAS A DELIBERATE OFFSET TO ALLOW OTPT SLOPE () TO BE SET INDEPENDENTLY FROM AN ARBITRARY HIGH TEMPERATRE CENTER POINT (1 C). THIS IS REQIRED BECASE THE SLOPE OF TYPE S THERMOCOPLES VARIES RAPIDLY WITH TEMPERATRE, INCREASING FROM 6µV/ C AT 5 C to 11µV/ C AT 1 C. NONLINEARITY LIMITS ACCRACY TO 3 C OVER THE 8 C TO 1 C RANGE EVEN WITH OFFSET CORRECTION 9

10 PACKAGE DESCRIPTIO J8 Package 8-Lead CERDIP (Narrow.3, Hermetic) (LTC DWG # ).5.68 ( ) FLL LEAD OPTION.3 BSC (7.6 BSC) CORNER LEADS OPTION ( PLCS).3.5 ( ) HALF LEAD OPTION.5 (.17) MIN.5 (.635) RAD TYP.5 (1.87) MAX ( ). (5.8) MAX.15.6 ( ).8.18 (.3.57) 15 NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE OR TIN PLATE LEADS.5.65 ( ).1.6 (.36.66).1 (.5) BSC MIN J8 81 OBSOLETE PACKAGE 1

11 PACKAGE DESCRIPTIO N8 Package 8-Lead PDIP (Narrow.3) (LTC DWG # ).3.35 ( ).5.65 ( ).13 ±.5 (3.3 ±.17).* (1.16) MAX (.3.381) ( ).65 (1.651) TYP.1 (.5) BSC NOTE: INCHES 1. DIMENSIONS ARE MILLIMETERS *THESE DIMENSIONS DO NOT INCLDE MOLD FLASH OR PROTRSIONS. MOLD FLASH OR PROTRSIONS SHALL NOT EXCEED.1 INCH (.5mm).55 ±.15* (6.77 ±.381).1 (3.8) MIN. (.58) ±.3 (.57 ±.76) MIN N8 1 S8 Package 8-Lead Plastic Small Outline (Narrow.15 Inch) (Reference LTC DWG # ).5 BSC.5 ± (.81 5.) NOTE MIN.16 ±.5.8. ( ) ( ) NOTE 3.3 ±.5 TYP RECOMMENDED SOLDER PAD LAYOT (.3.5).1. (.5.58) 5 8 TYP ( )..1 (.11.5).16.5 (.6 1.7) NOTE: INCHES 1. DIMENSIONS IN (MILLIMETERS).1.19 ( ) TYP. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLDE MOLD FLASH OR PROTRSIONS. MOLD FLASH OR PROTRSIONS SHALL NOT EXCEED.6" (.15mm).5 (1.7) BSC SO8 33 Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 11

12 TYPICAL APPLICATIO Grounded Thermocouple Amplifier with Positive Output 1k R3 1M R 1k R5 k FLL-SCALE ADJST V TYPE J* C1.1µF R6** 8.k GND R J R7 6.8k C.1µF V LTKAx V V OT TA7 * FOR BEST ACCRACY, THERMOCOPLE RESISTANCE SHOLD BE LESS THAN 5Ω ** SELECTED FOR C TO C RANGE OR EQIVALENT. SEE AMPLIFIER CONSIDERATIONS RELATED PARTS PART NMBER DESCRIPTION COMMENTS LT11 Picoamp Input Current Op Amp 1µV Max V OS, 8pA Max I OS LTC15 Zero Drift Amplifier 5µV Max V OS, A VOL 1V/µV Max LTC5 SOT-3 Zero Drift Amplifier 3µV Max V OS 1 Linear Technology Corporation 163 McCarthy Blvd., Milpitas, CA (8) 3-19 FAX: (8) LT/TP 113 1K REV B PRINTED IN SA LINEAR TECHNOLOGY CORPORATION 1988