LT1025ACN8#PBF

LT1025 Micropower Thermocouple Cold Junction Compensator U FEATURES ■ ■ ■ ■ ■ ■ DESCRIPTIO 80µA Supply Current 4V to 36V Operation 0.5°C Initial Accuracy (A Version) Compatible with Standard Thermocouples (E, J, K, R, S, T) Auxiliary 10mV/°C Output Available in 8-Lead PDIP and SO Packages U APPLICATIO S ■ ■ ■ Thermocouple Cold Junction Compensator Centigrade Thermometer Temperature Compensation Network The LT®1025 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 0.5°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 LT1025 will operate with a supply voltage from 4V to 36V. Typical supply current is 80µA, resulting in less than 0.1°C internal temperature rise for supply voltages under 10V. A 10mV/°C output is available at low impedance, in addition to the direct thermocouple voltages of 60.9µV/°C (E), 51.7µV/°C (J), 40.3µV/°C (K, T) and 5.95µV/°C (R, S). All outputs are essentially independent of power supply voltage. A special kit is available (LTK001) which contains an LT1025 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 LTKA0x. The LT1025 is available in either an 8-pin PDIP or 8-pin SO package for temperatures between 0°C and 70°C. , LTC and LT are registered trademarks of Linear Technology Corporation. W U BLOCK DIAGRA TYPICAL APPLICATIO Type K 10mV/°C Thermometer E 60.9µV/°C VIN R2 100Ω FULL-SCALE TRIM J 51.7µV/°C R3** 255k 1% + BOW* CORRECTION VOLTAGE BUFFER 10mV/°C TEMPERATURE SENSOR R, S 6µV/°C V+ R– COMMON V+ – LTKA0x†† VIN K – + LT1025 V0 10mV/°C GND *CORRECTS FOR BOW IN COLD JUNCTION, NOT IN PROBE (HOT JUNCTION) R1 1k 1% K,T 40.6µV/°C – GND R– + V– TYPE K VO R4* C1 0.1µF C2 0.1µF VOUT 10mV/°C V– *R4 ≤ 30µA , R4 IS NOT REQUIRED (OPEN) FOR LT1025 TEMPERATURES ≥ 0°C **SELECTED FOR 0°C TO 100°C RANGE †† OR LT1025 • BD01 V– EQUIVALENT. SEE “AMPLIFIER CONSIDERATIONS” LT1025 • TA01 1025fb 1 LT1025 W W W AXI U U U W PACKAGE/ORDER I FOR ATIO U ABSOLUTE RATI GS (Note 1) Input Supply Voltage .......................................... 36V Output Voltage (Forced)........................................ 5V Output Short-Circuit Duration ..................... Indefinite Operating Temperature Range LT1025AC, LT1025C ............................0°C to 70°C LT1025AM, LT1025M .................. – 55°C to 125°C Storage Temperature Range ............ – 55°C to 150°C ORDER PART NUMBER TOP VIEW E 60.9µV/°C 1 VIN 2 VO 3 10mV/°C GND 4 8 7 6 5 N8 PACKAGE 8-LEAD PDIP J 51.7µV/°C K, T 40.6µV/°C R, S 6µV/°C R– COMMON LT1025ACN8 LT1025CN8 LT1025CS8 S8 PACKAGE 8-LEAD PLASTIC SO S8 PART MARKING TJMAX = 150°C, θJA = 130°C/W (N8) TJMAX = 150°C, θJA = 190°C/W (S8) 1025 LT1025AMJ8 LT1025MJ8 J8 PACKAGE 8-LEAD CERDIP TJMAX = 150°C, θJA = 100°C/W 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 TA = 25°C. VS = 5V, Pin 5 tied to Pin 4, unless otherwise noted. PARAMETER CONDITIONS Temperature Error at 10mV/°C Output (Notes 4, 5) TJ = 25°C LT1025A LT1025 MIN Full Temperature Span Resistor Divider Accuracy (Notes 2, 4) VOUT = 10mV/°C LT1025A LT1025 TYP MAX UNITS 0.3 0.5 0.5 2.0 °C °C See Curve ● E J K, T R, S 60.6 51.4 40.3 5.8 60.9 51.7 40.6 5.95 61.3 52.1 41.0 6.2 µV/°C µV/°C µV/°C µV/°C E J K, T R, S 60.4 51.2 40.2 5.75 60.9 51.7 40.6 5.95 61.6 52.3 41.2 6.3 µV/°C µV/°C µV/°C µV/°C 50 50 80 100 150 200 µA µA µA °C/V 4V ≤ VIN ≤ 36V LT1025AC, LT1025C LT1025AM, LT1025M ● ● Line Regulation (Note 3) 4V ≤ VIN ≤ 36V ● 0.003 0.02 Load Regulation (Note 3) 0 ≤ IO ≤ 1mA ● 0.04 0.2 Supply Current Divider Impedance Change in Supply Current E J K, T R, S 4V ≤ VIN ≤ 36V Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Divider accuracy is measured by applying a 10.000V 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 30µA ≤ IO ≤ 1mA for TA ≤ 0°C. 2.5 2.1 4.4 3.8 0.01 °C kΩ kΩ kΩ kΩ 0.05 µA/V Note 4: To calculate total temperature error at individual thermocouple outputs, add 10mV/°C output error to the resistor divider error. Total error for type K output at 25°C with an LT1025A is 0.5°C plus (0.4µV/°C)(25°C)/ (40.6µV/°C) = 0.5°C + 0.25°C = 0.75°C. Note 5: Temperature error is defined as the deviation from the following formula: VOUT = 10mV(T) + (10mV)(5.5 • 10-4)(T – 25°C)2. The second term is a built-in nonlinearity designed to help compensate the nonlinearity of the cold junction. This “bow” is ≈ 0.34°C for a 25°C temperature change. 1025fb 2 LT1025 U W TYPICAL PERFOR A CE CHARACTERISTICS 10mV/°C Output Temperature Error LT1025A Supply Current 10 5 200 8 4 180 4 GUARANTEED LIMITS* LT1025 2 0 –2 –4 –6 –8 –10 50 25 0 75 100 –50 –25 JUNCTION TEMPERATURE (°C) 125 GUARANTEED LIMITS* LT1025A 3 2 140 1 0 –1 100 80 60 –3 40 –4 20 –5 50 25 0 75 100 –50 –25 JUNCTION TEMPERATURE (°C) TJ = 125°C 120 –2 LT1025 • G01 TJ = 25°C TJ = –55°C PIN 4 TIED TO PIN 5 125 0 0 10 15 20 25 30 SUPPLY VOLTAGE (V) 5 LT1025 • G02 35 40 LT1025 • G03 *ERROR CURVE FACTORS IN THE NONLINEARITY TERM BUILT IN TO THE LT1025. SEE THEORY OF OPERATION IN APPLICATION GUIDE SECTION U *ERROR CURVE FACTORS IN THE NONLINEARITY TERM BUILT IN TO THE LT1025. SEE THEORY OF OPERATION IN APPLICATION GUIDE SECTION DOES NOT INCLUDE 30µA PULL-DOWN CURRENT REQUIRED FOR TEMPERATURES BELOW 0°C 160 CURRENT (µA) 6 TEMPERATURE ERROR (°C) TEMPERATURE ERROR (°C) 10mV/°C Output Temperature Error LT1025 W U U APPLICATIO S I FOR ATIO The LT1025 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, T2. The Seebeck voltage, VS, is the product of the Seebeck coefficient α, and the temperature difference, T1 – T2; VS = α (T1 – T2). The junctions at T2 are commonly called the cold junction because a common practice is to immerse the T2 junction in 0°C ice/water slurry to make T2 independent of room temperature variations. Thermocouple tables are based on a cold-junction temperature of 0°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 VS such that the voltage sum is the same as if the T2 junction were at a constant 0°C instead of at room temperature. This voltage source is called a cold junction compensator. Its output is designed to be 0V at 0°C and have a slope equal to the Seebeck coefficient over the expected range of T2 temperatures. TEMPERATURE T1 TO BE MEASURED Fe Cu T2 CONSTANTAN } VS Cu LT1025 MUST BE LOCATED NEXT TO COLD JUNCTION FOR TEMPERATURE TRACKING LT1025 • AG01 Figure 1 To operate properly, a cold junction compensator must be at exactly the same temperature as the cold junction of the thermocouple (T2). Therefore, it is important to locate the LT1025 physically close to the cold junction with local temperature gradients minimized. If this is not possible, 1025fb 3 LT1025 U W U U 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 LT1025 can be located remotely from the user termination as shown in Figure 2. LT1025 Fe Fe Cu CN CN EXTENDER Cu AMPLIFIER “HOT” JUNCTION FRONT PANEL CONNECTOR “NEW” COLD JUNCTION temperatures below zero unless an external pull-down resistor is added to the VO output. This resistor can be connected to any convenient negative supply. It should be selected to sink at least 30µA of current. Suggested value for a – 5V supply is 150kΩ, and for a – 15V supply, 470kΩ. Smaller resistors must be used if an external load is connected to the 10mV/°C output. The LT1025 can source up to 1mA of current, but there is a trade-off with internal temperature rise. Internal Temperature Rise VOUT = αT + αß(T –25°)2 The LT1025 is specified for temperature accuracy assuming no internal temperature rise. At low supply voltages this rise is usually negligible (≈ 0.05°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 = (θJA) (V +)(IQ + IL), where θJA is thermal resistance from junction to ambient, (≈130°C/W), V+ is the LT1025 supply voltage, IQ is the LT1025 supply current (≈ 80µA) and IL 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 50µA pull-down current would yield, (130°C/W) (15V) (80µA + 50µA) = 0.32°C. This is a significant rise in some applications. It can be reduced by lowering supply voltage (a simple fix is to insert a 10V zener in the VIN lead) or the system can be calibrated and specified after an initial warm-up period of several minutes. ß ≈ 5.5 • 10– 4 Driving External Capacitance LT1025 • AG02 Figure 2 The four thermocouple outputs on the LT1025 are 60.9µV/°C (E), 51.7µV/°C (J), 40.6µV/°C (K and T), and 6µV/°C (R and S). These particular coefficients are chosen to match the room temperature (25°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 LT1025 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 25°C squared. The coefficient (ß) of the quadratic term is a compromise value chosen to offer improvement in all the outputs. The actual ß term which would be required to best compensate each thermocouple type in the temperature range of 0°C to 50°C is: E, 6.6 • 10–4; J, 4.8 • 10–4; K, 4.3 • 10–4; R, 1.9 • 10–3, S, 1.9 • 10–3; T, 1 • 10–3. The temperature error specification for the LT1025 10mV/°C output (shown as a graph) assumes a ß of 5.5 •10 –4. For example, an LT1025 is considered “perfect” if its 10mV/°C output fits the equation VO = 10mV(T) + (10mV)(5.5 • 10 –4)(T – 25°C)2. Operating at Negative Temperatures The LT1025 is designed to operate with a single positive supply. It therefore cannot deliver proper outputs for The direct thermocouple drive pins on the LT1025 (J, K, etc.) can be loaded with as much capacitance as desired, but the 10mV/°C output should not be loaded with more than 50pF 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 42 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 1025fb 4 LT1025 U W U U 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 LT1025, or the thermocouple amplifier. If a gradient cannot be eliminated, leads should be positioned isothermally, especially the LT1025 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 10mV/°C Output The LT1025 can be made to “stand on its head” to achieve a minus 10mV/°C output point. This is done as shown in Figure 3. The normal output (VO) 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 R2 15k VIN VO LT1025 D1 VZ ≈ 6V R1 = V + – VZ (≈6 V) V– , R2 = – 300µA + IL V /R1 + 280µA For ±15V supplies, with IL = 20µA maximum, R1 = 47k and R2 = 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 40µV/°C to 60µV/°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 LTKA0x is a bipolar design with extremely low offset (< 35µV), low drift (