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LT1025 Datasheet(PDF) 6 Page - Linear Technology |
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LT1025 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LT1025 1025fb Figure 4. Thermocouple Nonlinearity, 0 °C to 400°C Figure 5. Offset Curve Fitting 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 60Hz pick-up with reasonable capacitor values, input resistors in the 10k-100k range are needed. Under 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-stagethermocoupleamplifierswith10mV/ °Corhigher outputs. A type K amplifier, for instance, with 100mV/ °C output, needs a closed loop gain of ≈2,500. An ordinary op amp with a minimum open loop of 50,000 would have an initial gain error of (2,500)/(50,000) = 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 250,000. This gain is adequate for type R and S if output scaling is 10mV/ °C or less. Suggested Amplifier Types SUPPLY VOLTAGE THERMOCOUPLE ±15V ±5V SINGLE SUPPLY E, J, K, T LTKA0x LTKA0x LTC1050 LT1012 LT1012 LTC1052 LT1001 LT1001 LT1006 LTC1050 LTC1052 LT1006 R, S LTKA0x LTC1050 LTC1050 LT1012 LTC1052 LTC1052 LTKA0x LT1006 Thermocouple Nonlinearities Thermocouples are linear over relatively limited temperature spans if accuracies of better than 2 °C are needed. The graph in Figure 4 shows thermocouple nonlinearity for the temperature range of 0 °C to 400°C. Nonlinearities can be dealtwithinhardwarebyusingoffsets,breakpoints,orpower 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 470A. 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 temperature. The temperature range of interest is between TL and TH, with a calibration point at TM. If a simple amplifier is used and calibrated at TM, the output will be very high at TL and very low at TH. 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) (TH – TL)/(VH – VL) SF = Output scale factor, e.g., 10mV/ °C VH = Thermocouple output at TH VL = Thermocouple output at TL 2. Use precision resistors to set gain or calibrate gain by introducing a precision “delta” input voltage and trimming for proper “delta” output. APPLICATIO S I FOR ATIO TEMPERATURE ( °C) 0 20 15 12.5 10 5 100 200 250 LT1025 • G04 17.5 2.5 0 7.5 8 6 5 4 2 7 1 0 3 50 150 300 350 400 K →SCALE J →SCALE SCALE ←E SCALE ←T TEMPERATURE ( °C) 0 VL VH LT1025 • G05 T1/6 TH TL TM T5/6 THERMOCOUPLE SIMPLE AMPLIFIER OFFSET AMPLIFIER ERROR AFTER OFFSETTING ERROR BEFORE OFFSETTING |
Similar Part No. - LT1025_15 |
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Similar Description - LT1025_15 |
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