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AD8494ARMZ Scheda tecnica(PDF) 11 Page - Analog Devices |
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AD8494ARMZ Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 16 page AD8494/AD8495/AD8496/AD8497 Rev. 0 | Page 11 of 16 THEORY OF OPERATION THERMOCOUPLES A thermocouple is a rugged, low cost temperature transducer whose output is proportional to the temperature difference between a measurement junction and a reference junction. 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 compensated. 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 electronics. This connection forms a second junction: the reference junction, also called the cold junction. AD849x PCB TRACES REFERENCE JUNCTION MEASUREMENT JUNCTION THERMOCOUPLE WIRES Figure 26. Thermocouple Junctions To derive the temperature at the measurement junction (TMJ), the user must know the differential voltage created by the thermo- couple. The user must also know the error voltage generated by the temperature at the reference junction (TRJ). Compensating for the reference junction error voltage is typically called cold junction compensation. The electronics 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 thermocouple amplifiers provide a simple, low cost solution for measuring thermocouple temperatures. These amplifiers simplify many of the difficulties of measuring thermocouples. An integrated temperature sensor performs cold junction compensation. A fixed-gain instrumen- 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 0°C and 50°C on the reference and measurement junctions. Table 6 also shows the performance of the AD8494 amplifying the thermocouple voltage and compensating for the reference junction temperature changes, thus eliminating the error. Table 6. J Type Thermocouple Voltages and AD8494 Readings Measurement Junction Temperature (TMJ) Reference Junction Temperature (TRJ) Thermocouple Voltage AD8494 Reading 50°C 0°C +2.585 mV 250 mV 50°C 50°C 0 mV 250 mV 0°C 0°C 0 mV 0 mV 0°C 50°C −2.585 mV 0 mV AD8494/AD8495/AD8496/AD8497 ARCHITECTURE 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 A1 A2 AD8494/AD8495/ AD8496/AD8497 A3 SENSE ESD AND OVP ESD AND OVP COLD JUNCTION COMPENSATION 1M Ω THERMO- COUPLE Figure 27. Block Diagram The AD849x output is a voltage that is proportional to the tem- perature at the measurement junction of the thermocouple (TMJ). To derive the measured temperature from the AD849x output voltage, use the following transfer function: TMJ = (VOUT − VREF)/(5 mV/°C) An ideal AD849x achieves this output with an error of less than ±2°C, 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. VOUT = (TMJ × 5 mV/°C) + VREF To accommodate the nonlinear 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. |
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