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AD590JR Datenblatt(PDF) 10 Page - Analog Devices |
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AD590JR Datenblatt(HTML) 10 Page - Analog Devices |
10 / 16 page AD590 Rev. C | Page 10 of 16 GENERAL APPLICATIONS Figure 14 demonstrates the use of a low cost digital panel meter for the display of temperature on either the Kelvin, Celsius, or Fahrenheit scales. For Kelvin temperature, Pins 9, 4, and 2 are grounded; for Fahrenheit temperature, Pins 4 and 2 are left open. 6 5 9 4 2 OFFSET CALIBRATION 5V GAIN SCALING OFFSET SCALING 3 8 AD2040 GND AD590 + – Figure 14. Variable Scale Display The above configuration yields a 3-digit display with 1°C or 1°F resolution, in addition to an absolute accuracy of ±2.0°C over the −55°C to +125°C temperature range, if a one-temperature calibration is performed on an AD590K, AD590L, or AD590M. Connecting several AD590 units in series as shown in Figure 15 allows the minimum of all the sensed temperatures to be indicated. In contrast, using the sensors in parallel yields the average of the sensed temperatures. AD590 + – AD590 + – AD590 + – + VT MIN 10k Ω (0.1%) – + – AD590 + – + – + VT AVG 333.3 Ω (0.1%) – 5V 15V Figure 15. Series and Parallel Connection The circuit in Figure 16 demonstrates one method by which differential temperature measurements can be made. R1 and R2 can be used to trim the output of the op amp to indicate a desired temperature difference. For example, the inherent offset between the two devices can be trimmed in. If V+ and V− are radically different, then the difference in internal dissipation causes a differential internal temperature rise. This effect can be used to measure the ambient thermal resistance seen by the sensors in applications such as fluid-level detectors or anemometry. AD590L #2 + – AD590L #1 + – R4 10k Ω R3 10k Ω R1 5M Ω R2 50k Ω V+ (T1 – T2) × (10mV/°C) V– AD707A – + Figure 16. Differential Measurements Figure 17 is an example of a cold junction compensation circuit for a Type J thermocouple using the AD590 to monitor the reference junction temperature. This circuit replaces an ice-bath as the thermocouple reference for ambient temperatures between 15°C and 35°C. The circuit is calibrated by adjusting RT for a proper meter reading with the measuring junction at a known reference temperature and the circuit near 25°C. Using components with the TCs as specified in Figure 17, compensation accuracy is within ±0.5°C for circuit temperatures between 15°C and 35°C. Other thermocouple types can be accommodated with different resistor values. Note that the TCs of the voltage reference and the resistors are the primary contributors to error. + – REFERENCE JUNCTION IRON + – 7.5V AD590 AD580 CONSTANTAN MEASURING JUNCTION RESISTORS ARE 1%, 50PPM/°C METER + – – + CU 52.3 Ω 8.66k Ω VOUT RT 1k Ω Figure 17. Cold Junction Compensation Circuit for Type J Thermocouple Figure 18 is an example of a current transmitter designed to be used with 40 V, 1 kΩ systems; it uses its full current range of 4 mA to 20 mA for a narrow span of measured temperatures. In this example, the 1 µA/K output of the AD590 is amplified to 1 mA/°C and offset so that 4 mA is equivalent to 17°C and 20 mA is equivalent to 33°C. RT is trimmed for proper reading at an intermediate reference temperature. With a suitable choice of resistors, any temperature range within the operating limits of the AD590 may be chosen. |
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