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AD594AQ Scheda tecnica(PDF) 4 Page - Analog Devices |
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AD594AQ Scheda tecnica(HTML) 4 Page - Analog Devices |
4 / 8 page AD594/AD595 REV. C –4– CONSTANTAN (ALUMEL) IRON (CHROMEL) +5V TO +30V COMMON SPAN OF 5V TO 30V 0V TO –25V OVERLOAD DETECT G –TC +TC 12 3 4 5 67 13 12 11 10 AD594/ AD595 14 ICE POINT COMP. +A 98 G Figure 2. Dual Supply Operation With a negative supply the output can indicate negative tem- peratures and drive grounded loads or loads returned to positive voltages. Increasing the positive supply from 5 V to 15 V ex- tends the output voltage range well beyond the 750 °C temperature limit recommended for type J thermocouples (AD594) and the 1250 °C for type K thermocouples (AD595). Common-mode voltages on the thermocouple inputs must remain within the common-mode range of the AD594/AD595, with a return path provided for the bias currents. If the thermocouple is not remotely grounded, then the dotted line connections in Figures 1 and 2 are recommended. A resistor may be needed in this connection to assure that common-mode voltages induced in the thermocouple loop are not converted to normal mode. THERMOCOUPLE CONNECTIONS The isothermal terminating connections of a pair of thermo- couple wires forms an effective reference junction. This junction must be kept at the same temperature as the AD594/AD595 for the internal cold junction compensation to be effective. A method that provides for thermal equilibrium is the printed circuit board connection layout illustrated in Figure 3. IRON (CHROMEL) CONSTANTAN (ALUMEL) +T +C –T –C +IN –IN 114 –ALM +ALM 78 COMP COMMON V– VOUT V+ Figure 3. PCB Connections Here the AD594/AD595 package temperature and circuit board are thermally contacted in the copper printed circuit board tracks under Pins 1 and 14. The reference junction is now com- posed of a copper-constantan (or copper-alumel) connection and copper-iron (or copper-chromel) connection, both of which are at the same temperature as the AD594/AD595. The printed circuit board layout shown also provides for place- ment of optional alarm load resistors, recalibration resistors and a compensation capacitor to limit bandwidth. To ensure secure bonding the thermocouple wire should be cleaned to remove oxidation prior to soldering. Noncorrosive rosin flux is effective with iron, constantan, chromel and alumel and the following solders: 95% tin-5% antimony, 95% tin-5% silver or 90% tin-10% lead. FUNCTIONAL DESCRIPTION The AD594 behaves like two differential amplifiers. The out- puts are summed and used to control a high gain amplifier, as shown in Figure 4. 14 13 12 11 10 9 8 OVERLOAD DETECT ICE POINT COMP. +A G G –TC +TC 12 3 4 56 7 AD594/AD595 +IN +C +T COM –T –C V– –IN –ALM +ALM V+ COMP VO FB Figure 4. AD594/AD595 Block Diagram In normal operation the main amplifier output, at Pin 9, is con- nected to the feedback network, at Pin 8. Thermocouple signals applied to the floating input stage, at Pins 1 and 14, are ampli- fied by gain G of the differential amplifier and are then further amplified by gain A in the main amplifier. The output of the main amplifier is fed back to a second differential stage in an in- verting connection. The feedback signal is amplified by this stage and is also applied to the main amplifier input through a summing circuit. Because of the inversion, the amplifier causes the feedback to be driven to reduce this difference signal to a small value. The two differential amplifiers are made to match and have identical gains, G. As a result, the feedback signal that must be applied to the right-hand differential amplifier will pre- cisely match the thermocouple input signal when the difference signal has been reduced to zero. The feedback network is trim- med so that the effective gain to the output, at Pins 8 and 9, re- sults in a voltage of 10 mV/ °C of thermocouple excitation. In addition to the feedback signal, a cold junction compensation voltage is applied to the right-hand differential amplifier. The compensation is a differential voltage proportional to the Celsius temperature of the AD594/AD595. This signal disturbs the dif- ferential input so that the amplifier output must adjust to restore the input to equal the applied thermocouple voltage. The compensation is applied through the gain scaling resistors so that its effect on the main output is also 10 mV/ °C. As a result, the compensation voltage adds to the effect of the ther- mocouple voltage a signal directly proportional to the difference between 0 °C and the AD594/AD595 temperature. If the thermo- couple reference junction is maintained at the AD594/AD595 temperature, the output of the AD594/AD595 will correspond to the reading that would have been obtained from amplification of a signal from a thermocouple referenced to an ice bath. |
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