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AD693BQ 数据表(PDF) 10 Page - Analog Devices |
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AD693BQ 数据表(HTML) 10 Page - Analog Devices |
10 / 12 page AD693 REV. A –10– increasing the application voltages by adding resistance between Pins 14 and 3 will decrease the temperature span. An external voltage divider may also be used in conjunction with the circuit shown to produce any range of temperature spans as well as providing zero output (4 mA) for a non 0 temperature input. For example, measuring VX with respect to a voltage 2.385 times the excitation (rather than 2 times) will result in zero input to the Signal Amplifier when the RTD is at 100 °C (or 138.5 Ω). As suggested in Table I, the temperature span may also be adjusted by changing the voltage span of the Signal Amplifier. Changing the gain from 2 to 4, for example, will halve the temperature span to about 52 °C on the 4-20mA output configuration. (See section “Adjusting Input Span.”) The configuration for a three-wire RTD shown in Figure 17 can accommodate two-wire sensors by simply joining Pins 1 and 5 of the AD693. INTERFACING LOAD CELLS AND METAL FOIL STRAIN GAGES The availability of the on-chip Voltage Reference, Auxiliary Amplifier and 3 mA of excitation current make it easy to adapt the AD693 to a variety of load cells and strain gages. The circuit shown in Figure 18 illustrates a generalized approach in which the full flexibility of the AD693 is required to interface to a low resistance bridge. For a high impedance transducer the bridge can be directly powered from the 6.2 V Reference. Component values in this example have been selected to match the popular standard of 2 mV/V sensitivity and 350 Ω bridge resistance. Load cells are generally made for either tension and compression, or compression only; use of the 12 mA zero tap allows for operation in the tension and compression mode. An optional zero adjustment is provided with values selected for +2% FS adjustment range. Because of the low resistance of most foil bridges, the excitation voltage must be low so as not to exceed the available 4 mA zero current. About 1 V is derived from the 6.2 V Reference and an external voltage divider; the Aux-Amp is then used as a follower to make a stiff drive for the bridge. Similar applications with higher resistance sensors can use proportionally higher voltage. Finally, to accommodate the 2 mV/V sensitivity of the bridge, the full-scale span of the Signal Amplifier must be reduced. Using the load cell in both tension and compression with 1 V of excitation, therefore, dictates that the span be adjusted to 4 mV. By substituting in the expression, RS1 = 400 Ω/[(30 mV/S) – 1], the nominal resistance required to achieve this span is found to be 61.54 Ω. Calculate the minimum resistance required by subtracting 10% from 61.54 Ω to allow for the internal resistor tolerance of the AD693, leaving 55.38 Ω (See “Adjusting Input Span.”) The standard value of 54.9 Ω is used with a 20 Ω potentiometer for full-scale adjustment. If a load cell with a precalibrated sensitivity constant is to be used, the resultant full-scale span applied to the Signal Amplifier is found by multiplying that sensitivity by the excitation voltage. (In Figure 18, the excitation voltage is actually (10 k Ω/62.3 kΩ) (6.2 V) = 0.995 V). THERMOCOUPLE MEASUREMENTS The AD693 can be used with several types of thermocouple inputs to provide a 4-20 mA current loop output corresponding to a variety of measurement temperature ranges. Cold junction compensation (CJC) can be implemented using an AD592 or AD590 and a few external resistors as shown in Figure 19. From Table II simply choose the type of thermocouple and the appropriate average reference junction temperature to select values for RCOMP and RZ. The CJC voltage is developed across RCOMP as a result of the AD592 1 µA/K output and is added to the thermocouple loop voltage. The 50 Ω potentiometer is biased by RZ to provide the correct zero adjustment range appropriate for the divider and also translates the Kelvin scale of the AD592 to °Celsius. To calibrate the circuit, put the thermocouple in an ice bath (or use a thermocouple simulator set to 0) and adjust the potentiometer for a 4 mA loop current. The span of the circuit in °C is determined by matching the signal amplifier input voltage range to its temperature equivalent Figure 18. Utilizing the Auxiliary Amplifier to Drive a Load Cell, 12 mA ± 8 mA Output |
类似零件编号 - AD693BQ |
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类似说明 - AD693BQ |
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