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AD693AD Fiches technique(PDF) 6 Page - Analog Devices |
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AD693AD Fiches technique(HTML) 6 Page - Analog Devices |
6 / 12 page AD693 REV. A –6– if the 6.2 V of the reference is unsuitable. Configured as a simple follower, it can be driven from a user supplied voltage divider or the precalibrated outputs of the AD693 divider (Pins 3 and 4) to provide a stiff voltage output at less than the 6.2 level, or by incorporating a voltage divider as feedback around the amplifier, one can gain-up the reference to levels higher than 6.2 V. If large positive outputs are desired, IX, the Auxiliary Amplifier output current supply, should be strapped to either VIN or Boost. Like the Signal Amplifier, the Auxiliary requires about 3.5 V of headroom with respect to VIN at its input and about 2 V of difference between IX and the voltage to which VX is required to swing. The output stage of the Auxiliary Amplifier is actually a high gain Darlington transistor where IX is the collector and VX is the emitter. Thus, the Auxiliary Amplifier can be used as a V/I converter when configured as a follower and resistively loaded. IX functions as a high-impedance current source whose current is equal to the voltage at VX divided by the load resistance. For example, using the onboard 100 Ω resistor and the 75 mV or 150 mV application voltages, either a 750 µA or 1.5 mA current source can be set up for transducer excitation. The IX terminal has voltage compliance within 2 V of VX. If the Auxiliary Amplifier is not to be used, then Pin 2, the noninverting input, should be grounded. REVERSE VOLTAGE PROTECTION FEATURE In the event of a reverse voltage being applied to the AD693 through a current-limited loop (limited to 200 mA), an internal shunt diode protects the device from damage. This protection mode avoids the compliance voltage penalty which results from a series diode that must be added if reversal protection is required in high-current loops. Applying the AD693 CONNECTIONS FOR BASIC OPERATION Figure 10 shows the minimal connections for basic operation: 0–30 mV input span, 4–20 mA output span in the two-wire, loop-powered mode. If not used for external excitation, the 6.2 V reference should be loaded by approximately 1 mA (6.2 k Ω to common). USING AN EXTERNAL PASS TRANSISTOR The emitter of the NPN output section, IOUT, of the AD693 is usually connected to common and the negative loop connection (Pins 7 to 6). Provision has been made to reconnect IOUT to the base of a user supplied NPN transistor as shown in Figure 11. This permits the majority of the power dissipation to be moved off chip to enhance performance, improve reliability, and extend the operating temperature range. An internal hold-down resistor of about 3k is connected across the base emitter of the external transistor. The external pass transistor selected should have a BVCEO greater than the intended supply voltage with a sufficient power rating for continuous operation with 25 mA current at the supply voltage. Ft should be in the 10 MHz to 100 MHz range and β should be greater than 10 at a 20 mA emitter current. Some transistors that meet this criteria are the 2N1711 and 2N2219A. Heat sinking the external pass transistor is suggested. The pass transistor option may also be employed for other applications as well. For example, IOUT can be used to drive an LED connected to Common, thus providing a local monitor of loop fault conditions without reducing the minimum compliance voltage. ADJUSTING ZERO In general, the desired zero offset value is obtained by connecting an appropriate tap of the precision reference/voltage divider network to the inverting terminal of the V/I converter. As shown in Figure 9, precalibrated taps at Pins 14, 13 and 11 result in zero offsets of 0 mA, 4 mA and 12 mA, respectively, when connected to Pin 12. The voltages which set the 4 mA and 12 mA zero operating points are 15 mV and 45 mV negative with respect to 6.2 V, and they each have a nominal source resistance of 450 Ω. While these voltages are laser trimmed to high accuracy, they may require some adjustment to accommodate variability between sensors or to provide additional ranges. You can adjust zero by pulling up or down on the selected zero tap, or by making a separate voltage divider to drive the zero pin. The arrangement of Figure 12 will give an approximately linear adjustment of the precalibrated options with fixed limits. To find the proper resistor values, first select IA, the desired range Figure 10. Minimal Connection for 0–30 mV Unipolar Input, 4–20 mA Output |
Numéro de pièce similaire - AD693AD |
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Description similaire - AD693AD |
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