Datasheet AD693 (Analog Devices) - 6

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