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

Boost. Like the Signal Amplifier, the Auxiliary requires about

to swing.

The output stage of the Auxiliary Amplifier is actually a high

emitter. Thus, the Auxiliary Amplifier can be used as a V/I

converter when configured as a follower and resistively loaded.

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.

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

usually connected to common and the negative loop connection

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.

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

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

Figure 10. Minimal Connection for 0–30 mV Unipolar Input, 4–20 mA Output