AD693

REV. A

–8–

R

S

S

− 60 mV

− 1.0024









Figure 14 shows a scheme for adjusting the modified span and

This in effect, creates a divider with the same ratio as the

internal divider that sets the 4 mA zero level (–15 mV with

respect to 6.2 V). As long as the input signal remains zero the

voltage at Pin 12, the zero adjust, will remain at –15 mV with

respect to 6.2 V.

Figure 14. Adjusting for Spans between 60 mV and

loop. An attenuated portion of the input signal is now added

into the V/I zero to maintain the 75 mV maximum differential.

If there is some small offset at the input to the Signal Amplifier,

it may be necessary to repeat the two adjustments.

LOCAL-POWERED OPERATION FOR 0–20 mA OUTPUT

The AD693 is designed for local-powered, three-wire systems as

well as two-wire loops. All its usual ranges are available in three-

wire operation, and in addition, the 0–20 mA range can be used.

The 0-20 mA convention offers slightly more resolution and

may simplify the loop receiver, two reasons why it is sometimes

preferred.

The arrangement, illustrated in Figure 15, results in a 0–20 mA

transmitter where the precalibrated span is 37.5 mV. Con-

necting P1 to P2 will double the span to 75 mV. Sensor input

and excitation is unchanged from the two-wire mode except for

the 25% increase in span. Many sensors are ratiometric so that

an increase in excitation can be used instead of a span

adjustment.

In the local-powered mode, increases in excitation are made

the loop voltage may be permitted to fall to 6 volts at the

AD693, easing the trade-off between loop voltage and loop

with the local power supply current.

An alternative arrangement, allowing wide range span adjust-

previous formulae. The smallest value is then placed in series

with the wiper of the 1.5 k

Ω potentiometer shown in the figure.

The smaller value, 800

Ω, is then reduced by 10% to cover the

possible ranges of resistance in the AD693 and that value is put

in place.

Figure 13. Wide Range Span Adjustment

A number of other arrangements can be used to set the span as

long as they are compatible with the pretrimmed noninverting

gain of two. The span adjustment can even include thermistors

or other sensitive elements to compensate the span of a sensor.

In devising your own adjustment scheme, remember that you

should adjust the gain such that the desired span voltage at the

Signal Amplifier input translates to 60 mV at the output. Note

also that the full differential voltage applied to the V/I converter

is 75 mV; in the 4-20 mA mode, –15 mV is applied to the

inverting input (zero pin) by the Divider Network and +60 mV

is applied to the noninverting input by the Signal Amplifier. In

the 0–20 mA mode, the total 75 mV must be applied by the

Signal Amplifier. As a result, the total span voltage will be 25%

larger than that calculated for a 4-20 mA output.

Finally, the external resistance from P2 to 6.2 V should not be

made less than 1 k

Ω unless the voltage reference is loaded to at

least 1.0 mA. (A simple load resistor can be used to meet this

requirement if a low value potentiometer is desired.) In no case

should the resistance from P2 to 6.2 V be less than 200

Ω.

Input Spans Between 60 mV and 100 mV

Input spans of up to 100 mV can be obtained by adding an

offset proportional to the output signal into the zero pin of the

V/I converter. This can be accomplished with two resistors and

adjusted via the optional trim scheme shown in Figure 14. The

Signal Amplifier modifies the differential input voltage range

applied to the V/I converter.

This can be accomplished (power supply disconnected) by

measuring the resistance between the 4 mA of offset (Pin 13)

and common (Pin 6) with the 6.2 V reference (Pin 14) connected