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AD693

Loop-Powered 4–20 mA Sensor Transmitter

Analog Devices 

AD693

An alternative arrangement, allowing wide range span adjust-

ment between two set ranges, is shown in Figure 13. RS1 and

RS2 are calculated to be 90% of the values determined from the

previous formulae. The smallest value is then placed in series

with the wiper of the 1.5 kΩ potentiometer shown in the figure.

For example, to adjust the span between 25 mV and 40 mV, RS1

and RS2 are calculated to be 2000 Ω and 800 Ω, respectively.

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.

RE 2

=



RD 

S

−

S

60

mV

−

1.0024





and RE1 = 412 RE2

Figure 14 shows a scheme for adjusting the modified span and

4 mA offset via RE3 and RE4. The trim procedure is to first

connect both signal inputs to the 6.2 V Reference, set RE4 to

zero and then adjust RE3 so that 4 mA flows in the current loop.

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 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 kΩ 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.

Figure 14. Adjusting for Spans between 60 mV and

100 mV (RE1 and RE2) with Fine-Scale Adjust (RE3 and RE4)

After adjusting RE3 place the desired full scale (S) across the

signal inputs and adjust RE4 so that 20 mA flows in the current

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.

Input Spans Between 60 mV and 100 mV

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

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

transmitter where the precalibrated span is 37.5 mV. Con-

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

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

V/I converter. This can be accomplished with two resistors and and excitation is unchanged from the two-wire mode except for

adjusted via the optional trim scheme shown in Figure 14. The the 25% increase in span. Many sensors are ratiometric so that

resistor divider formed by RE1 and RE2 from the output of the

Signal Amplifier modifies the differential input voltage range

an increase in excitation can be used instead of a span

adjustment.

applied to the V/I converter.

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

In order to determine the fixed resistor values, RE1 and RE2, first

measure the source resistance (RD) of the internal divider network.

This can be accomplished (power supply disconnected) by

easier. Voltage compliance at the IIN terminal is also improved;

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

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

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

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

to common. The measured value, RD, is then used to calculate

RE1 and RE2 via the following formula:

resistance. Note that the load resistor, RL, should meter the

current into Pin 10, IIN, so as not to confuse the loop current

with the local power supply current.

–8–

REV. A

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