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AD693

Loop-Powered 4–20 mA Sensor Transmitter

Analog Devices 

AD693

Figure 11. Using an External Pass Transistor to Minimize Self-Heating Errors

of adjustment of the output current from nominal. Substitute

this value in the appropriate formula below for adjustment at the

4 mA tap.

RZ1 = (1.6 V/IA) – 400 Ω and

RZ2 = RZ1 × 3.1 V/(15 mV + IA × 3.75 Ω)

Use a similar connection with the following resistances for

adjustments at the 12 mA tap.

RZ1 = (4.8 V/IA) – 400 Ω and

RZ2 = RZ1 × 3.1 V/(45 mV + IA × 3.75 Ω)

These formulae take into account the ± 10% tolerance of tap

resistance and insure a minimum adjustment range of IA. For

example, choosing IA = 200 µA will give a zero adjustment range

of ± 1% of the 20 mA full-scale output. At the 4 mA tap the

maximum value of:

RZ1 = 1.6 V/200 µA – 400 Ω = 7.6 kΩ and

RZ2 = 7.6 kΩ × 3.1 V/(15 mV + 200 µA × 3.75 Ω) = 1.49 MΩ

0-to-75 mV signal in the 0-to-20 mA mode). The gain of this

amplifier is trimmed to 2.00 so that an input signal ranging from

0-to-30 mV will drive the V/I section to produce 4-to -20 mA.

Joining P1 and P2 (Pins 15 and 16) will reduce the Signal Ampli-

fier gain to one, thereby requiring a 60 mV signal to drive the V/I

to a full 20 mA span.

To produce spans less than 30 mV, an external resistor, RS1, can

be connected between P1 and 6.2 V. The nominal value is given

by:

RS1 =

400 Ω

30 mV − 1

S

where S is the desired span. For example, to change the span to

6 mV a value of:

RS1

=

400 Ω

30 mV − 1

= 100

Ω

6 mV

is required. Since the internal, 800 Ω gain setting resistors

exhibit an absolute tolerance of 10%, RS1 should be provided

with up to ± 10% range of adjustment if the span must be well

controlled.

For spans between 30 mV and 60 mV a resistor RS2 should be

connected between P1 and P2. The nominal value is given by:

RS2

=

400

Ω

1−

60 mV

S

30 mV −1





S

For example, to change the span to 40 mV, a value of:

Figure 12. Optional 4 mA Zero Adjustment (12 mA Trim

Available Also)

These can be rounded down to more convenient values of

7.5 kΩ and 1.3 MΩ, which will result in an adjustment range

comfortably greater than ± 200 µA.

ADJUSTING INPUT SPAN

Input Span is adjusted by changing the gain of the Signal

Amplifier. This amplifier provides a 0-to-60 mV signal to the

V/I section to produce the 4-to-20 mA output span (or a

RS 2

=

400

Ω

1

−

60

40

mV

mV

30 mV − 1





= 800

Ω

40 mV

is required. Remember that this is a nominal value and may

require adjustment up to ± 10%. In many applications the span

must be adjusted to accommodate individual variations in the

sensor as well as the AD693. The span changing resistor should,

therefore, include enough adjustment range to handle both the

sensor uncertainty and the absolute resistance tolerance of P1

and P2. Note that the temperature coefficient of the internal

resistors is nominally –17 ppm/°C, and that the external

resistors should be comparably stable to insure good tempera-

ture performance.

REV. A

–7–

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