AD693

REV. A

–7–

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.

Use a similar connection with the following resistances for

adjustments at the 12 mA tap.

Ω and

× 3.1 V/(45 mV + I

A

× 3.75 Ω)

These formulae take into account the

±10% tolerance of tap

of

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

maximum value of:

µA – 400 Ω = 7.6 kΩ and

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

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

Available Also)

These can be rounded down to more convenient values of

7.5 k

Ω 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

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.

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

by:

R

400

Ω

30 mV

S

−1

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

6 mV a value of:

R

400

Ω

30 mV

6 mV

−1

= 100 Ω

is required. Since the internal, 800

Ω gain setting resistors

with up to

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

controlled.

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

400

Ω 1−

60 mV

S









30 mV

S

−1

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

R

400

Ω 1−

60 mV

40 mV









30 mV

40 mV

−1

= 800 Ω

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.