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.
R
Z1
=
(1.6
V/I
A
) – 400
Ω
and
R
Z2
=
R
Z1
×
3.1
V/(15 mV
+
I
A
×
3.75
Ω)
Use a similar connection with the following resistances for
adjustments at the 12 mA tap.
R
Z1
= (4.8
V/I
A
) – 400
Ω
and
R
Z2
= R
Z1
×
3.1
V/(45 mV
+
I
A
×
3.75
Ω)
These formulae take into account the
±
10% tolerance of tap
resistance and insure a minimum adjustment range of I
A
. For
example, choosing I
A
= 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:
R
Z1
= 1.6 V/200
µA
– 400
Ω
= 7.6 kΩ and
R
Z2
= 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, R
S1
, can
be connected between P1 and 6.2 V. The nominal value is given
by:
400
Ω
R
S1
=
30
mV
−
1
S
where
S
is the desired span. For example, to change the span to
6 mV a value of:
400
Ω
R
S1
=
=
100
Ω
30
mV
−
1
6
mV
is required. Since the internal, 800
Ω
gain setting resistors
exhibit an absolute tolerance of 10%, R
S1
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 R
S2
should be
connected between P1 and P2. The nominal value is given by:
60
mV
400
Ω
1−
S
R
S2
=
30
mV
−1
S
For example, to change the span to 40 mV, a value of:
60
mV
400
Ω
1
−
40
mV
=
=
800
Ω
30
mV
−
1
40
mV
R
S2
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
REV. A
–7–
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.
AD [ ANALOG DEVICES ]
