Datasheet

REV. B
ADDAC80/ADDAC85/ADDAC87
–12–
DRIVING A RESISTOR LOAD BIPOLAR
The equivalent output circuit for a bipolar output voltage range
is shown in Figure 13, R
L
= R
LI
+ R
LS
. V
OUT
is determined by:
VmA
Rk
Rk
OUT
L
L
=±
×
+






1
322
322
.
.
Ω
Ω
(2)
where R
L
max = 11.18 kΩ and V
OUT
max = ±2.5 V
To achieve specified drift, connect the internal scaling resistors
(R
LI
) as shown in Table IV for the COB or CTC codes and add
an external metal film resistor (R
LS
) in series to obtain a full scale
output range of ±1 V. In this configuration, with R
LS
equal to
zero, the full scale range will be ±0.874 V.
1mA
CURRENT CONTROLLED
BY DIGITAL INPUT
3.22k
R
LI
1.2k
COMMON
V
OUT
R
LS
+
–
15
20
21
Figure 13. ADDAC80-CBI-I Connected for Bipolar
Output Voltage with Resistive Load
DRIVING AN EXTERNAL OP AMP
The current model ADDAC80 will drive the summing junction
of an op amp used as a current to voltage converter to produce
an output voltage. As seen in Figure 14,
VIR
OUT OUT F
=×
(3)
where I
OUT
is the ADDAC80 output current and R
F
is the feed-
back resistor. Using the internal feedback resistors of the current
model ADDAC80 provides output voltage ranges the same as
the voltage model ADDAC80. To obtain the desired output
voltage range when connecting an external op amp, refer to
Table V and Figure 14.
20V RANGE
5k
I
0 TO 2mA
6.6k
5k CBI
10V RANGE
AD509KH*
V
OUT
*FOR FAST SETTLING TIME
19
18
15
21
A
Figure 14. External Op Amp Using Internal
Feedback Resistors
OUTPUT LARGER THAN 20 V RANGE
For output voltage ranges larger than ± 10 V, a high voltage op
amp may be employed with an external feedback resistor. Use
I
OUT
values of ±l mA for bipolar voltage ranges and –2 mA for
unipolar voltage ranges (see Figure 15). Use protection diodes
when a high voltage op amp is used.
The feedback resistor, R
F
, should have a temperature coefficient
as low as possible. Using an external feedback resistor, overall
drift of the circuit increases due to the lack of temperature track-
ing between R
F
and the internal scaling resistor network. This will
typically add 50 ppm/°C + R
F
drift to total drift.
17
16
15
24
21
V
+
–
I
0 TO 2mA
V
REF
6.3V
6.6k
6.3k
*FOR OUTPUT VOLTAGE SWINGS UP TO 140V p-p
V
OUT
R
F
171K*
Figure 15. External Op Amp Using External
Feedback Resistors
Table IV. Current Model/Resistive Load Connections
1%
Metal Film R
LI
Connections Reference Bipolar Offset
Internal External
Digital Output Resistance Resistance Connect Connect Connect Connect Connect
Input Codes Range R
LI
(k)R
LS
Pin 15 to Pin 18 to Pin 20 to Pin 16 to Pin 17 to R
LS
CSB 0 to –2 V 0.968 210 Ω 20 19 and R
LS
15 24 Com (21) Between
Pin 18 and
Com (21)
COB or CTC ±1 V 1.2 249 Ω 18 19 R
LS
24 15 Between
Pin 20 and
Com (21)
CCD 0 to ±2 V 3 N/A NC 21 NC 24 NC N/A