Datasheet

DAC7731
15
SBAS249B
www.ti.com
V
CC
REF
OUT
REF
IN
REFADJ
V
REF
R
OFFSET
AGND
RFB2
RFB1
SJ
V
OUT
V
DD
V
SS
REFEN
RSTSEL
SCLK
CS
SDO
SDI
LDAC
RST
NC
TEST
DGND
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
DAC7731
100kΩ
1.0µF
(Other connections omitted for clarity.)
Low-Pass Reference Filter
FIGURE 14. Filtering the Internal Reference.
Figure 12 illustrates the typical minimum offset adjustment
ranges provided by forcing a current at SJ for a given output
voltage configuration.
OUTPUT R
POT2
R
1
R
S
I
SJ
NOMINAL
CONFIGURATION RANGE OFFSET
ADJUSTMENT
0V to +10V 10K 0 2.5M ±2µA ±25mV
–10V to +10V 10K 5K 1.5M ±2.2µA ±55mV
–5V to +5V 10K 20K 1M ±1.7µA ±21mV
TABLE V. Recommended External Component Values for
Symmetrical Offset Adjustment (V
REF
= 10V).
OFFSET ADJUST RANGE
–10V to +10V V
OUT
Configuration
min (75% of typ)
min (75% of typ)
typ
typ
50
25
0
–25
–50
Offset Adjustment at V
OUT
(mV)
–220–11
I
SJ
(µA)
0V to 10V and –5V to +5V
V
OUT
Configuration
FIGURE 12. Offset Adjustment Transfer Characteristic.
GAIN ADJUSTMENT
When using the internal reference of the DAC7731, gain
adjustment is performed by adjusting the device’s internal
reference voltage via the reference adjust pin, REFADJ. The
effect of a reference voltage change on the gain of the DAC
output can be seen in the generic equation (for unipolar
configuration):
V
OUT
= V
REFIN
• (N/65536)
Where N is represented in decimal format and ranges from
0 to 65535.
REFADJ can be driven by a low impedance voltage source
such as a unipolar, 0V to +10V DAC or a potentiometer (less
than 100kΩ), see Figure 11. Since the input impedance of
REFADJ is typically 50kΩ, the smaller the resistance of the
potentiometer, the more linear the adjustment will be. A 10kΩ
potentiometer is suggested if linearity of the reference adjust-
ment is of concern.
When the DAC7731’s internal reference is not used, gain
adjustments can be made via trimming the external refer-
ence applied to the DAC at REF
IN
. This can be accomplished
through using a potentiometer, unipolar DAC, or other means
of precision voltage adjustment to control the voltage pre-
sented to the DAC7731 by the external reference. Figure 13
and Table VI summarize the range of adjustment of the
internal reference via REFADJ.
REF
OUT
ADJUST RANGE
40
30
20
10
0
–10
–20
–30
–40
REF
OUT
Adjustment (mV)
0246810
REFADJ (V)
Typical REF
OUT
Adjustment Range
Minimum REF
OUT
Adjustment Range
FIGURE 13. Internal Reference Adjustment Transfer Charac-
teristic.
VOLTAGE AT REFADJ REF
OUT
VOLTAGE
REFADJ = 0V 10V + 25mV (min)
REFADJ = 5V or NC
(1)
10V
REFADJ = 10V 10V – 25mV (max)
NOTE: NC = Not Connected.
TABLE VI. Minimum Internal Reference Adjustment Range.
NOISE PERFORMANCE
Increased noise performance of the DAC output can be
achieved by filtering the voltage reference input to the DAC7731.
Figure 14 shows a typical internal reference filter schematic. A
low-pass filter applied between the REF
OUT
and REF
IN
pins can
increase noise immunity at the DAC and output amplifier. The
REF
OUT
pin can source a maximum of 50µA so care should be
taken in order to avoid overloading the internal reference output.