AD9713JP 查看數據表（PDF） - Analog Devices
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AD9713JP Datasheet PDF : 8 Pages
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ANALOG DEVICES fAX-ON-DEMAND HOTLINE - Page 28
AD9712/AD9713
Outputs
The Switch Network controls complementary current outputs
lOUT and lOUT' As indicated earlier, DcD. are decoded into IS
"thermometer code" lines which drive matched current sources.
the DAC output as shown in Figure 5. Reducing DAC full-scale
outpUt current degrades both linearity and settling time; there-
fore, the current divider method is preferable.
Ds and D6 control weighted current sources; and D7-D12 are
applied to the R-2R network.
This segmentation reduces frequency domain errors due to'
glitch impulse. Current is steered to either lOUT or lOUT in pro-
portion to the digital input code. The sum of the two currents is
always equal to the full-scale output current minus one LSB.
The current output can be converted to a voltage by resistive
loading as shown in Figure 4. Both louT and lOUT should be
.V."'
R"
RL
v"'"
V'OLL 8C.ILE ~ .5V
Yu.a"" =-5V
loaded equally for best overall performance. The voltage which
RL
is developed is the product of the output current and the value
of the load resistor.
O.'"F*
-uv
OBSO - eeL
L DRIVE
ETE LOGIC
Figure 5. IN Conversion Using Current Feedback Amp
The DAC output is not clamped at virtual ground in this config-
uration because of the series resistance RFF' The value of RFF is
selected according to the equation:
- RLIFS
V Full Seal<
R FB
+ I OFF
RL
RFF =
V Full Scale
R FB
+ I OFF
As an example, assume the following conditions:
RL 0=50 n
RFB = 1.5 kG
IFs = 20.48 mA
IOFF---VZerosC'.oIe- - 33. mA
RFB
Given these conditions, RFF = 103.6 fl
S'lSTI!M
CAOuICI
Figure 4. Typical Resistive Load Connection
When operating at the nominal full.scale current of 20.48 mA,
the voltage swing will be from 0 to -1.024 V across SOfl resis-
tors. Bipolar outputs are possible by sourcing a current equal to
half the DAC full-scale current into the load resistor.
An alternate method of converting the current oUtput to voltage
is by driving the summing node of an operational amplifier
directly with a feedback resistor selected according [0 "the
equation:
RFS = VOUT(FSI / lOUT (FS)
A current feedback amplifier such as the AD9610 offers signifi.
cantly faster settling and greater bandwidth than a conventional
voltage feedback op amp. The feedback resistor for the AD9610
must be 1.5 kfl or greater to maintain stability. This value for
RFB' along with the 20.48 mA full-scale output current, results
in a full-scale output of 30 V, which exceeds the output range of
the AD9610.
Full-scale output voltage can be reduced by either reducing the
DAC's full-scale output current, or by using a current divider at
Power and Grounding
Maintaining low noise on power supplies and ground is critical
for obtaining optimum results with the AD9712 or AD9713.
DACs are most often used in circuits which are predominantly
digitaL To preserve 12-bit performance, especially at conversion
speeds up to 100 MSPS, special precautions are necessary for
power supplies and grounding.
Ideally, the DAC should have a separate analog ground plane.
All ground pins of the DAC, as well as reference and analog
output components, should be tied directly to this analog
ground plane. The DAC's ground plane should be connected to
the system ground plane at a single point.
Ferrite beads, along with high frequency, low inductance decou-
piing capacitors, should be used for the supply connections ro
isolate digital switching currents from the DAC supply pins.
Separate isolation networks for the digital and analog supply
connections will further reduce supply noise coupling to the
oUtput.
Molded socket assemblies should be avoided even when proto-
typing circuits with the AD9712 or AD9713. When the DAC
cannot be directly soldered into the board, individual pin sock-
ets such as AMP #6-330808-0 (knock-out end), or #60330808-3
(open end) should be used. These have much less effect on
interlead capacitance than do molded assemblies.
-6-
REV. A
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