AD9781 Analog Devices, AD9781 Datasheet - Page 28

• Manufacturer Part Number: AD9781
• Description: Dual 14-Bit, LVDS Interface 500 MSPS DAC
• Manufacturer: Analog Devices

Specifications of AD9781

• Resolution (bits): 14bit
• Dac Update Rate: 500MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9780/AD9781/AD9783
impacted by this sinc roll-off depending on where the carrier is
placed, as shown in Figure 65.
Auxiliary DACs
Two auxiliary DACs are provided on the AD9780/AD9781/
AD9783. A functional diagram is shown in Figure 66. The
auxiliary DACs are current output devices with two output
pins, AUXP and AUXN. The active pin can be programmed to
either source or sink current. When either sinking or sourcing,
the full-scale current magnitude is 2 mA. The available compliance
range at the auxiliary DAC outputs depends on whether the output
is configured to sink or source current. When sourcing current,
the compliance voltage is 0 V to 1.6 V, but when sinking current,
the output compliance voltage is reduced to 0.8 V to 1.6 V. Either
output can be used, but only one output of the AUX DAC (P or
N) is active at any time. The inactive pin is always in a high
impedance state (>100 kΩ).
–10
–20
–30
–40
Figure 65. Transfer Function for Each Analog Operating Mode
0
0
0.5
NORMAL
MIX
(
1.0
f
S
)
RETURN-TO-ZERO
1
.5
2.0
Rev. A | Page 28 of 36
In a single sideband transmitter application, the combination of
the input referred dc offset voltage of the quadrature modulator
and the DAC output offset voltage can result in local oscillator
(LO) feedthrough at the modulator output, which degrades system
performance. The auxiliary DACs can be used to remove the dc
offset and the resulting LO feedthrough. The circuit configura-
tion for using the auxiliary DACs for performing dc offset
correction depends on the details of the DAC and modulator
interface. An example of a dc-coupled configuration with low-
pass filtering is shown in Figure 67.
Figure 67. DAC DC-Coupled to Quadrature Modulator with a Passive DC Shift
25Ω TO 50Ω
AD9783
DAC1 OR
V
DAC2
BIAS
0mA
2mA
0mA
2mA
TO
TO
Figure 66. Auxiliary DAC Functional Diagram
SOURCE
AD9783
DAC1 OR
FILTERING
OPTIONAL
SINK
DAC2
PASSIVE
OR
AUX
MODULATOR V+
QUADRATURE
NEGATIVE
POSITIVE
OR
25Ω TO 50Ω
QUAD MOD
I OR Q INPUTS
AUXP
AUXN