ADL5372
Rev. 0 | Page 14 of 24
APPLICATIONS INFORMATION
DAC MODULATOR INTERFACING
The ADL5372 is designed to interface with minimal components
to members of the Analog Devices family of DACs. These DACs
feature an output current swing from 0 to 20 mA, and the
interface described in this section can be used with any DAC
that has a similar output.
Driving the ADL5372 with a TxDAC
An example of the interface using the
AD9779 TxDAC is shown
in
Figure 29. The baseband inputs of the ADL5372 require a dc
bias of 500 mV. The average output current on each of the
outputs of the
AD9779 is 10 mA. Therefore, a single 50 Ω
resistor to ground from each of the DAC outputs results in an
average current of 10 mA flowing through each of the resistors,
thus producing the desired 500 mV dc bias for the inputs to the
ADL5372.
RBIP
50
RBIN
50
93
92
19
20
OUT1_N
OUT1_P
IBBN
IBBP
AD9779
F-MOD
RBQN
50
RBQP
50
84
83
23
24
OUT2_P
OUT2_N
QBBP
QBBN
06
51
1-
0
29
Figure 29. Interface Between the
AD9779 and ADL5372 with 50 Ω Resistors to
Ground to Establish the 500 mV DC Bias for the ADL5372 Baseband Inputs
The
AD9779 output currents have a swing that ranges from 0 to
20 mA. With the 50 Ω resistors in place, the ac voltage swing
going into the ADL5372 baseband inputs ranges from 0 V to 1 V.
A full-scale sine wave out of the
AD9779 can be described as a
1 V p-p single-ended (or 2 V p-p differential) sine wave with a
500 mV dc bias.
LIMITING THE AC SWING
There are situations in which it is desirable to reduce the ac
voltage swing for a given DAC output current. This can be
achieved through the addition of another resistor to the interface.
This resistor is placed in the shunt between each side of the
differential pair, as shown in
Figure 30. It has the effect of
reducing the ac swing without changing the dc bias already
established by the 50 Ω resistors.
RBIP
50
RBIN
50
93
92
19
20
IBBN
IBBP
AD9779
F-MOD
RBQN
50
RBQP
50
84
83
23
24
RSLI
100
RSLQ
100
OUT1_N
OUT1_P
OUT2_P
OUT2_N
QBBP
QBBN
06
51
1-
0
30
Figure 30. AC Voltage Swing Reduction Through the Introduction
of a Shunt Resistor Between Differential Pair
The value of this ac voltage swing limiting resistor is chosen
based on the desired ac voltage swing.
Figure 31 shows the
relationship between the swing-limiting resistor and the peak-
to-peak ac swing that it produces when 50 Ω bias-setting
resistors are used.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
10
100
1000
10000
D
IFF
E
R
E
N
TI
A
L
S
W
IN
G
(
V
p-
p
)
RL ()
065
11
-03
1
Figure 31. Relationship Between the AC Swing-Limiting Resistor and the
Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors
FILTERING
It is necessary to low-pass filter the DAC outputs to remove
images when driving a modulator. The interface for setting up
the biasing and ac swing that was discussed in the
Limiting theAC Swing section lends itself well to the introduction of such a
filter. The filter can be inserted between the dc bias setting
resistors and the ac swing-limiting resistor. Doing so establishes
the input and output impedances for the filter.