adl5372 Analog Devices, Inc., adl5372 Datasheet - Page 14

• Manufacturer Part Number: adl5372
• Description: 1500 Mhz To 2500 Mhz Quadrature Modulator
• Manufacturer: Analog Devices, Inc.

ADL5372
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
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.
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.
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
AD9779
OUT1_N
OUT2_N
OUT1_P
OUT2_P
92
93
84
83
RBQN
RBQP
RBIP
RBIN
50Ω
50Ω
50Ω
50Ω
AD9779
23
24
TxDAC is shown
19
20
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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.
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 the
AC 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.
AD9779
Figure 31. Relationship Between the AC Swing-Limiting Resistor and the
OUT1_P
OUT1_N
OUT2_N
OUT2_P
Figure 30. AC Voltage Swing Reduction Through the Introduction
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors
0
10
93
92
84
83
of a Shunt Resistor Between Differential Pair
RBQN
RBQP
RBIN
RBIP
50Ω
50Ω
50Ω
50Ω
100
R
L
(Ω)
RSLQ
100Ω
100Ω
RSLI
1000
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24
19
20
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QBBP
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10000