ADF4112 AD [Analog Devices], ADF4112 Datasheet - Page 13

• Manufacturer Part Number: ADF4112
• Description: 300 MHz to 1000 MHz Quadrature Modulator
• Manufacturer: AD [Analog Devices]

APPLICATIONS INFORMATION
DAC MODULATOR INTERFACING
The ADL5370 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 ADL5370 with an Analog Devices TxDAC®
An example of the interface using the
in Figure 31. The baseband inputs of the ADL5370 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
ADL5370.
Figure 29. Interface Between the AD9779 and ADL5370 with 50 Ω Resistors to
The AD9779 output currents have a swing that ranges from 0 mA
to 20 mA. With the 50 Ω resistors in place, the ac voltage swing
going into the ADL5370 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 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.
Ground to Establish the 500 mV DC Bias for the ADL5370 Baseband Inputs
AD9779
OUT1_P
OUT1_N
OUT2_N
OUT2_P
93
92
84
83
RBQN
RBQP
RBIN
RBIP
50Ω
50Ω
50Ω
50Ω
AD9779
TxDAC is shown
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ADL5370
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QBBP
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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.
An example is shown in Figure 32 with a third-order elliptical
filter with a 3 dB frequency of 3 MHz. Matching input and output
impedances makes the filter design easier, so the shunt resistor
chosen is 100 Ω, producing an ac swing of 1 V p-p differential.
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
Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
10
92
93
84
83
of a Shunt Resistor Between Differential Pair
RBQN
RBQP
RBIP
RBIN
50Ω
50Ω
50Ω
50Ω
100
R
L
(Ω)
RSLQ
100Ω
100Ω
RSLI
1000
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ADL5370
ADL5370
QBBN
QBBP
IBBN
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10000