ADL5372 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADL5372' PDF : 24 Pages View PDF
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 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.
AD9779
93
OUT1_P
92
OUT1_N
RBIP
50Ω
RBIN
50Ω
F-MOD
19
IBBP
20
IBBN
OUT2_N
OUT2_P
84
RBQN
50Ω
RBQP
83 50Ω
23 QBBN
24
QBBP
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.
Data Sheet
AD9779
93
OUT1_P
92
OUT1_N
RBIP
50Ω
RBIN
50Ω
RSLI
100Ω
F-MOD
19
IBBP
20
IBBN
OUT2_N
OUT2_P
84
RBQN
50Ω
RBQP
83 50Ω
RSLQ
100Ω
23 QBBN
24
QBBP
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
RL (Ω)
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 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.
Rev. A | Page 14 of 24
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