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ADL5375-05ACPZ-WP Datasheet(PDF) 24 Page - Analog Devices
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ADL5375-05ACPZ-WP Datasheet(HTML) 24 Page - Analog Devices
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Rev. C | Page 24 of 36
The two pull-up inductors of the Zbias provide two 50 Ω source
impedances in combination with R1 resistor in parallel for the
filter. While the ADL5375 is specified to be driven by a single-
ended LO, the LOIP and LOIN input pins are naturally
differential. Therefore, the differential LO drive from the
ADF4350 is more desirable.
The output power from the ADF4350 can be set to −4 dBm,
−1 dBm,+2 dBm, and +5 dBm using Register 4 Bits[D2:D1] and
−6 dBm to +7 dBm LO drive level for ADL5375 is recommended.
If the physical distance between the PLL and the IQ modulator
is significant, the filter should be placed adjacent to the IQ
modulator, and two 50 Ω traces should be run between the
devices (since there is a 50 Ω impedance looking from each of
the filter inputs back to each of the PLL outputs).
The ADL5375 evaluation board can be reconfigured for
differential drive and also includes component pads in its LO
path to accommodate a harmonic filter. The ADF4350 evaluation
board can also be configured to provide a differential output and
can be connected directly to the ADL5375 evaluation board.
Optimizing the interface between a PLL LO and I/Q modulator
is discussed further in CN-0134 Broadband Low EVM Direct
Conversion Transmitter: How to Optimize the Interface
Between a PLL LO and I/Q Modulator.
DAC MODULATOR INTERFACING
Driving the ADL5375-05 with a TXDAC®
The ADL5375-05 is designed to interface with minimal
components to members of the Analog Devices, Inc. TxDAC
families. These dual-channel differential current output DACs
feature an output current swing from 0 mA to 20 mA. The
interface described in this section can be used with any DAC
that has a similar output.
An example of an interface using the AD9122 TxDAC is shown
in Figure 58. The baseband inputs of the ADL5375-05 require a
dc bias of 500 mV. The nominal midscale current on each of the
outputs of the AD9122 is 10 mA. Therefore, a single 50 Ω resis-
tor 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
Figure 58. Interface Between the AD9122 and ADL5375-05 with 50 Ω
Resistors to Ground to Establish the 500 mV DC Bias for the ADL5375-05
The AD9122 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 ADL5375-05 baseband inputs ranges from 0 V to
1 V. A full-scale sine wave out of the AD9122 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 59. It has the effect of
reducing the ac swing without changing the dc bias already
established by the 50 Ω resistors.
Figure 59. AC Voltage Swing Reduction Through the Introduction
of a Shunt Resistor Between Differential Pair
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