Analog Devices ADRV9001 User Manual page 278

Reference Manual ADRV9001
PRINTED CIRCUIT BOARD LAYOUT RECOMMENDATIONS
Figure 261. ADRV9001 DC Bias Configuration for the Transmitter Output Using Wire-Wound Chokes
Figure 262. ADRV9001 DC Bias Configuration for the Transmitter Output Using a Center-Tapped Transformer
The ADRV9001 evaluation board provides flexibility to configure each transmitter output to work with either a center-tapped transformer (balun)
or a set of two closely matched wire-wounded chokes. The center-tapped transformer passes the bias voltage directly to the transmitter outputs
through each differential input. This configuration offers the lowest component count.
In some cases, the desired balun does not provide a DC connection to the transmitter output lines. To support this situation, the ADRV9001
evaluation board provides the placeholders for RF chokes connected to the VANA1_1P8 (for Tx1 output) and VANA2_1P8 (for Tx2 output)
supply. It also provides the placeholders for AC-coupling capacitors to prevent creating a DC short through the balun to ground.
Impedance matching networks on the balun single-ended port are usually required to achieve optimum performance. In addition, AC-coupling is
often required on the single-ended side if the balun contains a DC path from one of the transmitter's differential outputs to the single-ended port.
Careful planning is required to select the transmitter balun. If a transmitter balun is selected that requires a set of external DC bias chokes, it is
necessary to find the optimum compromise between the choke physical size, choke DC resistance (RDCR), and the balun pass band insertion
loss. Refer to the relevant section of this document for more information on selecting the transmitter output balun and RF choke as well as
recommendations for matching circuit.
Transmitter Balun DC Supply Options
Each transmitter requires approximately 200 mA supplied through an external connection. The PCB layout of the ADRV9001 board allows the
use of external chokes to provide 1.8 V power domain to the ADRV9001 outputs. This allows users to try different baluns that do not have a DC
center tap pin to supply the bias voltage to the transmitter outputs.
To reduce switching transients when attenuation settings change, the balun DC feed is powered directly by the 1.8 V plane. The geometry of
the 1.8 V plane is designed so that each balun or each pair of chokes is associated with its transmitter output. The VANA1_1P8 is used to
power the Tx1 output and VANA2_1P8 is used to power the Tx2 output.
If careful layout and isolation of the DC supply is not followed, it can adversely affect transmitter-transmitter isolation. Figure 263 shows the
power-supply layout configuration used on the ADRV9001 board to achieve the desired transmitter-transmitter isolation performance. This
image illustrates star connection from the common 1.8 V analog power plane.
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