AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 38

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

AD9775
of the AD9775 while meeting other system level objectives (i.e.,
cost, power) is recommended. The op amp’s differential gain,
its gain setting resistor values, and full-scale output swing capa-
bilities should all be considered when optimizing this circuit. R
is only necessary if level shifting is required on the op amp out-
put. In Figure 53, AVDD, which is the positive analog supply for
both the AD9775 and the op amp, is also used to level shift the
differential output of the AD9775 to midsupply (i.e., AVDD/2).
INTERFACING THE AD9775 WITH THE AD8345
QUADRATURE MODULATOR
The AD9775 architecture was defined to operate in a transmit
signal chain using an image reject architecture. A quadrature
modulator is also required in this application and should be
designed to meet the output characteristics of the DAC as much
as possible. The AD8345 from Analog Devices meets many of
the requirements for interfacing with the AD9775. As with any
DAC output interface, there are a number of issues that have to
be resolved. Among the major issues are the following.
DAC Compliance Voltage/Input Common-Mode Range
The dynamic range of the AD9775 is optimal when the DAC
outputs swing between ± 1.0 V. The input common-mode range
of the AD8345, at 0.7 V, allows optimum dynamic range to be
achieved in both components.
Gain/Offset Adjust
The matching of the DAC output to the common-mode input
of the AD8345 allows the two components to be dc-coupled,
with no level shifting necessary. The combined voltage offset of
the two parts can therefore be compensated for via the AD9775
programmable offset adjust. This allows excellent LO cancella-
tion at the AD8345 output. The programmable gain adjust
allows for optimal image rejection as well.
The AD9775 evaluation board includes an AD8345 and recom-
mended interface (Figures 59 and 60). On the output of the
AD9775, R9 and R10 convert the DAC output current to a
voltage. R16 may be used to do a slight common-mode shift if
necessary. The (now voltage) signal is applied to a low pass
reconstruction filter to reject DAC images. The components
installed on the AD9775 provide a 35 MHz cutoff, but may be
changed to fit the application. A balun (Mini-Circuits ADTL1-12)
is used to cross the ground plane boundary to the AD8345.
Another balun (Mini-Circuits ETC1-1-13) is used to couple
the LO input of the AD8345. The interface requires a low ac
impedance return path from the AD8345, so a single connec-
tion between the AD9775 and AD8345 ground planes is
recommended.
The performance of the AD9775 and AD8345 in an image reject
transmitter, reconstructing three WCDMA carriers, can be seen
in Figure 54. The LO of the AD8345 in this application is 800 MHz.
Image rejection (50 dB) and LO feedthrough (–78 dBFS) have
been optimized with the programmable features of the AD9775.
The average output power of the digital waveform for this test
was set to –15 dBFS to account for the peak-to-average ratio of
the WCDMA signal.
Win95 and Win98 are a registered trademarks of Microsoft Corporation.
OPT
–38–
EVALUATION BOARD
The AD9775 evaluation board allows easy configuration of the
various modes, programmable via the SPI port. Software is
available for programming the SPI port from either Win95
Win98
ture modulator and support circuitry that allows the user to
optimally configure the AD9775 in an image reject transmit
signal chain.
Figures 55 through 58 describe how to configure the evaluation
board in the one and two port input modes with the PLL
enabled and disabled. Refer to Figures 59 through 68, the
schematics, and the layout for the AD9775 evaluation board for
the jumper locations described below. The AD9775 outputs can
be configured for various applications by referring to the follow-
ing instructions.
DAC Single-Ended Outputs
Remove transformers T2 and T3. Solder jumper links JP4 or
JP28 to look at the DAC1 outputs. Solder jumper links JP29 or
JP30 to look at the DAC2 outputs. Jumpers 8 and 13–17 should
remain unsoldered. The jumpers JP35–JP38 may be used to
ground one of the DAC outputs while the other is measured
single-ended. Optimum single-ended distortion performance is
typically achieved in this manner. The outputs are taken from
S3 and S4.
DAC Differential Outputs
Transformers T2 and T3 should be in place. Note that the lower
band of operation for these transformers is 300 kHz to 500 kHz.
Jumpers 4, 8, 13–17, and 28–30 should remain unsoldered. The
outputs are taken from S3 and S4.
Using the AD8345
Remove transformers T2 and T3. Jumpers JP4 and 28–30 should
remain unsoldered. Jumpers 13–16 should be soldered. The
desired components for the low pass interface filter L6, L7, C55,
and C81 should be in place. The LO drive is connected to the
AD8345 via J10 and the balun T4; and the AD8345 output is
taken from J9.
Figure 54. AD9775/AD8345 Synthesizing a Three-
Carrier WCDMA Signal at an LO of 800 MHz
®
–100
. The evaluation board also contains an AD8345 quadra-
–10
–20
–30
–40
–50
–60
–70
–80
–90
762.5
0
782.5
FREQUENCY – MHz
802.5
822.5
842.5
®
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