AD9148BPCZ AD [Analog Devices], AD9148BPCZ Datasheet - Page 44

• Manufacturer Part Number: AD9148BPCZ
• Description: Quad 16-Bit,1 GSPS, TxDAC+ Digital-to-Analog Converter
• Manufacturer: AD [Analog Devices]

AD9148
DEVICE SYNCHRONIZATION
SYNCHRONIZING MULTIPLE DEVICES
System demands may require that the outputs of multiple DACs
be synchronized with each other or with a system clock. Systems
that support transmit diversity or beam-forming, where multiple
antennas are used to transmit a correlated signal, require multiple
DAC outputs to be phase aligned with each other. Systems with
a time-division multiplexing transmit chain may require one or
more DACs to be synchronized with a system-level reference clock.
Multiple devices are considered synchronized to each other
when the state of the clock generation state machines is identical
for all parts and time aligned data is being read from the FIFOs
of all parts simultaneously. Devices are considered synchronized to
a system clock when there is a fixed and known relationship
between the clock generation state machine and the data being
read from the FIFO and a particular clock edge of the system
clock. The AD9148 has provisions for enabling multiple devices to
be synchronized to each other or to a system clock.
The AD9148 supports synchronization in two different modes,
data rate mode and FIFO rate mode. The two modes are
distinguished by the lowest rate clock that the synchronization
logic attempts to synchronize. In data rate mode, the input data
rate represents the lowest synchronized clock. In FIFO rate mode,
the FIFO rate, which is the data rate divided by the FIFO depth
of 8, represents the lowest rate clock. The advantage of the FIFO
SYSTEM CLOCK
Figure 52. Typical Circuit Diagram for Synchronizing Devices with Clock Multiplication Enabled
FPGA
CLOCK DRIVER
LOW SKEW
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MATCHED
rate synchronization is increased setup and hold times of DCI
relative to the CLK input. When in data rate synchronization
mode, the elasticity of the FIFO is not used to absorb timing
variations between the data source and DAC, resulting in
tighter setup and hold time requirements.
The method chosen for providing the DAC sampling clock directly
impacts the synchronization methods available. When the device
clock multiplier is used, only data rate synchronization is
available. When the DAC sampling clock is sourced directly,
both data rate mode and FIFO rate mode synchronization are
available.
SYNCHRONIZATION WITH CLOCK MULTIPLICATION
When using the clock multiplier to generate the DACCLK, the
REFCLK/SYNC input signal acts as both the reference clock for
the PLL-based clock multiplier and as the synchronization
signal. To synchronize devices, the REFCLK/ SYNC signal must
be distributed with low skew to all of the devices to be
synchronized. Skew between the REFCLK/SYNC signals of
different devices show up directly as a timing mismatch at the
DAC outputs.
The frequency of the REFCLK/SYNC signal is typically equal to
the input data rate. The FRAME signal and DCI signals can be
created in the FPGA along with the data. A circuit diagram of a
typical configuration is shown in Figure 52.
REFCLK/SYNC
FRAME
DCI
REFCLK/SYNC
FRAME
DCI
Preliminary Technical Data
OUT1
OUT2