Datasheet

AD9734/AD9735/AD9736
Rev. A | Page 35 of 72
THEORY OF OPERATION
The AD9736, AD9735, and AD9734 are 14-bit, 12-bit, and
10-bit DACs that run at an update rate up to 1.2 GSPS. Input
data can be accepted up to the full 1.2 GSPS rate, or a 2×
interpolation filter can be enabled (2× mode) allowing full
speed operation with a 600 MSPS input data rate. The DATA
and DATACLK_IN inputs are parallel LVDS, meeting the IEEE
reduced swing LVDS specifications with the exception of input
hysteresis. The DATACLK_IN input runs at one-half the input
DATA rate in a double data rate (DDR) format. Each edge of
DATACLK_IN transfers DATA into the AD9736, as shown in
Figure 79.
The DACCLK−/DACCLK+ inputs (Pin E1 and Pin F1) directly
drive the DAC core to minimize clock jitter. The DACCLK
signal is also divided by 2 (1× and 2× mode), then output as the
DATACLK_OUT. The DATACLK_OUT signal clocks the data
source. The DAC expects DDR LVDS data (DB<13:0>) aligned
with the DDR input clock (DATACLK_IN) from a circuit simi-
lar to the one shown in
Figure 96. Table 19 shows the clock
relationships.
Table 19. AD973x Clock Relationship
MODE DACCLK DATACLK_OUT DATACLK_IN DATA
1× 1.2 GHz 600 MHz 600 MHz 1.2 GSPS
2× 1.2 GHz 600 MHz 300 MHz 600 MSPS
Maintaining correct alignment of data and clock is a common
challenge with high speed DACs, complicated by changes in
temperature and other operating conditions. Using the
DATACLK_OUT signal to generate the data allows most of the
internal process, temperature, and voltage delay variation to be
cancelled. The AD973x further simplifies this high speed data
capture problem with two adaptive closed-loop timing
controllers.
One timing controller manages the LVDS data and data clock
alignment (LVDS controller), and the other manages the LVDS
data and DACCLK alignment (sync controller).
The LVDS controller locates the data transitions and delays the
DATACLK_IN so that its transition is in the center of the valid
data window. The sync controller manages the FIFO that moves
data from the LVDS DATACLK_IN domain to the DACCLK
domain.
Both controllers can operate in manual mode under external
processor control, in surveillance mode where error conditions
generate external interrupts, or in automatic mode where errors
are automatically corrected.
The LVDS and sync controllers include moving average filtering
for noise immunity and variable thresholds to control activity.
Normally, the controllers are set to run in automatic mode,
making any necessary adjustments without dropping or dupli-
cating samples sent to the DAC. Both controllers require initial
calibration prior to entering automatic update mode.
The AD973x analog output changes 35 DACCLK cycles after
the input data changes in 1× mode with the FIFO disabled. The
FIFO adds up to eight additional cycles of delay. This delay is
read from the SPI port. Internal clock delay variation is less
than a single DACCLK cycle at 1.2 GHz (833 ps).
Stopping the AD973x DATACLK_IN while the DACCLK is still
running can lead to unpredictable output signals. This occurs
because the internal digital signal path is interleaved. The last
two samples clocked into the DAC continue to be clocked out
by DACCLK even after DATACLK_IN has stopped. The result-
ing output signal is at a frequency of one-half f
DAC,
and the
amplitude depends on the difference between the last two
samples.
Control of the AD973x functions is via the serially programmed
registers listed in
Table 9. Optionally, a limited number of func-
tions can be directly set by external pins in pin mode.