AD9734/AD9735/AD9736
Rev. A | Page 55 of 72
SYNCHRONIZATION TIMING
When more than one AD973x must be synchronized or when
a constant group delay must be maintained, the internal
controllers cannot be used. If the FIFO is enabled, the delay
between multiple AD973x devices is unknown. If the
DATACLK_OUT from multiple devices is used, there is an
uncertainty of two DACCLK periods because the initial phase
of DATACLK_OUT with respect to DACCLK cannot be
controlled. This means one DAC must be used to provide
DATACLK_OUT for all synchronized DACs and all timing
must be externally managed. The following timing information
allows system timing to be calculated so that multiple AD973xs
can be synchronized.
DATACLK_OUT changes relative to the rising edge of
DACCLK+ and is delayed, as shown in
Figure 101. Because
DACCLK is divided by 2 to create DATACLK_OUT, the phase
of DATACLK_OUT can be 0° or 180°. There is no way to
predict or control this relationship. It can be different after each
power cycle and is not affected by hardware or software resets.
tDDCO
DACCLK
DATACLK_OUT
04862-099
Figure 101. DACCLK to DATACLK_OUT Delay
The incoming data is de-interleaved internally as shown in
the de-interleaved data paths. Each edge of DATACLK_IN
latches an incoming sample in two alternating registers. The
DATACLK_IN to data setup and hold definitions are illustrated
in
Figure 102. All the data input must be valid during the setup-
and-hold period. External skew effectively increases the setup
and hold times that the data source must meet.
04862-
100
DATACLK_IN
OR DATACLK_OUT
DATA_IN
tDH
tDSU
Figure 102. Standard Definitions for DATACLK_IN or DATACLK_OUT to
Data Setup and Hold, SD = 0
While correct DATA_IN vs. DATACLK_IN timing is critical,
the transition of the incoming data to the DACCLK domain is
equally critical. By referencing the incoming DATA and
DATACLK_IN timing to the DATACLK_OUT signal, some
timing uncertainty can be removed. The DATACLK_OUT
timing very closely tracks the timing of the DACCLK-
controlled registers. Any variation in the path delay affects both
paths in almost the same way. If DATACLK_OUT is not used,
the full DACCLK to DATACLK_OUT path variation reduces
the external timing margin. Figure 101 shows a simplified view
of the internal clocking scheme with the relevant delay paths.
The internal architecture is interleaved such that each phase has
twice as long to make the transition across the clock domains.
This results in an extremely narrow window where the
incoming data must be held stable.
Figure 102. These parameters were measured for a sample of
five devices from five silicon lots. Worst-case fast and slow skew
lots were included in addition to the nominal (or average) lot.
The typical 40°C to typical +85°C spread illustrates the
variability with temperature for a single lot. Adding in lot-to-lot
variation with the fast and slow lots indicates the worst-case
spread in timing.
The timing varies such that all of the parameters move in the
same direction. For example, if the DATACLK_IN to data setup
time is fast, the hold time is similarly fast. The DACCLK to
DATACLK_OUT delay and the DATACLK_OUT to data setup
and hold is also at the fast end of the range.
Note that the polarities of setup-and-hold values in
Table 28conform to the standard convention of setup time occurring
prior to the latching edge and hold time occurring after the
Table 28. AD973x Clock and Data Timing Parameters
Symbol and Definition
Fast 40°C
Typ 40°C
All +25°C
Typ +85°C
Slow +85°C
Unit
tDDCO DACCLK to DATACLK_OUT Delay
+1650
+1800
+1890
+2050
+2350
ps
tDCISU DATACLK_IN to DATA Setup
100
120
150
170
220
ps
tDCIH DATACLK_IN to DATA Hold
+210
+220
+240
+280
+360
ps
tDISU DATACLK_OUT to DATA Setup
+1310
+1440
+1611
+1710
+1970
ps
tDIH DATACLK_OUT to DATA Hold
1250
1360
1548
1640
1890
ps