TO DAC CORE 4
0: RISING OR FALLING EDGE
TRIGGERED FOR I OR Q DATA.
1, 2, 3, 4: RISING EDGE TRIGGERED.
Figure 81. Simplified Diagram of AD9114/AD9115/AD9116/AD9117 Timing
DIGITAL DATA LATCHING AND RETIMER SECTION
The AD9114/AD9115/AD9116/AD9117 have two clock inputs,
DCLKIO and CLKIN. The CLKIN is the analog clock whose
jitter affects DAC performance, and the DCLKIO is a digital
clock, probably from an FPGA that needs to have a fixed
relationship with the input data to ensure that the data is
picked up correctly by the flip-flops on the pads.
Figure 81 is a simplified diagram of the entire data capture
system in the AD9114/AD9115/AD9116/AD9117. The double
data rate input data, DB[13:0], is latched at the pads/pins either
on the rising edge or the falling edge of the DCLKIO-INT clock, as
determined by IRISING, the SPI bit. IFIRST, the SPI bit, deter-
mines which channel data is latched first (that is, I or Q). The
captured data is then retimed to the internal clock (CLKIN-INT)
in the retimer block before being sent to the final analog DAC
core (D-FF (4)), which controls the current steering output
switches. All delay blocks depicted in Figure 81 are noninverting,
and any wires without an explicit delay block can be assumed
to have no delay for the purpose of understanding.
Only one channel is shown in Figure 81 with the DATA pads
(DB[13:0]) serving as double data rate pads for both channels.
The default PINMD and SPI settings are IE = high (closed) and
OE = low (open). These settings are enabled when RESET/PINMD
(Pin 35) is held high. In this mode, the user has to supply both
DCLKIO and CLKIN. In PINMD, it is also recommended that the
DCLKIO and the CLKIN be in-phase for proper functioning of
the DAC, which can easily be ensured by tying the pins together
on the PCB. If the user can access the SPI, settling IE low (that
is, IE is high) causes the CLKIN to be used as the DCLKIO also.
Settling OE high in the SPI allows the user to get a DCLKIO
output from the CLKIN input for use in the user’s PCB system.
It is strongly recommended that IE = OE = high not be used
even though the device may appear to function correctly.
The AD9114/AD9115/AD9116/AD9117 have an internal data
retimer circuit that compares the CLKIN-INT and DCLKIO-INT
clocks and, depending on their phase relationship, selects a
retimer clock (RETIMER-CLK) to safely transfer data from the
DCLKIO used at the chip’s input interface to the CLKIN used to
clock the analog DAC cores (D-FF (4)).
The retimer selects one of the three phases shown in Figure 82.
The retimer is controlled by the SPI bits is shown in Table 15.
1/4 PERIOD 1/2 PERIOD
Figure 82. RETIMER-CLK Phases
Note that in most cases, more than one retimer phase works,
and in such cases, the retimer arbitrarily picks one phase that
works. The retimer cannot pick the best or safest phase. If the
user has a working knowledge of the exact phase relationship
between DCLKIO and CLKIN (and thus DCLKIO-INT and
CLKIN-INT, because the delay is approximately the same for
both clocks and equal to DELAY1), then the retimer can be
forced to this phase with CLKMODEN = 1 as described in
Table 15 and the following paragraphs.
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