AD9774
–15–
REV. B
DVDD
DIGITAL
INPUT
Figure 32. Equivalent Digital Input
Since the AD9774 is capable of being updated up to 32 MSPS,
the quality of the clock and data input signals are important in
achieving the optimum performance. Operating the AD9774
with reduced logic swings and a corresponding digital supply
(DVDD) will result in the lowest data feedthrough and on-chip
digital noise. The drivers of the digital data interface circuitry
should be specified to meet the minimum setup and hold times
of the AD9774 as well as its required min/max input logic level
thresholds.
Digital signal paths should be kept short and run lengths matched
to avoid propagation delay mismatch. The insertion of a low
value resistor network (i.e., 20
to 100 ) between the AD9774
digital inputs and driver outputs may be helpful in reducing any
overshooting and ringing at the digital inputs that contribute to
data feedthrough.
The external clock driver circuitry should provide the AD9774
with a low jitter clock input meeting the min/max logic levels
while providing fast edges. Fast clock edges will help minimize
any jitter that will manifest itself as phase noise on a recon-
structed waveform. Thus, the clock input should be driven by
the fastest logic family suitable for the application.
SLEEP AND SNOOZE MODE OPERATION
The AD9774 has a SLEEP function that turns off the output
current and reduces the supply current to less than 5 mA over
the specified supply range of 2.7 V to 5.5 V and temperature
range. This mode can be activated by applying a logic level “1”
to the SLEEP pin. The AD9774 takes less than 0.1
s to power
down and approximately 6.4
s to power back up.
The SNOOZE mode should be considered as an alternative
power-savings option if the power-up characteristics of the
SLEEP mode are unsuitable. This mode, which is also activated
by applying a logic level “1” to the SNOOZE pin, disables the
AD9774’s digital filters only, resulting in significant power
savings. Both the SLEEP and SNOOZE pins should be tied to
DCOM if power savings is not required.
POWER DISSIPATION
The power dissipation, PD, of the AD9774 is dependent on
several factors, including: (1) AVDD, PLLVDD, and DVDD,
the power supply voltages; (2) IOUTFS, the full-scale current
output; (3) fCLOCK, the update rate; and (4) the reconstructed
digital input waveform. The power dissipation is directly pro-
portional to the analog supply current, IAVDD, and the digital
supply current, IDVDD. IAVDD is directly proportional to IOUTFS,
as shown in Figure 33, and is insensitive to fCLOCK.
Conversely, IDVDD is dependent on both the digital input wave-
form, fCLOCK, and digital supply DVDD. Figures 34 and 35
show IDVDD as a function of full-scale sine wave output ratios
(fOUT/fCLOCK) for various update rates with DVDD = 5 V and
DVDD = 3 V, respectively. Note, how IDVDD is reduced by more
than a factor of 2 when DVDD is reduced from 5 V to 3 V.
IOUTFS – mA
30
0
220
4
6
8
1012
141618
25
20
15
10
5
I AVDD
–
mA
Figure 33. IAVDD vs. IOUTFS
RATIO –
fOUT/fCLOCK
200
180
20
0.01
1.0
0.10
I DVDD
–
mA
100
80
60
40
140
120
160
0
32MSPS
16MSPS
8MSPS
4MSPS
Figure 34. IDVDD vs. Ratio @ DVDD = 5 V
RATIO –
fOUT/fCLOCK
100
0
0.01
1.0
0.10
I DVDD
–
mA
90
50
80
70
60
40
30
20
10
32MSPS
16MSPS
8MSPS
4MSPS
Figure 35. IDVDD vs. Ratio @ DVDD = 3 V
For those applications requiring the AD9774 to operate under the
following conditions: (1) AVDD, PLLVDD and DVDD = +5 V;
(2) fCLOCK > 25 MSPS; and (3) ambient temperatures > 70°C;
proper thermal management via a heatsink or thermal epoxy is
recommended.