Datasheet

AD9754
–13–
REV. A
INPUT CLOCK AND DATA TIMING RELATIONSHIP
SNR in a DAC is dependent on the relationship between the
position of the clock edges and the point in time at which the
input data changes. The AD9754 is positive edge triggered, and
so exhibits SNR sensitivity when the data transition is close to
this edge. In general, the goal when applying the AD9754 is to
make the data transitions close to the negative clock edge. This
becomes more important as the sample rate increases. Figure 23
shows the relationship of SNR to clock placement.
TIME (ns) OF DATA CHANGE RELATIVE TO
RISING CLOCK EDGE
68
–8
SNR – dB
64
60
56
52
48
44
40
–6 –4 –2
0
246810
F
S
= 125MSPS
F
S
= 65MSPS
Figure 23. SNR vs. Clock Placement @ f
OUT
= 10 MHz
SLEEP MODE OPERATION
The AD9754 has a power-down function that turns off the
output current and reduces the supply current to less than
8.5 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. This digital input also con-
tains an active pull-down circuit that ensures the AD9754 re-
mains enabled if this input is left disconnected. The AD9754
takes less than 50 ns to power down and approximately 5 µs to
power back up.
POWER DISSIPATION
The power dissipation, P
D
, of the AD9754 is dependent on
several factors, including: (1) AVDD and DVDD, the power
supply voltages; (2) I
OUTFS
, the full-scale current output; (3)
f
CLOCK
, the update rate; and (4) the reconstructed digital input
waveform. The power dissipation is directly proportional to the
analog supply current, I
AVDD
, and the digital supply current,
I
DVDD
. I
AVDD
is directly proportional to I
OUTFS,
as shown in
Figure 24, and is insensitive to f
CLOCK
.
Conversely, I
DVDD
is dependent on both the digital input wave-
form, f
CLOCK
, and digital supply DVDD. Figures 25 and 26
show I
DVDD
as a function of full-scale sine wave output ratios
(f
OUT
/f
CLOCK
) for various update rates with DVDD = 5 V and
DVDD = 3 V, respectively. Note, how I
DVDD
is reduced by more
than a factor of 2 when DVDD is reduced from 5 V to 3 V.
I
OUTFS
– mA
35
5
2204 6 8 10 12 14 16 18
30
25
20
15
10
I
AVDD
– mA
Figure 24. I
AVDD
vs. I
OUTFS
RATIO (f
CLOCK
/f
OUT
)
18
16
0
0.01 10.1
I
DVDD
– mA
8
6
4
2
12
10
14
5MSPS
25MSPS
50MSPS
100MSPS
125MSPS
Figure 25. I
DVDD
vs. Ratio @ DVDD = 5 V
RATIO (f
CLOCK
/f
OUT
)
8
0
0.01 10.1
I
DVDD
– mA
6
4
2
5MSPS
25MSPS
50MSPS
100MSPS
125MSPS
Figure 26. I
DVDD
vs. Ratio @ DVDD = 3 V