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AD9648
Rev. 0 | Page 30 of 44
Jitter Considerations
High speed, high resolution ADCs are sensitive to the quality
of the clock input. The degradation in SNR from the low fre-
quency SNR (SNRLF) at a given input frequency (fINPUT) due to
jitter (tJRMS) can be calculated by
SNRHF = 10 log[(2π × fINPUT × tJRMS)2 + 10
)
10
/
(
LF
SNR
]
In the previous equation, the rms aperture jitter represents the
clock input jitter specification. IF undersampling applications
are particularly sensitive to jitter, as illustrated in
Figure 59.80
75
70
65
60
55
50
45
1
10
100
1k
FREQUENCY (MHz)
S
NR
(d
B
F
S)
0.5ps
0.2ps
0.05ps
1.0ps
1.5ps
2.0ps
2.5ps
3.0ps
09975-
080
Figure 59. SNR vs. Input Frequency and Jitter
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of th
e AD9648.To avoid modulating the clock signal with digital noise, keep
power supplies for clock drivers separate from the ADC output
driver supplies. Low jitter, crystal-controlled oscillators make the
best clock sources. If the clock is generated from another type of
source (by gating, dividing, or another method), it should be
retimed by the original clock at the last step.
CHANNEL/CHIP SYNCHRONIZATION
T
he AD9648 has a SYNC input that offers the user flexible
synchronization options for synchronizing sample clocks
across multiple ADCs. The input clock divider can be enabled
to synchronize on a single occurrence of the SYNC signal or on
every occurrence. The SYNC input is internally synchronized
to the sample clock; however, to ensure there is no timing
uncertainty between multiple parts, the SYNC input signal should
be externally synchronized to the input clock signal, meeting the
setup and hold times shown in
Table 5. Drive the SYNC input
using a single-ended CMOS-type signal.
POWER DISSIPATION AND STANDBY MODE
the
AD9648 is proportional to its sample rate. The digital
power dissipation of the CMOS outputs are determined
primarily by the strength of the digital drivers and the load
on each output bit.
The maximum DRVDD current (IDRVDD) can be calculated as
IDRVDD = VDRVDD × CLOAD × fCLK × N
where N is the number of output bits (30, in the case of the
This maximum current occurs when every output bit switches
on every clock cycle, that is, a full-scale square wave at the Nyquist
frequency of fCLK/2. In practice, the DRVDD current is estab-
lished by the average number of output bits switching, which
is determined by the sample rate and the characteristics of the
analog input signal.
Reducing the capacitive load presented to the output drivers can
minimize digital power consumption. The data in
Figure 60 was
taken in CMOS mode using the same operating conditions as those
used for the Power Supplies and Power Consumption specifications
40
60
80
100
120
140
160
180
200
220
0
10
20
30
40
50
60
70
80
90
100
5
25
45
65
85
105
125
PO
W
ER
(m
W
)
SU
PP
L
Y
CURRE
NT
(
A)
ENCODE RATE (MSPS)
IAVDD
TOTAL POWER
IDRVDD
09975-
070
Figure 60. AD9648-125 Power and Current vs. Clock Rate (1.8 V CMOS
Output Mode)
40
60
80
100
120
140
160
180
200
0
10
20
30
40
50
60
70
80
90
5
25
45
65
85
105
PO
W
ER
(m
W
)
SU
PP
L
Y
CURRE
NT
(
A)
ENCODE RATE (Msps)
IAVDD
IDRVDD
TOTAL POWER
09975-
066
Figure 61. AD9648-105 Power and Current vs. Clock Rate (1.8 V CMOS
Output Mode)