
Data Sheet
AD9653
Rev. 0 | Page 27 of 40
ADC output driver supplies to avoid modulating the clock signal
with digital noise. Low jitter, crystal-controlled oscillators make
the best clock sources. If the clock is generated from another
type of source (by gating, dividing, or other methods), it should
be retimed by the original clock at the last step.
performance as it relates to ADCs.
Figure 69. Ideal SNR vs. Input Frequency and Jitter
POWER DISSIPATION AND POWER-DOWN MODE
proportional to its sample rate. The digital power dissipation
does not vary significantly because it is determined primarily by
the DRVDD supply and bias current of the LVDS output drivers.
Figure 70. Analog Core Power vs. fSAMPLE for fIN = 9.7 MHz, Four Channels
port or by asserting the PDWN pin high. In this state, the ADC
typically dissipates 2 mW. During power-down, the output
drivers are placed in a high impedance state. Asserting the
PDWN pin low returns th
e AD9653 to its normal operating
mode. Note that PDWN is referenced to the digital output
driver supply (DRVDD) and should not exceed that supply
voltage.
Low power dissipation in power-down mode is achieved by
shutting down the reference, reference buffer, biasing networks,
and clock. Internal capacitors are discharged when entering
power-down mode and then must be recharged when returning
to normal operation. As a result, wake-up time is related to the
time spent in power-down mode, and shorter power-down
cycles result in proportionally shorter wake-up times. When
using the SPI port interface, the user can place the ADC in
power-down mode or standby mode. Standby mode allows the
user to keep the internal reference circuitry powered when
faster wake-up times are required. See th
e Memory Map section
for more details on using these features.
DIGITAL OUTPUTS AND TIMING
Th
e AD9653 differential outputs conform to the ANSI-644 LVDS
standard on default power-up. This can be changed to a low power,
reduced signal option (similar to the IEEE 1596.3 standard) via the
SPI. The LVDS driver current is derived on chip and sets the
output current at each output equal to a nominal 3.5 mA. A 100 Ω
differential termination resistor placed at the LVDS receiver
inputs results in a nominal 350 mV swing (or 700 mV p-p
differential) at the receiver.
When operating in reduced range mode, the output current is
reduced to 2 mA. This results in a 200 mV swing (or 400 mV p-p
differential) across a 100 Ω termination at the receiver.
The
AD9653 LVDS outputs facilitate interfacing with LVDS
receivers in custom ASICs and FPGAs for superior switching
performance in noisy environments. Single point-to-point net
topologies are recommended with a 100 Ω termination resistor
placed as close to the receiver as possible. If there is no far-end
receiver termination or there is poor differential trace routing,
timing errors may result. To avoid such timing errors, it is
recommended that the trace length be less than 24 inches and
that the differential output traces be close together and at equal
lengths. An example of the FCO and data stream with proper
the LVDS output timing example in reduced range mode.
Figure 71. LVDS Output Timing Example in ANSI-644 Mode (Default)
1
10
100
1000
16 BITS
14 BITS
12 BITS
30
40
50
60
70
80
90
100
110
120
130
0.125ps
0.25ps
0.5ps
1.0ps
2.0ps
ANALOG INPUT FREQUENCY (MHz)
10 BITS
8 BITS
RMS CLOCK JITTER REQUIREMENT
S
N
R
(
d
B)
10538
-067
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
20
40
60
80
100
120
A
NAL
O
G
C
O
RE
P
O
W
E
R
(
W
)
SAMPLE RATE (MSPS)
VREF =1.3V
VREF =1.0V
10
538-
06
8
10
538-
0
69
D0 500mV/DIV
D1 500mV/DIV
DCO 500mV/DIV
FCO 500mV/DIV
4ns/DIV