
AD9269
Rev. 0 | Page 23 of 40
If a low jitter clock source is not available, another option is to
ac couple a differential PECL signal to the sample clock input
excellent jitter performance.
100
0.1F
240
240
50k
50k
CLK–
CLK+
CLOCK
INPUT
CLOCK
INPUT
ADC
AD951x
PECL DRIVER
0
853
8-
01
9
Figure 51. Differential PECL Sample Clock (Up to 480 MHz)
A third option is to ac couple a differential LVDS signal to the
clock drivers offer excellent jitter performance.
100
0.1F
50k
50k
CLK–
CLK+
ADC
CLOCK
INPUT
CLOCK
INPUT
AD951x
LVDS DRIVER
0
853
8-
020
Figure 52. Differential LVDS Sample Clock (Up to 480 MHz)
In some applications, it may be acceptable to drive the sample
clock inputs with a single-ended 1.8 V CMOS signal. In such
applications, drive the CLK+ pin directly from a CMOS gate, and
bypass the CLK pin to ground with a 0.1 μF capacitor (see
OPTIONAL
100
0.1F
50
1
150
RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
VCC
1k
1k
CLOCK
INPUT
AD951x
CMOS DRIVER
08
53
8-
02
1
Figure 53. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
Input Clock Divider
The AD9269 contains an input clock divider with the ability
to divide the input clock by integer values between 1 and 6.
Optimum performance is obtained by enabling the internal
duty cycle stabilizer (DCS) when using divide ratios other than
1, 2, or 4.
The AD9269 clock divider can be synchronized using the external
SYNC input. Bits[2:1] in Register 0x100 allow the clock divider
to be resynchronized on every SYNC signal or only on the first
SYNC signal after the register is written. A valid SYNC causes
the clock divider to reset to its initial state. This synchronization
feature allows multiple parts to have their clock dividers aligned
to guarantee simultaneous input sampling.
Clock Duty Cycle
Typical high speed ADCs use both clock edges to generate
a variety of internal timing signals and, as a result, may be
sensitive to clock duty cycle. Commonly, a ±5% tolerance is
required on the clock duty cycle to maintain dynamic
performance characteristics.
The AD9269 contains a duty cycle stabilizer (DCS) that retimes
the nonsampling (falling) edge, providing an internal clock
signal with a nominal 50% duty cycle. This allows the user to
provide a wide range of clock input duty cycles without affecting
the performance of the AD9269. Noise and distortion perform-
ance are nearly flat for a wide range of duty cycles with the DCS
80
79
78
77
76
70
71
72
73
74
75
30
35
40
45
50
55
60
65
70
S
N
R
(
d
BF
S
)
POSITIVE DUTY CYCLE (%)
08
53
8-
0
64
DCS OFF
DCS ON
Figure 54. SNR vs. Duty Cycle Stabilizer On/Off
Jitter in the rising edge of the input is still of concern and is not
easily reduced by the internal stabilization circuit. The duty
cycle control loop does not function for clock rates of less than
20 MHz nominally. The loop has a time constant associated
with it that must be considered in applications in which the
clock rate can change dynamically. A wait time of 1.5 μs to 5 μs
is required after the dynamic clock frequency increases or decreases
before the DCS loop is relocked to the input signal.