Datasheet

AD9648
Rev. 0 | Page 29 of 44
If a low jitter clock source is not available, another option is to
ac couple a differential PECL signal to the sample clock input
pins, as shown in Figure 55. The AD9510/AD9511/AD9512/
AD9513/AD9514/AD9515/AD9516/AD9517 clock drivers offer
excellent jitter performance.
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
240Ω240Ω
50kΩ 50kΩ
CLK–
CLK+
CLOCK
INPUT
CLOCK
INPUT
ADC
AD951x
PECL DRIVER
09975-061
Figure 55. Differential PECL Sample Clock (Up to 1 GHz)
A third option is to ac couple a differential LVDS signal to the
sample clock input pins, as shown in Figure 56. The AD9510/
AD9511/AD9512/AD9513/AD9514/AD9515/AD9516/AD9517
clock drivers offer excellent jitter performance.
100Ω
0.1µF
0.1µF
0.1µF
0.1µF
50kΩ 50kΩ
CLK–
CLK+
ADC
CLOCK
INPUT
CLOCK
INPUT
AD951x
LVDS DRIVER
09975-062
Figure 56. Differential LVDS Sample Clock (Up to 1 GHz)
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
Figure 57).
OPTIONAL
100Ω
0.1µF
0.1µF
0.1µF
50Ω
1
1
50Ω RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
V
CC
1kΩ
1kΩ
CLOCK
INPUT
AD951x
CMOS DRIVER
09975-063
Figure 57. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
Input Clock Divider
The AD9648 contains an input clock divider with the ability
to divide the input clock by integer values between 1 and 8.
The AD9648 clock divider can be synchronized using the
external SYNC input. Bit 1 and Bit 2 of Register 0x3A 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 AD9648 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 AD9648. Noise and distortion perform-
ance are nearly flat for a wide range of duty cycles with the DCS
on, as shown in Figure 58.
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 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 a dynamic clock frequency increase or decrease
before the DCS loop is relocked to the input signal.
09975-076
40
45
50
55
60
65
70
75
80
35 40 45 50 55 60 65
SNR (dBFS)
POSITIVE DUTY CYCLE (%)
SNR (DCS OFF)
SNR (DCS ON)
Figure 58. SNR vs. DCS On/Off