Datasheet

AD9629
Rev. 0 | Page 20 of 32
CLOCK INPUT CONSIDERATIONS
For optimum performance, clock the AD9629 sample clock inputs,
CLK+ and CLK−, with a differential signal. The signal is typi-
cally ac-coupled into the CLK+ and CLK− pins via a transformer
or capacitors. These pins are biased internally (see Figure 45)
and require no external bias.
0.9V
AVDD
2pF 2pF
CLK–CLK+
08540-016
Figure 45. Equivalent Clock Input Circuit
Clock Input Options
The AD9629 has a very flexible clock input structure. The clock
input can be a CMOS, LVDS, LVPECL, or sine wave signal.
Regardless of the type of signal being used, clock source jitter is
of great concern, as described in the Jitter Considerations section.
Figure 46 and Figure 47 show two preferred methods for clock-
ing the AD9629. The CLK inputs support up to 4× the rated
sample rate when using the internal clock divider feature. A low
jitter clock source is converted from a single-ended signal to a
differential signal using either an RF transformer or an RF balun.
0.1µF
0.1µF
0.1µF0.1µF
SCHOTTKY
DIODES:
HSMS2822
CLOCK
INPUT
50Ω
100Ω
CLK–
CLK+
ADC
Mini-Circuits
®
ADT1-1WT, 1:1 Z
XFMR
08540-017
Figure 46. Transformer-Coupled Differential Clock (3 MHz to 200 MHz)
0.1µF
0.1µF1nF
CLOCK
INPUT
1nF
50Ω
CLK–
CLK+
SCHOTTKY
DIODES:
HSMS2822
ADC
08540-018
Figure 47. Balun-Coupled Differential Clock (Up to 4× Rated Sample Rate)
The RF balun configuration is recommended for clock frequencies
between 80 MHz and 320 MHz, and the RF transformer is recom-
mended for clock frequencies from 3 MHz to 200 MHz. The
back-to-back Schottky diodes across the transformer/balun
secondary limit clock excursions into the AD9629 to ~0.8 V p-p
differential.
This limit helps prevent the large voltage swings of the clock
from feeding through to other portions of the AD9629 while
preserving the fast rise and fall times of the signal that are critical
to a low jitter performance.
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 48. 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
08540-019
Figure 48. Differential PECL Sample Clock (Up to 4× Rated Sample Rate)
A third option is to ac couple a differential LVDS signal to the
sample clock input pins, as shown in Figure 49. 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
08540-020
Figure 49. Differential LVDS Sample Clock (Up to 4× Rated Sample Rate)
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 F capacitor (see
Figure 50).
OPTIONAL
100Ω
0.1µF
0.1µF
0.1µF
50Ω
1
1
50Ω RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
V
CC
1kΩ
1kΩ
C
LOCK
INPUT
AD951x
CMOS DRIVER
08540-021
Figure 50. Single-Ended 1.8 V CMOS Input Clock (Up to 200 MHz)
Input Clock Divider
The AD9629 contains an input clock divider with the ability
to divide the input clock by integer values of 1, 2, or 4.