AD9520-4/PCBZ Analog Devices Inc, AD9520-4/PCBZ Datasheet - Page 82
![BOARD EVAL FOR AD9520-4](/photos/9/8/90843/analog_devices_ad9520-4_pcbz_sml.jpg)
AD9520-4/PCBZ
Manufacturer Part Number
AD9520-4/PCBZ
Description
BOARD EVAL FOR AD9520-4
Manufacturer
Analog Devices Inc
Specifications of AD9520-4/PCBZ
Design Resources
Synchronizing Multiple AD9910 1 GSPS Direct Digital Synthesizers (CN0121) Phase Coherent FSK Modulator (CN0186)
Main Purpose
Timing, Clock Generator
Embedded
No
Utilized Ic / Part
AD9520-4
Primary Attributes
1.4 ~ 1.8 GHz Output Frequency
Secondary Attributes
Accepts CMOS, LVDS, or LVPECL References Up to 250 MHz
Silicon Manufacturer
Analog Devices
Application Sub Type
PLL Clock Synthesizer
Kit Application Type
Clock & Timing
Silicon Core Number
AD9520-0, AD9520-2, AD9520-2
Silicon Family Name
AD9520-X
Rohs Compliant
Yes
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
AD9520-4
APPLICATIONS INFORMATION
FREQUENCY PLANNING USING THE AD9520
The AD9520 is a highly flexible PLL. When choosing the PLL
settings and version of the AD9520, the following guidelines
should be kept in mind.
The AD9520 has four frequency dividers: the reference (or R)
divider, the feedback (or N) divider, the VCO divider, and the
channel divider. When trying to achieve a particularly difficult
frequency divide ratio requiring a large amount of frequency
division, some of the frequency division can be done by either
the VCO divider or the channel divider, thus allowing a higher
phase detector frequency and more flexibility in choosing the
loop bandwidth.
Within the AD9520 family, lower VCO frequencies generally
result in slightly better jitter. The difference in integrated jitter
(from 12 kHz to 20 MHz offset) for the same output frequency
is usually less than 150 fs over the entire VCO frequency range
(1.4 GHz to 2.95 GHz) of the AD9520 family. If the desired
frequency plan can be achieved with a version of the AD9520
that has a lower VCO frequency, choosing the lower frequency
part results in the best phase noise and the lowest jitter. However,
choosing a higher VCO frequency can result in more flexibility
in frequency planning.
Choosing a nominal charge pump current in the middle of the
allowable range as a starting point allows the designer to increase or
decrease the charge pump current, and thus allows the designer
to fine-tune the PLL loop bandwidth in either direction.
ADIsimCLK is a powerful PLL modeling tool that can be
downloaded from
for determining the optimal loop filter for a given application.
USING THE AD9520 OUTPUTS FOR ADC CLOCK
APPLICATIONS
Any high speed ADC is extremely sensitive to the quality of the
sampling clock of the AD9520. An ADC can be thought of as a
sampling mixer, and any noise, distortion, or timing jitter on
the clock is combined with the desired signal at the analog-to-
digital output. Clock integrity requirements scale with the analog
input frequency and resolution, with higher analog input
frequency applications at ≥14-bit resolution being the most
stringent. The theoretical SNR of an ADC is limited by the ADC
resolution and the jitter on the sampling clock. Considering an
ideal ADC of infinite resolution where the step size and
quantization error can be ignored, the available SNR can be
expressed approximately by
where:
f
t
A
J
is the rms jitter on the sampling clock.
is the highest analog frequency being digitized.
SNR
(dB)
=
20log
www.analog.com
⎛
⎜
⎜
⎝
2
π
f
1
A
t
J
⎞
⎟
⎟
⎠
and is a very accurate tool
Rev. 0 | Page 82 of 84
Figure 67 shows the required sampling clock jitter as a function
of the analog frequency and effective number of bits (ENOB).
See the AN-756 application note and the AN-501 application note
at www.analog.com.
Many high performance ADCs feature differential clock inputs
to simplify the task of providing the required low jitter clock on
a noisy PCB. (Distributing a single-ended clock on a noisy PCB
can result in coupled noise on the sample clock. Differential
distribution has inherent common-mode rejection that can
provide superior clock performance in a noisy environment.)
The differential LVPECL outputs of the AD9520 enable clock
solutions that maximize converter SNR performance.
The input requirements of the ADC (differential or single-
ended, logic level termination) should be considered when
selecting the best clocking/converter solution.
LVPECL CLOCK DISTRIBUTION
The LVPECL outputs of the AD9520 provide the lowest jitter
clock signals available from the AD9520. The LVPECL outputs
(because they are open emitter) require a dc termination to bias
the output transistors. The simplified equivalent circuit in
Figure 51 shows the LVPECL output stage.
In most applications, an LVPECL far-end Thevenin termination
(see Figure 68) or Y-termination (see Figure 69) is recommended.
In both cases, VS of the receiving buffer should match the VS_DRV.
If not, ac coupling is recommended (Figure 70).
LVPECL Y-termination is an elegant termination scheme that
uses the fewest components and offers both odd- and even-mode
impedance matching. Even-mode impedance matching is an
important consideration for closely coupled transmission lines
at high frequencies. Its main drawback is that it offers limited
flexibility for varying the drive strength of the emitter-follower
LVPECL driver. This can be an important consideration when
driving long trace lengths but is usually not an issue.
110
100
90
80
70
60
50
40
30
10
Figure 67. SNR and ENOB vs. Analog Input Frequency
f
A
100
(MHz)
SNR = 20log
2πf
1
A
t
J
1k
18
16
14
12
10
8
6