AD9850 Analog Devices, AD9850 Datasheet - Page 13

• Manufacturer Part Number: AD9850
• Description: CMOS/ 125 MHz Complete DDS Synthesizer
• Manufacturer: Analog Devices

REV. E
PCB LAYOUT INFORMATION
The AD9850/CGPCB and AD9850/FSPCB evaluation boards
(Figures 27–30) represent typical implementations of the
AD9850 and exemplify the use of high frequency/high resolu-
tion design and layout practices. The printed circuit board that
contains the AD9850 should be a multilayer board that allows
dedicated power and ground planes. The power and ground
planes should be free of etched traces that cause discontinuities
in the planes. It is recommended that the top layer of the multi-
layer board also contain interspatial ground plane, which makes
ground available for surface-mount devices. If separate analog
and digital system ground planes exist, they should be con-
nected together at the AD9850 for optimum results.
Avoid running digital lines under the device as these will couple
noise onto the die. The power supply lines to the AD9850
should use as large a track as possible to provide a low-impedance
path and reduce the effects of glitches on the power supply line.
Fast switching signals like clocks should be shielded with
ground to avoid radiating noise to other sections of the board.
Avoid crossover of digital and analog signal paths. Traces on
opposite sides of the board should run at right angles to each
other. This will reduce the effects of feedthrough through the
circuit board. Use microstrip techniques where possible.
Good decoupling is also an important consideration. The analog
(AVDD) and digital (DVDD) supplies to the AD9850 are
independent and separately pinned out to minimize coupling
between analog and digital sections of the device. All analog
and digital supplies should be decoupled to AGND and DGND,
respectively, with high quality ceramic capacitors. To achieve
best performance from the decoupling capacitors, they should
be placed as close as possible to the device, ideally right up
against the device. In systems where a common supply is used to
drive both the AVDD and DVDD supplies of the AD9850, it is
recommended that the system’s AVDD supply be used.
Analog Devices, Inc., applications engineering support is avail-
able to answer additional questions on grounding and PCB
layout. Call 1-800-ANALOGD.
DAC Output
IOUT IOUTB
V
CC
DATA (7) –
W CLK
FQ UD
Comparator Output
W32=0
Figure 25. Serial-Load Power-Down Sequence
Figure 26. AD9850 I/O Equivalent Circuits
V
CC
W33=0
QOUT/
QOUTB
W34=1
W35=X
–13–
Evaluation Boards
Two versions of evaluation boards are available for the AD9850,
which facilitate the implementation of the device for bench-
top analysis, and serve as a reference for PCB layout. The
AD9850/FSPCB is intended for applications where the device
will primarily be used as frequency synthesizer. This version
facilitates connection of the AD9850’s internal D/A converter
output to a 50 Ω spectrum analyzer input; the internal com-
parator on the AD9850 DUT is not enabled (see Figure 28 for
electrical schematic of AD9850/FSPCB). The AD9850/CGPCB
is intended for applications using the device in the clock genera-
tor mode. It connects the AD9850’s DAC output to the internal
comparator input via a single-ended, 42 MHz low-pass, 5-
pole Elliptical filter. This model facilitates the access of the
AD9850’s comparator output for evaluation of the device as a
frequency- and phase-agile clock source (see Figure 29 for
electrical schematic of AD9850/CGPCB).
Both versions of the AD9850 evaluation boards are designed to
interface to the parallel printer port of a PC. The operating
software runs under Microsoft
friendly and intuitive format for controlling the functionality
and observing the performance of the device. The 3.5" floppy
provided with the evaluation board contains an executable file
that loads and displays the AD9850 function-selection screen.
The evaluation board may be operated with +3.3 V or +5 V
supplies. The evaluation boards are configured at the factory for
an external reference clock input; if the onboard crystal clock
source is used, remove R2.
All trademarks are the property of their respective holders.
VINP/
VINN
W36=X
Comparator Input
W37=X
V
CC
W38=X
W39=X
DIGITAL
®
Windows and provides a user-
IN
Digital Inputs
V
AD9850
CC