Datasheet

AD9851
–17
–
PCB LAYOUT INFORMATION
The AD9851/CGPCB and AD9851/FSPCB evaluation boards
(Figures 22 through 25 and TPCs 1 and 2) represent typical
implementations of the AD9851 and exemplify the use of high
frequency/high res o lu tion design and layout practices. The print-
ed circuit board that contains the AD9851 should be a multilayer
board that allows dedicated power and ground planes. The power
and ground planes should (as much as possible) be free of etched
traces that cause discontinuities in the planes. It is recommended
that the top layer of the board also contain an interspatial ground
plane that makes ground available without vias for the surface-
mount devices. If separate analog and digital system ground
planes exist, they should be con nect ed together at the AD9851
eval u a tion board for optimum per for mance.
Avoid running digital lines under the device as these will couple
unnecessary noise onto the die. The power supply lines to the
AD9851 should use as large a trace as possible to provide a low-
impedance path and reduce the effects of switching currents on
the power supply line. Fast switching signals like clocks should
use microstrip, controlled im ped ance techniques where pos-
si ble. 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 crosstalk between the lines.
Good power supply decoupling is also an important con sid -
er ation. The analog (AVDD) and digital (DVDD) supplies
to the AD9851 are independent and separately pinned out to
min i mize coupling between analog and digital sections of the
device. All analog and digital supply pins should be decoupled
to AGND and DGND, respectively, with high quality ceramic
chip ca pac i tors. To achieve best performance from the decou-
pling ca pac i tors, they should be placed as close as possible to
the device. In systems where a common supply is used to drive
both the AVDD and DVDD supplies of the AD9851, it is rec-
om mend ed that the system’s AVDD supply be used.
Analog Devices applications engineering support is available to
answer additional questions on grounding and PCB layout. Call
1-800-ANALOGD.
EVALUATION BOARDS
Two versions of the AD9851 evaluation board are available. The
evaluation boards facilitate easy implementation of the device for
bench-top analysis and serve as a reference for PCB layout.
The AD9851/FSPCB is intended for applications where the
device will primarily be used as a frequency synthesizer. This
version is optimized for connection of the AD9851 internal D/A
converter output to a 50

spectrum analyzer input. The in ter nal
comparator of the AD9851 is made available for use via wire hole
access. The comparator inputs are externally pulled to opposing
voltages to prevent comparator chatter due to  oating inputs. The
DDS DAC output is un ltered and no reference oscillator is pro-
vided. This is done in recognition of the fact that many users may
 nd an oscillator to be a liability rather than an asset. See Figure 22
for an electrical schematic.
The AD9851/CGPCB is intended for applications using the
device as a CMOS output clock generator. It connects the
AD9851 DAC output to the internal comparator input via a
single-ended, 70 MHz low pass, 7th order, elliptic  lter. To
minimize output jitter of the comparator, special attention has
been given to the low-pass  lter design. Primary considerations
were input and output impedances (200

) and a very steep roll-
off characteristic to attenuate unwanted, nearby alias sig nals. The
high impedance of the  lter allows the DAC to de vel op 1 V p-p
(with 10 mA) across the two 200

resistors at the input and
output of the  lter. This voltage is entirely suf  cient to opti-
mally drive the AD9851 comparator. This  lter was designed
with the assumption that the AD9851 DDS is at full clock
speed (180 MHz). If this is not the case,  lter spec i  ca tions
may need to change to achieve proper attenuation of an tic i pat ed
alias signals. BNC connectors allow convenient ob ser va tion
of the comparator CMOS output and input, as well as that of
the DAC. No reference oscillator is provided for reasons stated
above. This model allows easy evaluation of the AD9851 as
a frequency and phase-agile CMOS output clock source (see
Figure 24 for electrical schematic).
Jitter Reduction Note
The AD9851/CGPCB has a wideband DDS fundamental out put,
dc to 70 MHz, and the on-chip comparator has even more band-
width. To op ti mize low jitter performance users should con sid er
band pass  ltering of the DAC output if only a narrow band width
is re quired. This will reduce jitter caused by spu ri ous, nonhar-
monic signals above and below the desired signal. Low er ing
the ap plied V
DD
helps in re duc ing com par a tor switch ing noise
by reducing

V/

T of the com par a tor outputs. For op ti mum
jitter per for mance, users should avoid the very busy digital en vi -
ron ment of the on-chip com par a tor and opt for an external, high
speed com par a tor.
Both versions of the AD9851 evaluation boards are designed
to interface to the parallel printer port of a PC. The operating
software (C++) runs under Microsoft
®
Windows
®
(Windows
3.1 and Windows 95); Windows NT
®
not supported and pro-
vides a user-friend ly and intuitive format for controlling the
functionality and ob serv ing the performance of the device.
The 3.5-in disk provided with the evaluation board contains
an executable  le that displays the AD9851 function-selection
screen. The eval u a tion board may be operated with 3.0 V or
5 V sup plies. Eval u a tion boards are con gured at the factory
for an external clock input. If the op tion al on-board crystal
clock source is installed, resistor R2 (50

) must be removed.
EVALUATION BOARD INSTRUCTIONS
Required Hardware/Software
Personal computer operating in Windows 3.1
or 95 en vi ron ment
(does not support Windows NT)
Printer port, 3.5-in  oppy drive, mouse, and Centronics com-
pat i ble printer cable, 3 V to 5 V voltage supply
Crystal clock oscillator or high frequency signal generator (sine
wave output) with dc offset capability
AD9851 Evaluation Board Software disk and AD9851/FSPCB or
AD9851/CGPCB Evaluation Board
Setup
Copy the contents of the AD9851 disk onto the host PCs hard
drive. (There are two  les, WIN9851.EXE version 1.x and
Bwcc.dll.) Connect the printer cable from the computer to the
eval u a tion board. Use a good quality cable as some cables do not
connect every wire that the printer port supports.
REV. D