AD9851 datasheet(13/23 Pages) AD

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AD9851

–13–

REV. C

PCB LAYOUT INFORMATION

The AD9851/CGPCB and AD9851/FSPCB evaluation boards

(Figures 22–25) represent typical implementations of the AD9851

and exemplify the use of high frequency/high resolution design

and layout practices. The printed 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 dis-

continuities 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 connected together at the AD9851 evaluation board

for optimum performance.

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 impedance techniques where possible.

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 consider-

ation. The analog (AVDD) and digital (DVDD) supplies to the

AD9851 are independent and separately pinned-out to mini-

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 capaci-

tors. To achieve best performance from the decoupling capaci-

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 recommended 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

Ω spectrum analyzer input. The inter-

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 floating

inputs. The DDS DAC output is unfiltered and no reference

oscillator is provided. This is done in recognition of the fact that

many users may find their presence to be a liability rather than

an asset. See Figure 22 for 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 filter. To

minimize output jitter of the comparator, special attention has

been given to the low pass filter design. Primary considerations

were input and output impedances (200

Ω) and a very steep

roll-off characteristic to attenuate unwanted, nearby alias sig-

nals. The high impedance of the filter allows the DAC to de-

velop 1 V p-p (with 10 mA) across the two 200

Ω resistors at

the input and output of the filter. This voltage is entirely suffi-

cient to optimally drive the AD9851 comparator. This filter was

designed with the assumption that the AD9851 DDS is at full

clock speed (180 MHz). If this is not the case, filter specifica-

tions may need to change to achieve proper attenuation of

anticipated alias signals. BNC connectors allow convenient

observation 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

bandwidth. To optimize low jitter performance users should

consider bandpass filtering of the DAC output if only a narrow

bandwidth is required. This will reduce jitter caused by spuri-

ous, nonharmonic signals above and below the desired signal.

Lowering the applied VDD helps in reducing comparator switch-

ing noise by reducing

∆V/∆T of the comparator outputs. For

optimum jitter performance, users should avoid the very busy

digital environment of the on-chip comparator and opt for an

external, high speed comparator.

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

® (3.1 and

Windows 95, NT is NOT supported) and provides a user-

friendly and intuitive format for controlling the functionality

and observing the performance of the device. The 3.5" disk

provided with the evaluation board contains an executable file

that displays the AD9851 function-selection screen. The

evaluation board may be operated with +3.0 V or +5 V sup-

plies. Evaluation boards are configured at the factory for an

external clock input. If the optional on-board crystal clock

source is installed, resistor R2 (50

Ω) must be removed.

EVALUATION BOARD INSTRUCTIONS

Required Hardware/Software

Personal computer operating in Windows 3.1 or “95” environ-

ment (does not support Windows NT).

Printer port, 3.5" floppy drive, mouse and Centronics compat-

ible 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 files, WIN9851.EXE version 1.x and

Bwcc.dll). Connect the printer cable from computer to the

evaluation board. Use a good quality cable as some cables do

not connect every wire that the printer port supports.

Windows is a registered trademark of Microsoft Corporation.

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