AD9877-EB datasheet(34/37 Pages) AD

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AD9877

Rev. B | Page 33 of 36

PCB DESIGN CONSIDERATIONS

Although the AD9877 is a mixed-signal device, it should be

treated as an analog component. The on-chip digital circuitry is

specially designed to minimize the impact the digital switching

noise has on the operation of the analog circuits. The power,

grounding, and layout recommendations in this section will

help provide the best performance from the MxFE.

COMPONENT PLACEMENT

Chances for obtaining the best performance from the MxFE are

greatly increased if the three following guidelines of component

placement are followed.

• Manage the path of return currents flowing into the ground

plane so that high frequency switching currents from the

digital circuits do not flow onto the ground plane under the

MxFE or analog circuits.

• Keep noisy digital signal paths and sensitive receive signal

paths as short as possible.

• Keep digital (noise generating) and analog (noise

susceptible) circuits as far away from each other as possible.

To best manage the return currents, pure digital circuits that

generate high switching currents should be closest to the power

supply entry. This keeps the highest frequency return current

paths short and prevents them from traveling over the sensitive

MxFE and analog portions of the ground plane. Also, these

circuits should be generously bypassed at each device, further

reducing the high frequency ground currents. The MxFE

should be placed adjacent to the digital circuits, such that the

ground return currents from the digital sections do not flow

into the ground plane under the MxFE. The analog circuits

should be placed furthest from the power supply.

The AD9877 has several pins that are used to decouple sensitive

internal nodes. These pins are REFIO, REFB8, REFT8, REFB12,

and REFT12. The decoupling capacitors connected to these

points should have low ESR and ESL. These capacitors should

be placed as close as possible to the MxFE and be connected

directly to the analog ground plane.

The resistor connected to the FSADJ pin and the RC network

connected to the PLLFILT pin should also be placed close to the

device and connected directly to the analog ground plane.

POWER PLANES AND DECOUPLING

The AD9877 evaluation board demonstrates a good power

supply distribution and decoupling strategy. The board has four

layers: two signal layers, one ground plane, and one power

plane. The power plane is split into a 3 VDD section used for

the 3 V analog supply pins of the AD9877 and a VANLG

section that supplies the higher voltage analog components on

the board.

That 3 VDD section typically has the highest frequency

currents on the power plane and should be kept the farthest

from the MxFE and analog sections of the board. The DVDD

portion of the plane brings the current used to power the digital

portion of the MxFE to the device. This should be treated

similarly to the 3 VDD power plane and be kept from going

underneath the MxFE or analog components. The MxFE

should sit above the AVDD portion of the power plane.

The AVDD and DVDD power planes can be fed from the same

low noise voltage source. They should be decoupled from each

other, however, to prevent the noise generated in the DVDD

portion of the MxFE from corrupting the AVDD supply. This

can be done by using ferrite beads between the voltage source

and DVDD and between the source and AVDD. Both DVDD

and AVDD should have a low ESR, bulk decoupling capacitor

on the MxFE side of the ferrite as well as low ESR, low ESL

decoupling capacitors on each supply pin (for example, the

AD9877 requires 17 power supply decoupling caps). The

decoupling capacitors should be placed as close as possible to

the MxFE supply pins. An example of the proper decoupling is

shown in the AD9877 evaluation board schematic.

GROUND PLANES

In general, if the component placing guidelines discussed in the

Component Placement section can be implemented, it is best to

have at least one continuous ground plane for the entire board.

All ground connections should be made as short as possible.

This results in the lowest impedance return paths and the

quietest ground connections.

If the components cannot be placed in a manner that keeps the

high frequency ground currents from traversing under the

MxFE and analog components, it may be necessary to put

current steering channels into the ground plane to route the

high frequency currents around these sensitive areas. These

current steering channels should be made only when and where

necessary.

AD9877-EB Datasheet(PDF) 34 Page - Analog Devices

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