AD9879-EB datasheet(28/32 Pages) AD

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AD9879

Rev. A | Page 28 of 32

PCB DESIGN CONSIDERATIONS

Although the AD9879 is a mixed-signal device, the part 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 AD9879 has several pins which are used to decouple

sensitive internal nodes. These pins are REFIO, REFB10,

REFT10, 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 AD9879 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 which is used for

the 3 V digital logic circuits, a DVDD section that is used to

supply the digital supply pins of the AD9879, an AVDD section

that is used to supply the analog supply pins of the AD9879, and

a VANLG section that supplies the higher voltage analog

components on the board. The 3 VDD section typically has the

highest frequency currents on the power plane and should be

kept the furthest 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 3VDD 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 de-

coupling capacitors on each supply pin (for example, the

AD9879 requires 17 power supply decoupling caps). The

decoupling caps should be placed as close as possible to the

MxFE supply pins. An example of the proper decoupling is

shown in the AD9875 evaluation board schematic.

AD9879-EB Datasheet(PDF) 28 Page - Analog Devices

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