AD9879-EB datasheet(29/32 Pages) AD

29 / 32 page

AD9879

Rev. A | Page 29 of 32

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.

SIGNAL ROUTING

The digital Rx and Tx signal paths should be kept as short as

possible. Also, these traces should have a controlled impedance

of about 50 Ω. This prevents poor signal integrity and the high

currents that can occur during undershoot or overshoot caused

by ringing. If the signal traces cannot be kept shorter than

approximately 1.5 inches, then series termination resistors

(33 Ω to 47 Ω) should be placed close to all signal sources. It is

advisable to series terminate all clock signals at their source

regardless of trace length.

The receive (I IN, Q IN, and RF IN) signals are the most

sensitive signals on the entire board. Careful routing of these

signals is essential for good receive path performance. The Rx±

signals form a differential pair and should be routed together as

a pair. By keeping the traces adjacent to each other, noise

coupled onto the signals appears as common-mode and is

largely rejected by the MxFE receive input. Keeping the driving

point impedance of the receive signal low and placing any low-

pass filtering of the signals close to the MxFE further reduces

the possibility of noise corrupting these signals.

AD9879-EB Datasheet(PDF) 29 Page - Analog Devices