11

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AFE2124

LAYOUT

The analog front end of an HDSL system has two conflicting

requirements. It must accept and deliver moderately high

rate digital signals and it must generate, drive, and convert

precision analog signals. To achieve optimal system perfor-

mance with the AFE2124, both the digital and the analog

sections must be treated carefully in board layout design.

The power supply for the digital section of the AFE2124 can

range from 3.3V to 5V. This supply should be decoupled to

digital ground with ceramic 0.1

µF capacitors placed as close

placed between pins 7 and 8 and the second capacitor,

between pins 41 and 42. Ideally, both a digital power supply

plane and a digital ground plane should run up to and

underneath the digital pins of the AFE2124 (pins 1 through

by a wide printed circuit board (PCB) trace. A digital ground

plane underneath all digital pins is strongly recommended.

The remaining portion of the AFE2124 should be considered

analog. All AGND pins should be connected directly to a

connected to an analog 5V power plane. Both of these planes

should have a low impedance path to the power supply. The

analog power supply pins should be decoupled to analog

ground with ceramic 0.1

µF capacitors placed as close to the

AFE2124 as possible. One 10

µF tantalum capacitor should

also be used with each AFE2124 between the analog supply

and analog ground.

Ideally, all ground planes and traces and all power planes

and traces should return to the power supply connector

before being connected together (if necessary). Each ground

and power pair should be routed over each other, should not

overlap any portion of another pair, and the pairs should be

separated by a distance of at least 0.25 inch (6mm). One

exception is that the digital and analog ground planes should

be connected together underneath the AFE2124 by a small

trace.