REV. A

–24–

AD9876

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

decoupling capacitors on each supply pin (i.e., the AD9876

requires five power supply decoupling caps, one each on Pins 5,

38, 47, 14, and 35). The decoupling caps should be placed as close

to the MxFE supply pins as possible. An example of the proper

decoupling is shown in the AD9876 evaluation board schematic.

Ground Planes

In general, if the component placing guidelines discussed earlier

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 will result in the lowest impedance

return paths and the quietest ground connections.

If the components cannot be placed in a manner that will keep 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, the impedance of these traces should have

a controlled characteristic impedance of about 50

Ω. This will

prevent 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 about 1.5 inches, series

termination resistors (33

Ω to 47 Ω) should be placed close to

all signal sources. It is a good idea to series-terminate all clock

signals at their source, regardless of trace length.

The receive Rx

and Rx

signals are the most sensitive

signals on the entire board. Careful routing of these signals

is essential for good receive path performance. The Rx

and

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 will appear as common mode and

will be 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 will

further reduce the possibility of noise corrupting these signals.

OUTLINE DIMENSIONS

48-Lead Plastic Quad Flatpack [LQFP]

1.4 mm Thick

(ST-48)

Dimensions shown in millimeters

TOP VIEW

(PINS DOWN)

1

12

13

25

24

36

37

48

0.27

0.22

0.17

0.50

BSC

7.00

BSC

SEATING

PLANE

1.60 MAX

0.75

0.60

0.45

VIEW A

7

3.5

0

0.20

0.09

1.45

1.40

1.35

0.15

0.05

0.08 MAX

COPLANARITY

VIEW A

ROTATED 90 CCW

PIN 1

INDICATOR

9.00 BSC

COMPLIANT TO JEDEC STANDARDS MS-026BBC

SEATING

PLANE

Revision History

Location

Page

10/02—Data Sheet changed from REV. 0 to REV. A.

Changes to to Table IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Changes to REGISTER 3—CLOCK SOURCE CONFIGURATION section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Updated OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24