LM4805 datasheet(15/18 Pages) NSC

15 / 18 page

Application Information (Continued)

GENERAL MIXED-SIGNAL LAYOUT

RECOMMENDATION

This section provides practical guidelines for mixed signal

PCB layout that involves various digital/analog power and

ground traces. Designers should note that these are only

"rule-of-thumb" recommendations and the actual results will

depend heavily on the final layout.

Power and Ground Circuits

For 2 layer mixed signal design, it is important to isolate the

digital power and ground trace paths from the analog power

and ground trace paths. Star trace routing techniques (bring-

ing individual traces back to a central point rather than daisy

chaining traces together in a serial manner) can have a

major impact on low level signal performance. Star trace

routing refers to using individual traces to feed power and

ground to each circuit or even device. This technique will

take require a greater amount of design time but will not

increase the final price of the board. The only extra parts

required may be some jumpers.

Single-Point Power / Ground Connection

The analog power traces should be connected to the digital

traces through a single point (link). A "Pi-filter" can be helpful

in minimizing high frequency noise coupling between the

analog and digital sections. It is further recommended to

place digital and analog power traces over the correspond-

ing digital and analog ground traces to minimize noise cou-

pling.

Placement of Digital and Analog Components

All digital components and high-speed digital signals traces

should be located as far away as possible from analog

components and circuit traces.

Avoiding Typical Design / Layout Problems

Avoid ground loops or running digital and analog traces

parallel to each other (side-by-side) on the same PCB layer.

When traces must cross over each other do it at 90 degrees.

Running digital and analog traces at 90 degrees to each

other from the top to the bottom side as much as possible will

minimize capacitive noise coupling and crosstalk.

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FIGURE 3. Demo Board Reference Schematic

LM4805 bảng dữ liệu(PDF) 15 Page - National Semiconductor (TI)