LMH6560 datasheet(20/23 Pages) NSC

20 / 23 page

Application Notes (Continued)

Discontinuities in a Ground Plane

A ground plane with traces routed over this plane results in

the build up of an electric field between the trace and the

ground plane as seen in Figure 13. This field is build up over

the entire routing of the trace. For the highest performance

the ground plane should not be interrupted because to do so

will cause the field lines to follow a roundabout path. In

Figure 17 it was necessary to interrupt the ground plane with

the blue crossing trace. This interruption causes the return

current to follow a longer route than the signal path follows to

overcome the discontinuity.

If needed it is possible to bypass the interruption with traces

that are parallel to the signal trace in order to reduce the

negative effects of the discontinuity in the ground plane. In

doing so, the current in the ground plane closely follows the

signal trace on the return path as can be seen in Figure 18.

Care must be taken not to place too many traces in the

ground plane or the ground plane effectively vanishes such

that even bypasses are unsuccessful in reducing negative

effects.

If the overall density becomes too high it is better to make a

design which contains additional metal layers such that the

ground planes actually function as ground planes. The costs

for such a pcb are increased but the payoff is in overall

effectiveness and ease of design.

Ground Planes at Top and Bottom Layer of a PCB

In addition to the bottom layer ground plane another useful

practice is to leave as much copper as possible at the top

layer. This is done to reduce the amount of copper to be

removed from the top layer in the chemical process. This

causes less pollution of the chemical baths allowing the

manufacturer to make more pcb’s with a certain amount of

chemicals. Connecting this upper copper to ground provides

additional shielding and signal performance is enhanced.

For lower frequencies this is specifically true. However, at

higher frequencies other effects become more and more

important such that unwanted coupling may result in a re-

duction in the bandwidth of a circuit. In the design of a test

circuit for the LMH6559 this effect was clearly noticeable and

the useful bandwidth was reduced from 1500MHz to around

850MHz.

As can be seen in Figure 19 the presence of a copper field

close to the transmission line to and from the buffer causes

unwanted coupling effects which can be seen in the dip at

about 850MHz. This dip has a depth of about 5dB for the

case when all of the unused space is filled with copper. In

case of only one area being filled with copper this dip is

about 9dB.

PCB BOARD LAYOUT AND COMPONENT SELECTION

Sound practice in the area of high frequency design requires

that both active and passive components be used for the

purposes for which they were designed. It is possible to

amplify signals at frequencies of several hundreds of MHz

using standard through hole resistors. Surface mount de-

vices, however, are better suited for this purpose. Surface

mount resistors and capacitors are smaller and therefore

parasitics are of lower value and therefore have less influ-

ence on the properties of the amplifier. Another important

issue is the pcb itself, which is no longer a simple carrier for

all the parts and a medium to interconnect them. The pcb

board becomes a real component itself and consequently

contributes its own high frequency properties to the overall

performance of the circuit. Sound practice dictates that a

20064260

FIGURE 17.

20064261

FIGURE 18.

20064262

FIGURE 19.

LMH6560 Datasheet(PDF) 20 Page - National Semiconductor (TI)

LMH6560 Datasheet(HTML) 20 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL