Datasheet

10M
100M 1G 2G
FREQUENCY (Hz)
-12
-9
-6
-3
0
3
GAIN (dB)
V
S
= 10V
WITH COPPER FIELD
V
+
GND
SIGNAL IN
RETURN PATH
LMH6559
SNOSA57C –APRIL 2003–REVISED MARCH 2013 www.ti.com
Figure 53.
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 reduction 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.
Figure 54.
As can be seen in Figure 54 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.
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