LMH6559_06 Datasheet, PDF (18/22 Pages) National Semiconductor (TI) – High-Speed, Closed-Loop Buffer

Application Notes (Continued)
Careful attention to power line distribution leads to improved
overall circuit performance. This is especially valid for analog
circuits which are more sensitive to spurious noise and other
unwanted signals.
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FIGURE 13.
This effect works to both sides because the circuit will not
generate radiation but the circuit is also not sensible if ex-
posed to a certain radiation level. The same is also notice-
able when placing components flat on the printed circuit
board. Standard through hole components when placed up-
right can act as an antenna causing an electric field which
could be picked up by a nearby upright component. If placed
directly at the surface of the pcb this influence is much lower.
The Effect Of Variation For e_r
When using pcb material the e_r has a certain shift over the
used frequency spectrum, so if necessary to work with very
accurate trace impedances one must taken into account for
which frequency region the design has to be functional.
Figure 14 (http://www.isola.de) gives an example what the
drift in e_r will be when using the pcb material produced by
Isola. If working at frequencies of 100MHz then a 50Ω trace
has a width of 3.04mm for standard 1.6mm FR4 pcb mate-
rial, and the same trace needs a width of 3.14mm. for
frequencies around 10GHz.
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FIGURE 15.
As demonstrated in Figure 15 the power lines are routed
from both sides on the pcb. In this case a current loop is
created as indicated by the dotted line. This loop can act as
an antenna for high frequency signals which makes the
circuit sensitive to R_F radiation. A better way to route the
power traces can be seen in the following setup. (see Figure
16)
FIGURE 14.
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Routing Power Traces
Power line traces routed over a pcb should be kept together
for best practice. If not a ground loop will occur which may
cause more sensitivity to radiation. Also additional ground
trace length may lead to more ringing on digital signals.
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FIGURE 16.
In this arrangement the power lines have been routed in
order to avoid ground loops and to minimize sensitivity to
noise etc. The same technique is valid when routing a high
frequent signal over a board which has no ground plane. In
that case is it good practice to route the high frequency
signal alongside a ground trace. A still better way to create a
pcb carrying high frequency signals is to use a pcb with a
ground plane or planes.
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