LMH6560 datasheet(18/23 Pages) NSC

18 / 23 page

Application Notes (Continued)

noticeable. This causes a distinctive dip in the output fre-

quency sweep and this dip varies depending upon the par-

ticular capacitor as seen in Figure 10.

To minimize peaking due to C

L a series resistor for the

purpose of isolation from the output stage should be used. A

low valued resistor will minimize the influence of such a load

capacitor. In a 50

Ω system as is common in high frequency

circuits a 50

Ω series resistor is often used. Usage of the

series resistor, as seen in Figure 11 eliminates the peaking

but not the dip. The dip will vary with the particular capacitor.

Using a resistor in series with a capacitor creates a rolloff of

6db/octave. Choice of a higher valued resistor, for example

500

Ω to 1kΩ, and a capacitor of hundreds of pf’s provides

the expected response at lower frequencies. However, at

high frequencies the internal inductance is appreciable and

forms with the capacitor a series LC combination.

USING GROUND PLANES

A ground plane on a printed circuit board provides for a low

ohmic connection everywhere on the board for use in con-

necting supply voltages or grounds. Multilayer boards often

make use of inner conductive layers for routing supply volt-

ages. These supply voltage layers form a complete plane

rather than using discrete traces to connect the different

points together for the specified supply. Signal traces on the

other hand are routed on outside layers both top and bottom.

This allows for easy access for measurement purposes.

Fortunately, only very high density boards have signal layers

in the middle of the board. In an earlier section, the formula

for Z was derived as:

(6)

The width of a trace is determined by the thickness of the

board. In the case of a multilayer board the thickness is the

space between the trace and the first supply plane under this

trace layer. By common practice, layers do not have to be

evenly divided in the construction of a pcb. Refer to Figure

12. The design of a transmission line design over a pcb is

based upon the thickness of the different internal layers and

the er of the board material. The pcb manufacturer can

supply information about important specifications. For ex-

ample, a nominal 1.6mm thick pcb produces a 50

Ω trace for

a calculated width of 2.9mm. If this layer has a thickness of

0.35mm and for the same er, the trace width for 50

Ω should

be of 0.63mm, as calculated from Equation 7, a derivation

from Equation 6.

(7)

Using a trace over a ground plane has big advantages over

the use of a standard single or double sided board. The main

advantage is that the electric field generated by the signal

transported over this trace is fixed between the trace and the

ground plane e.g. there is almost no possibility of radiation.

20064251

FIGURE 10.

20064252

FIGURE 11.

20064255

FIGURE 12.

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