LAYOUT CONSIDERATIONS

Whenever questions about layout arise, use the evaluation

board as a guide. Evaluation boards are shipped with

sample requests.

To reduce parasitic capacitances ground and power planes

should be removed near the input and output pins. Compo-

nents in the feedback loop should be placed as close to the

device as possible. For long signal paths controlled imped-

ance lines should be used, along with impedance matching

at both ends.

Bypass capacitors should be placed as close to the device

as possible. Bypass capacitors from each rail to ground are

applied in pairs. The larger electrolytic bypass capacitors

can be located anywhere on the board; however, the smaller

ceramic capacitors should be placed as close to the device

as possible.

VIDEO PERFORMANCE

The LMH6723/LMH6724/LMH6725 has been designed to

provide good performance with both PAL and NTSC com-

posite video signals. The LMH6723/LMH6724/LMH6725 is

specified for PAL signals. Typically, NTSC performance is

marginally better due to the lower frequency content of the

signal. Performance degrades as the loading is increased;

therefore, best performance will be obtained with back ter-

minated loads. The back termination reduces reflections

from the transmission line and effectively masks transmis-

sion line and other parasitic capacitances from the amplifier

output stage. Figure 4 shows a typical configuration for

driving a 75

Ω cable. The amplifier is configured for a gain of

2 to make up for the 6dB of loss in R

SINGLE 5V SUPPLY VIDEO

With a 5V supply the LMH6723/LMH6724/LMH6725 is able

to handle a composite NTSC video signal, provided that the

signal is AC coupled and level shifted so that the signal is

centered around V

POWER DISSIPATION

Follow these steps to determine the maximum power dissi-

pation for the LMH6723/LMH6724/LMH6725:

1.

Calculate the quiescent (no-load) power: P

(V

2.

Calculate the RMS power dissipated in the output stage:

P

are the voltage and current across the external load and

V

3.

Calculate the total RMS power: P

The maximum power that the LMH6723/LMH6724/LMH6725

package can dissipate at a given temperature can be de-

rived with the following equation:

P

θ

ture (˚C) and

θ

ambient, for a given package (˚C/W). For the SOIC-8 pack-

age

θ

SOIC-14 has a

θ

θ

160˚C/W.

ESD PROTECTION

The LMH6723/LMH6724/LMH6725 is protected against

electrostatic discharge (ESD) on all pins. The LMH6723/

LMH6725 will survive 2000V Human Body Model or 200V

Machine Model events.

Under closed loop operation the ESD diodes have no effect

on circuit performance. There are occasions, however, when

the ESD diodes will be evident. If the LMH6723/LMH6724/

LMH6725 is driven into a slewing condition the ESD diodes

will clamp large differential voltages until the feedback loop

restores closed loop operation. Also, if the device is powered

down and a large input signal is applied, the ESD diodes will

conduct.

20078943

FIGURE 8. Composite Amplifier Frequency Response

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