PC Board Layout Considerations

For optimum performance, an adequate ground plane, isola-

tion between channels, good supply bypassing and minimiz-

ing unwanted feedback are necessary. Also, the length of the

signal traces from the preamplifier to the LM2453 and from

the LM2453 to the CRT cathode should be as short as pos-

sible.

Due to the high gain and bandwidth of the LM2453, the part

may oscillate in a monitor if feedback occurs around the

video channel through the chassis wiring. To prevent this,

leads to the video amplifier input circuit should be shielded,

and input circuit wiring should be spaced as far as possible

from output circuit wiring.

IMPORTANT LAYOUT CONSIDERATIONS

The NSC reference design can be used directly or it can be

used as a guide for future layouts. Note the location of the

following components:

•

C48 —V

and the device ground.

•

C8 —V

ground.

•

C34,C35, C46 —V

V

the direct path from the capacitor ground to the LM2453

ground.

•

C25-27 — clamp output bypass caps. Very important for a

stable DC clamp voltage and for protecting the clamp out-

puts from damage due to a CRT arc.

The routing of the LM2453 video outputs to the CRT is very

critical to achieving optimum performance.

Figure 20 shows

the routing and component placement from pin 1 of the

LM2453 to the blue cathode. Note that the components are

placed so that they almost line up from the output pin of the

LM2453 to the blue cathode pin of the CRT connector. This

is done to minimize the length of the video path between

these two components. Note also that D16, D17, R17 and

D7 are placed to minimize the size of the video nodes that

they are attached to. This minimizes parasitic capacitance in

the video path and also enhances the effectiveness of the

protection diodes. The anode of protection diode D17 is con-

nected directly to a section of the ground plane that has a

short and direct path to the LM2453 ground pins. The cath-

ode of D16 is connected to V

pacitor C34, which is connected to the same section of the

ground plane as D17. The diode placement and routing is

very important for minimizing the voltage stress on the

LM2453 video outputs during an arc over event. Lastly, no-

tice that S1 is placed very close to the blue cathode and is

tied directly to the ground under the CRT connector.

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