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 LM2413 and

from the LM2413 to the CRT cathode should be as short as

possible. The red video trace from the LM1283 to the

LM2413 input should be considered as short as possible on

a PCB layout. If possible, position the pre-amp in such a way

where the traces from its output connects to the driver input

the most direct and shortest path. The following references

are recommended for video board designers:

Ott, Henry W., “Noise Reduction Techniques in Electronic

Systems”, John Wiley & Sons, New York, 1976.

“Guide to CRT Video Design”, National Semiconductor Appli-

cation Note 861.

“Video Amplifier Design for Computer Monitors”, National

Semiconductor Application Note 1013.

Pease,

Robert A.,

“Troubleshooting Analog

Circuits”,

Butterworth-Heinemann, 1991.

Because of its high small signal bandwidth, the part may os-

cillate in a monitor if feedback occurs around the video chan-

nel through the chassis wiring. To prevent this, leads to the

video amplifier input circuit should be shielded, and input cir-

cuit wiring should be spaced as far as possible from output

circuit wiring.

NSC Demonstration Board

Figure 11 and Figure 12 show routing and component place-

ment on the NSC LM1283/2413 demonstration board. The

schematic of the board is shown in

Figure 10. This board

provides a good example of a layout that can be used as a

guide for future layouts. Note the location of the following

components:

•

C79 —V

and ground pins. (

Figure 12)

•

C55 —V

ground. (

Figure 12)

•

C75 and C77 —V

and V

(

Figure 11)

The routing of the LM2413 outputs to the CRT is very critical

to achieving optimum performance.

Figure 13 shows the

routing and component placement from pin 1 to the blue

cathode. Note that the components are placed so that they

almost line up from the output pin of the LM2413 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 D14, D15, R29 and D13 are placed to keep

the size of the video nodes to a minimum. This minimizes

parasitic capacitance in the video path and also enhances

the effectiveness of the protection diodes. The anode of pro-

tection diode D14 is connected directly to a section of the

ground plane that has a short and direct path to the LM2413

ground pins. The cathode of D15 is connected to V

close to decoupling capacitor C55 (see

Figure 13) which is

connected to the same section of the ground plane as D14.

The diode placement and routing is very important for mini-

mizing the voltage stress on the LM2413 during an arc over

event. Lastly, notice that S3 is placed very close to the blue

cathode and is tied directly to CRT ground.

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