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PCB layout considerations

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5

PCB layout considerations

The decoupling capacitors should be placed as close as possible to the supply pins of the

device as it helps to minimize the noise due to switching and other sources. It is important to

ensure that the PCB tracks have resistances of 75 Ohm and are well matched for all the

inputs and outputs. This minimizes the impedance discontinuities between the analyzer

probes and measurement points on the board and thus reduces the reflections. From an

application perspective, image ghosting may occur if there is an impedance mismatch. The

75 Ohm termination on both the sides of the switch is provided by the network analyzer

itself. Load capacitance is placed close to the output pins of the device when required for

respective measurements.

5.1

Supply and ground effects

noise isolation and decoupling. For both high and low frequency noise filtering, a 10 µF

electrolytic capacitor is placed in parallel with a 100 nF ceramic capacitor. The ground plane

and good solid connections to it significantly improve the performance of the device. It is a

good practice to place as many vias from the top layer to the ground plane as possible to

lower the resistance between the contact and the ground plane, thereby reducing noise.

5.2

PCB demo board

This board is manufactured to do real video signal application level tests. A typical

configuration of the switch is shown below (to be added) for the tests. The BOM and the

board schematics + layout files (Gerber files) are available upon request and will be placed

in the Appendix section of the application note. The PCB demo board allows switching

between RGB, YPbPr inputs and also two composite (CVBS) inputs. The inputs come from

the video sources and the output of the switch is connected to the display device like a TV or

PC monitor.

6

Conclusion

In this way a single product finds use in a wide range of applications to serve various needs.

ST's STMAV340 is a perfect low-cost, high-performance solution which provides system

developers with simple and effective ways to do video signal switching without impacting the

quality of the image.