AN2471

PCB layout recommendation

7/12

3

PCB layout recommendation

For the purpose of maintaining 75

Ω track for all the analog lines, a 4-layer structure is

recommended. All the analog tracks are routed on the top and bottom layers of the board.

The middle two layers are used for both analog and digital ground.

The middle two layers are identical and separated into two sections, analog and digital

grounds. They are separated by a slot area and are only connected at the top layer through

ferrite bead. This ensures the digital noise will not get coupled to the analog portion.

All the analog tracks are referenced to the analog ground and all the digital tracks are

referenced to the digital ground. In this evaluation board SEL1&SEL2, that in the actual

system would come from the digital portion of the system, are included in the analog

portion. Since these two select digital lines will be mostly static during operation, it will not

cause any disruption to the analog portion.

STMAV335 supply is connected to VCC5V analog and GND analog. To further guarantee

the stability of STMAV335 supply, a 100 nF capacitor is used to decouple the VCC pin and

should be placed as close as possible to the pin. The stability of the analog supply lines is of

significant importance since the bias level of STMAV335 is referenced to it. Any instability on

the analog supply will affect the switch characteristics.

Each set of video signals, RGB, YPbPr, and YC (S-VIDEO) are routed with the same track

length. This will guarantee minimum channel-to-channel delay that will lead to minimum

display distortion.

Figure 2.

Recommended board stack-up for 75

Ω impedance