When using an analog switch to select between CVBS analog video signals I noticed that the cross-talk performance is degraded compared with the datasheet specification. What could be the root cause?
The datasheet specifications for cross-talk and off-isolation are guaranteed for signals applied to the analog switch that are within the analog signal range specification of the device.
In the case of CVBS signals, especially when AC coupled, the signal can have both positive and negative components. For single supply analog switches, the analog signal range is generally 0V(GND) to VDD or less.
When a negative signal is applied to the switch that is off, the negative portion of the video signal will partially turn on the switch causing the signal to leak though the switch and cause degradation in off-isolation adn crosstalk.
The solution is to either use a different switch withan analog signal range that covers both positive and negative signals(i.e. duall supply switches) or use a DC restoring circuit before the switch to shift the video signal above the ground level.
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