In the past few years, the number of video sources connected to a
single display has increased steadily, make the video signal switching
must in most video system. In a typicaly home entertainment systems, for
example, a set-top box (STB) or digital video recorder (DVR) cable or
statellite TV, VCR, DVD players, a video game console, and a PC all feed
a single display. The ability to switch multiple video sources to a
single display extends to cars as well, where video sources include
the vehicle entertainment system, rearview camera, DVD player,
navigation system, and auxiliary video input.
Traditional
CMOS multiplexers and switches suffer several disadvantages at video
frequencies, where their on resistance introduces distortion, degrades
differential gain and phase performance, and interacts with the terminal
resistor to the attenuation of the incoming video signal and
affect intensity. System designers to solve this problem by adding
external buffer added gain, increasing the drive capability.
Video multiplexing can be simplified by using high speed video amplifiers with a disable mode. When the optical amplifier
is disabled, its output stage into a high impedance state. This is
different from their low power consumption mode, greatly reduces the
power consumption, but leave the state of the output stage is undefined.
High-speed
video amplifiers have all the key features required to make them ideal
for this function. Their high input impedance does not affect the
characteristic impedance of the transmission line, thus allowing back
termination. Because they are video amplifiers, they have inherently
good video specifications, including differential gain and phase, slew
rate, bandwidth and 0.1-dB flatness.
In a mux
configuration, the disabled channels present a high-impedance load to
the single active channel. The gain setting and feedback resistors load
the active amplifier, but their values are large compared to the 150-ohm
video load, so their effect is negligible.
3:1 Video Multiplexer
Video multiplexer
is used to encodes the multi channel video signals and convert them to
optical signals to transmit on optical fibers. The ADA4853-3 has
independent disable controls, making it suitable for use as a low-cost
3:1 buffered -output video mux. Its output impedance is greater
than 2-kohms at 10 MHz, so the amplifier outputs can be connected to
form a 3:1 mux with excellent switching behavior and great isolation
characteristics. Operating on a single 5-V supply, the configuration
shown in Figure 1 provides 14-MHz bandwidth (0.1-dB), gain of +2, and
58-dB off-channel isolation at 10 MHz. Its 10-μs channel-to-channel
switching time supports CVBS analog video applications.
Figure 1. 3:1 Video Multiplexer
High-Performance 2:1 Video Multiplexer
Figure
2 shows a high-performance 2:1 mux. The two input amplifiers are
configured as unity gain followers, while the output amplifier is set
for a gain of +2. The ability to shut-down both stages allows this mux
to achieve the excellent input-to-output off-isolation shown in Figure
3. Switching time in this configuration is 45 μs.
Conclusion:
High-speed
video amplifiers with a single disabled needle is very suitable for
simple structure, low cost video multiplexers and switches for compound
and high resolution video. They are the ideal replace CMOS switch, it is
more cost effective than video multiplexer. Be sure to consider using
high-speed video amplifiers if your system requires video switching
function.
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