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 fiber optic 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
Digital 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.
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.