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Low-Cost Video Multiplexing Using High-Speed Amplifiers

By Don Nisbett [don.nisbett@analog.com]

Over the past few years, the number of video sources connected to a single display has increased steadily, making video signal switching a necessity in most video systems. In a typical home entertainment system, for example, a set-top box (STB) or digital video recorder (DVR) for cable or satellite TV, a VCR, a DVD player, 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 (VES), 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 termination resistor to attenuate the incoming video signal and affect the luminance. System designers solve this issue by adding external buffers to add gain and increase drive capability.

Video multiplexing can be simplified by using high-speed video amplifiers with a disable mode. When the amplifier is disabled, its output stage goes into a high-impedance state. This differs from the power-down mode, which significantly lowers the power consumption, but leaves the state of the output stage 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. Some high-speed video amplifiers that possess these key features are the AD8013, AD8029, and AD8063. Table 1 shows a representative list of muxable video amplifiers.

3:1 Video Multiplexer
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 Mux

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.

Figure 2. 2:1 Video Mux

Figure 3. Off-isolation of 2:1 Mux using the ADA4853-3

2:1 Video Multiplexer with SAG Correction
Signal amplitude gain (SAG) correction is used to provide low-frequency compensation for the high-pass filter formed by the 150-ohm video load of a back-terminated cable and the output coupling capacitor. Traditional ac-coupling uses a large, expensive coupling capacitor, making it costly and wasting valuable PCB space. SAG correction allows two small, low-cost capacitors to replace the one large ac-coupling capacitor. Figure 4 shows a high-performance 2:1 multiplexer with SAG correction. The compensation network includes C1, C2, R11 and R12. Field tilt is a measure of the voltage droop (tilt) that occurs on the ac-coupling capacitor when a constant luma signal is applied. This droop is caused by the small discharge current created by the 75-ohm load resistor. The capacitor values shown are optimized to achieve the equivalent field tilt of a 220-μF ac-coupling capacitor. A typical 220-μF tantalum ac-coupling capacitor occupies 28 mm² and costs $0.50 in high volume. The typical 47-μF and 22-μF capacitors used for SAG correction occupy about 0.72 mm² and 0.4 mm² and cost as little as $0.10 each in high volumes.

Figure 4. 2:1 Mux with SAG Correction

Figure 5. Frequency Response of 2:1 Mux with SAG Correction

Conclusion:
High-speed video amplifiers with individual disable pins are excellent for constructing simple, low-cost video multiplexers and switches for composite and high-resolution video. They are ideal for replacing CMOS switches, and are more cost effective than video multiplexers. Be sure to consider using high-speed video amplifiers if your system requires video switching capability.

Table 1. Muxable High-Speed Amplifiers

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