Multiplexer amplifiers form large, multiplane-multiplexer structures
Edited by Bill Travis
Bill Stutz, Maxim Integrated Products, Sunnyvale, CA -- EDN, December 26, 2002
The variety of video sources available to a home-video-switching system has grown from a few composite inputs to many multisignal sources. These sources include cable, HDTV, satellite dishes, VCR, DVD, video games in broadcast, and multi-PC or graphic KVM (keyboard-video-mouse) applications. Each requires an N×M-to-1 multiplexer, in which M is the number of sources and N is the number of channels that make up the signal. As an example, 16 RGB or Y, Pb, and Pr sources require a 3×16-to-1 multiplexer. Constructing such a multiplexer is difficult, and programming the source selection requires that you combine the individual 16-to-1 multiplexer control with the three channels (Table 1). You can configure a group of analog multiplexers as a large, multiplane video multiplexer that easily selects multichannel video sources, such as YC, RGB, and Y Pb Pr. The cited 3×16-to-1 multiplexer comprises six 8-to-1 multiplexers (Figure 1) controlled by a 4-bit binary code for source selection. (MAX4315 ICs include a 2V/V fixed-gain output buffer.)
The only external circuitry required is an SN7404 hex inverter for inverting the shutdown signals and 75Ω source and load resistors for implementing unity gain when driving a back-terminated load. Substituting a MAX4312 eight-channel video multiplexer with variable-gain output buffer allows variable gain and rejects input common-mode voltages. The high bandwidth and slew rate of these ICs make them ideal for selecting standard video and high-definition broadcast video, as well as graphics sources with UXGA and higher resolutions for KVM applications. The design requires no additional buffering, because the ICs can directly drive 150Ω back-terminated coaxial cable to within less than 0.75V of the supply rails, using single or dual supplies. Their 40-nsec switching speed and 10-mV p-p glitch voltage allow, in addition to source selection, insertion of on-screen display, closed captioning, and teletext in broadcast and graphics video.
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