Circuits & schematics

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Electronic circuits and electronic schematics

deo multiplexers route video from several sources to a single channel. Low-end consumer products use CMOS analog switches and multiplexers, such as the 4066 and 4051. Unfortunately, these devices have a series on-resistance that ranges from approximately 100Ω to 1 kΩ, a resistance that is not constant with video level and that appears in series with the signal. The traditional way of solving this problem is by buffering the analog-switch outputs with transistor stages. With this approach, the characteristics of the CMOS switch and the buffer stage degrade video performance. However, if you forget the multiplexing action for a moment and consider just the buffer-amplifier function, you will see that a better approach exists. It must present high enough input impedance to the switch that a 1-kΩ switch resistance is inconsequential and that variation in resistance of the switch with IRE (Institute of Radio Engineers) level produces no luminance shifts. Figure 1 shows a configuration using high-speed op amps in a video-multiplexing application.

High-speed op amps have plenty of bandwidth for video applications. By using an op amp that has 20 or more times the video bandwidth, roll-off and phase shift at 6 MHz are negligible. An op amp has high input impedance in the noninverting mode. You can terminate it for 75Ω input impedance by connecting a simple resistor. Two equal resistors create a gain of two in the noninverting configuration. The gain compensates for a 75Ω back-termination resistor on the op amp's output. The overall stage gain is therefore one. Now, consider the multiplexing function. Powering down an op amp disables the output, producing a 0V (0 IRE) black level on its output. Its output can therefore connect in parallel with the outputs of other op amps, because it contributes no luminance or sync pulses. This feature enables op amps to operate as video multiplexers. The multiplexer in Figure 1 shows a three-position, single-pole rotary switch. This switch could be a "break-before-make" or an electronic switching system, perhaps with an intelligent infrared interface.

To test the quality of signals passed through the video multiplexer, this design uses the Lucasfilm THX (www.thx.com) test patterns on one video input and a high-quality NTSC program source on another input. When the contrast/picture test goes through the video multiplexer as the active source, the presence of the op amp as a buffer has no effect on black and white levels. No bleeding or blooming occurs. Any crosstalk results in a visible brightening of the center of the picture in the video-program source, but none occurs. You set the brightness level with the video multiplexer, not in the circuit. Then, you insert the video buffer into the signal path. You'll find that brightness level does not change. The brightness setup test is also an ideal way to test for crosstalk between two video channels. Crosstalk would show up on the black background as a "ghost" image of the program material on the inactive channel; however, none occurs. No color shifts appear in the SMPTE (Society of Motion Picture and Television Engineers) bars with or without the video multiplexer in the signal chain. The color-bar patterns would also produce color shifts in the other channel if crosstalk were a factor. Human skin is the toughest color to get "right," and any change in skin tone arising from color crosstalk is apparent. No flesh-tone color shifts occur in the test.

Title: Video multiplexer uses high-speed op amps
Source: http://www.edn.com
Published on: 2005-06-19
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