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AD8182AR-REEL7 датащи(PDF) 9 Page - Analog Devices |
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AD8182AR-REEL7 датащи(HTML) 9 Page - Analog Devices |
9 / 12 page AD8180/AD8182 –9– REV. B Picture-in-Picture or Pixel Switching Many high end display systems require simultaneous display of two video pictures (from two different sources) on one screen. Video conferencing is one such example. In this case the remote site might be displayed as the main picture with a picture of the local site “inset” for monitoring purposes. The circuit in Fig- ure 23 could also be used to implement this “picture-in-picture” application. Implementing a picture-in-picture algorithm is difficult for several reasons. Both sources are being displayed simultaneously (i.e., during the same frame), both sources are in real time, and both must be synchronized. Figure 24 shows the raster scan- ning that takes place in all monitors. During every horizontal scan that includes part of the inset, the source must be switched twice (i.e., from main to inset and from inset to main). To avoid screen artifacts, it is critical that switching is clean and fast. The AD8180 and AD8182, in the above application, switch and settle to 0.1% accuracy in 14 ns. We quadratically add this value to the 10 ns settling time of the AD8001, and get an over- all settling time of 17.2 ns. This yields a sharp, artifact-free border between the inset and the main video. INSET VIDEO MULTIPLEXER MUST SWITCH CLEANLY ON EACH CROSSING MAIN VIDEO Figure 24. “Picture-in-Picture,” Pixel Switching Color Document Scanner Figure 25 shows a block diagram of a Color Document Scan- ner. Charge Coupled Devices (CCDs) find widespread use in scanner applications. A monochrome CCD delivers a serial stream of voltage levels, each level being proportional to the light shining on that cell. In the case of the color image scanner shown, there are three output streams, representing red, green and blue. Interlaced with the stream of voltage levels is a voltage representing the reset level (or black level) of each cell. A Corre- lated Double Sampler (CDS) subtracts these two voltages from each other in order to eliminate the relatively large offsets which are common with CCDs. CONTROL AND TIMING AD876 8/10-BIT 20MSPS A/D AD8182 OUT A OUT B CDS CDS CDS REFERENCE R G B C C D IN0 A IN1 A IN1 B IN0 B 100 4:1 MUX TRUTH TABLE SEL A, SEL B ENA, ENB OUTA, OUTB 0 0 1 1 0 1 0 1 IN0A IN0B IN1A IN1B Figure 25. Color Document Scanner The next step in the data acquisition process involves digitizing the three signal streams. Assuming that the analog to digital converter chosen has a fast enough sample rate, multiplexing the three streams into a single ADC is generally more eco- nomic than using one ADC per channel. In the example shown, we use the two 2-to-1 multiplexers in the AD8182 to create a 4-to-1 multiplexer. The enable control pins on the multiplexers allow the outputs to be wired directly together. Because of its high bandwidth, the AD8182 is capable of driv- ing the switched capacitor input stage of the AD876 without additional buffering. In addition to having the required the bandwidth, it is necessary to consider the settling time of the multiplexer. In this case, the ADC has a sample rate of 20 MHz which corresponds to a sampling period of 50 ns. Typically, one phase of the sampling clock is used for conversion (i.e., all levels are held steady) and the other phase is used for switch- ing and settling to the next channel. Assuming a 50% duty cycle, the signal chain must settle within 25 ns. With a settling time to 0.1% of 14 ns, the multiplexer easily satisfies this criterion. In the example shown, the fourth (spare) channel of the AD8182 is used to measure a reference voltage. This voltage would probably be measured less frequently than the R, G and B signals. Multiplexing a reference voltage offers the advantage that any temperature drift effects caused by the multiplexer will equally impact the reference voltage and the to-be- measured signals. If the fourth channel is unused, it is good design practice to tie this input to ground. |
Аналогичный номер детали - AD8182AR-REEL7 |
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Аналогичное описание - AD8182AR-REEL7 |
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