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AD8184-EB Datasheet(PDF) 9 Page - Analog Devices |
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AD8184-EB Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page AD8184 –9– REV. 0 Color Document Scanner Figure 23 shows a block diagram of a Color Document Scanner. Charge Coupled Devices (CCDs) find widespread use in scan- ner applications. A monochrome CCD delivers a serial stream of voltages levels, each level being proportional to the light shin- ing 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 repre- senting the reset level (or black level) of each cell. A Correlated Double Sampler (CDS) subtracts these two voltages from each other in order to eliminate the relatively large offsets common with CCDs. CONTROL & TIMING CDS CDS 0.1µF 10µF VINB VREF SENSE VINA AD9220 10/12-BIT 10MSPS A/D CONVERTER OUT CDS A1 A0 AD8184 R G ENABLE B CCD REFERENCE Figure 23. 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 economi- cal than using one ADC per channel. In the example shown, we use the AD8184 as the multiplexer. Because of its high bandwidth, the AD8184 is capable of driving the switched capacitor input stage of the AD9220 without addi- tional buffering. In addition to having the required bandwidth, it is necessary to consider the settling time of the multiplexer. In this case, the ADC has a sample rate of 10 MHz, which corre- sponds to a sampling period of 100 ns. Typically, one phase of the sampling clock is used for conversion (i.e., all levels are held steady) and the other is used for switching and settling to the next channel. Assuming a 50% duty cycle, the signal chain must settle within 50 ns. With a settling time to 0.1% of 15 ns, the multiplexer easily satisfies this criterion. In the example shown, the fourth (spare) channel of the AD8184 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 sig- nals. If the fourth channel is unused, it is good design practice to permanently tie it to ground. A 4 4 Crosspoint Switch While large crosspoint arrays are best constructed using highly integrated devices such as the AD8116, 16 × 16 crosspoint switch, smaller or irregular sized arrays can be constructed using 4-to-1 multiplexers such as the AD8184. The circuit below shows a 4 × 4 array, constructed using the AD8184 and buff- ered using the AD8079, a dual, fixed gain of 2 or 2.2, video amplifier. 750 Ω 750 Ω OUT 1/2 AD8079* AD8184 IN0-IN3 OUT0 750 Ω 750 Ω OUT 1/2 AD8079* AD8184 IN0-IN3 OUT1 750 Ω 750 Ω OUT 1/2 AD8079* AD8184 IN0-IN3 OUT2 750 Ω 750 Ω OUT 1/2 AD8079* AD8184 IN0-IN3 OUT3 IN0-3 4 4 4 4 4 *AD8079 IS A DUAL, FIXED GAIN OF 2 AMPLIFIER Figure 24. 4 × 4 Crosspoint Switch |
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