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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AD8174AR-REEL Fiches technique(PDF) 11 Page - Analog Devices |
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AD8174AR-REEL Fiches technique(HTML) 11 Page - Analog Devices |
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11 / 16 page  AD8170/AD8174 –11– REV. 0 Color Document Scanner 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 Correlated Double Sampler (CDS) subtracts these two voltages from each other in order to eliminate the relatively large offsets which are common with CCDs. 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 economic than using one ADC per channel. In the example shown, the AD8174 is used to multiplex the red, green and blue channels into the AD876, an 8- or 10-bit 20 MSPS ADC. Because of its high bandwidth, the AD8174 is capable of driving the switched capacitor input stage of the AD876 without additional buffering. In addition to 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 switching 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 15 ns, the multiplexer easily satisfies this criterion. In the example shown, the fourth (spare) channel of the AD8174 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 the input permanently to ground. AD8174 REFERENCE R G B A0 A1 SD ENABLE IN0 IN1 IN2 IN3 1k Ω (G = +1) CCD CDS CDS CDS CONTROL AND TIMING AD876 8/10-BIT 20MSPS A/D VOUT –VIN Figure 27. Color Document Scanner +1 AD8174 +1 +1 +1 2 2 +VS –VS GND GND 1 2 3 4 5 6 78 9 10 11 12 13 14 0.1µF +5V 10µF VOUT 549 Ω RT* + 10µF 0.1µF 75 Ω –5V IN3 75 Ω IN2 75 Ω IN1 IN0 75 Ω +5V 549 Ω A1 RT* A0 RT* A2 + SD ENABLE A1 A0 +1 AD8174 +1 +1 +1 2 2 +VS –VS GND GND 1 2 3 4 5 6 78 9 10 11 12 13 14 0.1µF +5V 10µF 549 Ω + 10µF 0.1µF 75 Ω –5V IN7 75 Ω IN6 75 Ω IN5 IN4 75 Ω +5V 549 Ω + SD ENABLE A1 A0 *OPTIONAL RBT 75 Ω Figure 26. 8-to-1 Multiplexer |
Numéro de pièce similaire - AD8174AR-REEL |
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Description similaire - AD8174AR-REEL |
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