AD8184 Просмотр технического описания (PDF) - Analog Devices

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AD8184

700 MHz, 5 mA 4-to-1 Video Multiplexer

Analog Devices 

AD8184

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.

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.

AD8184

4

IN0-IN3 OUT

1/2 AD8079*

OUT0

CONTROL & TIMING

750Ω

750Ω

R

G

CCD

B

CDS

CDS

CDS

REFERENCE

A0 A1 ENABLE

OUT

AD8184

10µF

0.1µF

AD9220

10/12-BIT

10MSPS

A/D

CONVERTER

VINA

VINB

VREF

SENSE

AD8184

4

IN0-IN3 OUT

4

IN0-3

AD8184

4

IN0-IN3 OUT

1/2 AD8079*

750Ω

750Ω

OUT1

1/2 AD8079*

OUT2

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.

750Ω

750Ω

AD8184

4

IN0-IN3

OUT

1/2 AD8079*

750Ω

750Ω

OUT3

*AD8079 IS A DUAL, FIXED GAIN OF 2 AMPLIFIER

Figure 24. 4 × 4 Crosspoint Switch

REV. 0

–9–

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