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

Figure 27. Color Document Scanner

+1

AD8174

+1

+1

+1

2

2

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

IN3

75

Ω

IN2

75

Ω

IN1

IN0

75

Ω

+5V

549

Ω

A1

A0

A2

+

SD

ENABLE

A1

A0

+1

AD8174

+1

+1

+1

2

2

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

75

Ω

Figure 26. 8-to-1 Multiplexer