AD8170/AD8174
+
IN0
75Ω
1
IN1
75Ω
3
IN2
75Ω
0.1µF
–5V
+
10µF
IN3
75Ω
A0
+
IN4
75Ω
1
IN5
75Ω
3
IN6
75Ω
0.1µF
–5V
+
10µF
IN7
75Ω
+1
2
AD8174
+1
+V
S
14
13
12
2
10µF
0.1µF
+5V
R
BT
75Ω
V
OUT
549Ω
SD
+5V
A2
R
T
*
549Ω
2 GND
+1
4 GND
5
+1
11
LOGIC
10
9
8
ENABLE
A1
A0
6 –V
S
7
+1
2
A1
R
T
*
R
T
*
AD8174
+1
+V
S
14
13
12
11
LOGIC
10
9
8
SD
2 GND
10µF
0.1µF
+5V
549Ω
+5V
549Ω
4 GND
5
+1
ENABLE
A1
A0
6 –V
S
7
+1
2
*
OPTIONAL
Figure 26. 8-to-1 Multiplexer
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.
CONTROL AND TIMING
A0 A1 SD
ENABLE
R
CDS
IN0
CCD
G
CDS
AD8174
IN1
V
OUT
CDS
REFERENCE
IN2
IN3
–V
IN
1kΩ
(G = +1)
AD876
8/10-BIT
20MSPS
A/D
B
Figure 27. Color Document Scanner
REV. 0
–11–
AD [ ANALOG DEVICES ]
