AD8184 Analog Devices, AD8184 Datasheet - Page 8

• Manufacturer Part Number: AD8184
• Description: 700 MHz/ 5 mA 4-to-1 Video Multiplexer
• Manufacturer: Analog Devices

AD8184
Chip capacitors should be used for supply bypassing. One end
of the capacitor should be connected to the ground plane and
the other within 1/4 inch of each power pin. An additional large
(4.7 F–10 F) tantalum capacitor should be connected in par-
allel with each of the smaller capacitors for low impedance sup-
ply bypassing over a broad range of frequencies.
Signal traces should be as short as possible. Stripline or
microstrip techniques should be used for long (longer than
about 1 inch) signal traces. These should be designed with a
characteristic impedance of 50
minated at each end using surface mount components.
Careful layout is imperative to minimize crosstalk. Guards
(ground or supply traces) must be run between all signal traces
to limit direct capacitive coupling. Input and output signal lines
should fan out away from the mux as much as possible. If mul-
tiple signal layers are available, a buried stripline structure hav-
ing ground plane above, below and between signal traces will
have the best crosstalk performance.
Return currents flowing through termination resistors can also
increase crosstalk if these currents flow in sections of the finite-
impedance ground circuit shared between more than one input
or output. Minimizing the inductance and resistance of the ground
plane can reduce this effect, but further care should be taken in po-
sitioning the terminations. Terminating cables directly at the con-
nectors will minimize the return current flowing on the board, but
the signal trace between the connector and the mux will look like
an open stub and will degrade the frequency response. Moving the
termination resistors close to the input pins will improve the fre-
quency response, but the terminations from neighboring inputs
should not have a common ground return.
IN0
IN1
IN2
IN3
75
75
75
75
or 75
4
1
3
5
6
2
7
and be properly ter-
GND
GND
GND
Figure 22. A Buffered 4-to-1 Multiplexer
+1
+1
+1
+1
AD8184
DECODER
+V
–V
NC
S
S
14
13
12
11
10
9
8
–8–
APPLICATIONS
A Buffered 4-to-1 Multiplexer
In applications where the output of a multiplexer must drive a
back-terminated 75
to buffer the output of the AD8184. In the example in Figure
22, this is accomplished using the AD8009 high speed current
feedback op amp. The amplifier is configured with a gain of 2 to
compensate for the signal halving due to termination at the multi-
plexer input. This gives the overall circuit a gain of unity.
If lower speed can be tolerated, a number of other amplifiers
can replace the AD8009 op amp in the above circuit. In general
there is a trade-off between bandwidth and power consumption.
Table II summarizes the bandwidth and power consumption
characteristics of these op amps.
Op Amp Comments
AD8009 Highest Bandwidth, (G = +2) = 700 MHz, I
AD8001 Lower Power Consumption, Bandwidth (G = +2) =
AD8011 Lower Power Consumption, Bandwidth (G = +2) =
AD8079 Fixed Gain Dual Amplifier (2 or 2.2), Bandwidth =
AD8005 Lowest Power Consumption, Bandwidth (G = +2) =
10µF
0.1µF
10µF
0.1µF
Table II. Amplifier Options for Multiplexer Buffering
+V
A0
–V
14 mA
440 MHz, I
210 MHz, I
260 MHz, I
170 MHz, I
S
S
A1
681
AD8009
line (R
SY
SY
SY
SY
–V
+V
681
S
S
= 5 mA
= 1 mA
= 5 mA Per Amp
= 400 A
10µF
10µF
L
0.1µF
0.1µF
= 75
+ 75 ), it is necessary
75
V
OUT
SY
REV. 0
=