AN9508: Video Multiplexer Delivers Lower Signal Degradation

850MHz Video Multiplexer Delivers Lower Signal
Degradation (EL5166)
®
Application Note
Video multiplexers pose a difficult design challenge. They
must perform several functions, such as matching the input
line impedance, signal amplification, signal switching, and
driving the output line, without degrading or adding noise
and transients to the signal. Typically, the signal flows
through the multiplexer where it’s degraded by the
multiplexers errors. In this design, the signal flows through
the op amp and thus isn’t degraded by the multiplexer.
This circuit can multiplex several sources like VCRs, tuners,
or cameras into a single monitor, see Figure 1. The EL5166
performs all of the multiplexer and amplification functions with
the aid of the TTL decoder. It exceeds the gain flatness,
differential phase, and differential gain specifications for
NTSC video, without adding the offset voltages, gain
variability, or transients associated with multiplexers.
Turning our attention to op amp U1, R1 terminates the input
cable in its characteristic impedance, which usually is 75Ω in
video systems. R4 back terminates the output cable in its
characteristic impedance of 75Ω. Because the cable
termination is 75Ω, it forms a voltage divider with R4 , which
has a gain of 0.5. The op amp is configured for a gain of two,
therefore the circuit has an overall gain of one when driving
a double terminated cable. The value of R3 can be changed
according to the formula G = 0.5 (1 + R2/R3). R2 determines
the video performance of the op amp, so it should not be
changed. The circuits U2 through U4, perform similarly.
If more than one video output is needed, R4, R8, R12, and
R16 can be paralleled with 75Ω resistors. Each resistor is
connected from the respective op amp output to a second
video output that can drive another 75Ω cable.
U5 is configured as a two-to-four line decoder, with A0 and
A1 acting as the select inputs and E3 as the enable input. All
of the amplifiers are disabled when E3 is low, so there’s no
output signal. When E3 is high, the select inputs determine
the video input that’s connected to the video output. If E3 is
used to disable the outputs when the select inputs are
changed, there will be minimal bus contention transients
during switching. However, if hot switching is desired, a
break-before-make delay circuit should be placed in series
with the YX lines. Because all of the signal switching occurs
within the EL5166, the amplifier’s differential phase and gain
parameters (0.03 degrees and 0.03%, respectively)
determine the circuit’s performance.
October 12, 2004
VIDEO
INPUT
#1
3
+
4 -
R1
75
3
+ 1
4 U2
5
Y0
Y1
Y2
U5
A0
SELECT
INPUTS
A1
Y3
E3
E1 E2 A2 ENABLE
INPUT
-5V
VIDEO
INPUT
#3
3
4
R9
75
U3
2
+
-
1
R12
75
5
R10
681
R11
681
+5V
VIDEO
INPUT
#4
3
4
U4
R13
75
R15
681
0.1µF
R8
75
R6
681
R7
681
+5V IN
VIDEO OUTPUT
TO 75Ω LOAD
R2
681
R3
681
R5
75
R4
75
5
U1
VIDEO
INPUT
#2
1
AN9508.2
6
+
-
1
R16
75
5
R14
681
+5V
+
10µF
-5V IN
0.1µF
-5V
10µF
+
NOTES:
1. U1, U2, U3, U4 are EL5166
2. All resistors in Ω
3. U5 is CD74HC238
4. Use ground plane
FIGURE 1. SEVERAL DIFFERENT SOURCES, SUCH AS
VCRs, TUNERS, OR CAMERAS, CAN BE
MULTIPLEXED INTO A SINGLE MONITOR USING
THIS VIDEO MULTIPLEXER. WITH THE AID OF
TTL DECODER, THE EL5166 PERFORMS ALL
MULTIPLEXER AND AMPLIFICATION
FUNCTIONS.
It’s easy to multiplex any number of channels with this
scheme because the single and dual versions of the op amp
help minimize the number of ICs required for a given design.
In addition, the decoder can be easily extended to 3 to 8
using the same IC or 4 to 16 using a different decoder. The
circuit given here switches in less than a microsecond.
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CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 9508
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