AN9507: Video Cable Drivers Save Board Space, Increase

Video Distribution Amplifier Saves Board Space,
Increases Bandwidth (EL8108)
®
October 12, 2004
Application Note
Designing video cable drivers seems to be a fairly simple
task. Just buy an amplifier with enough bandwidth, high
output current, a gain of two or greater (eliminating most
buffers) to counteract attenuation from back-terminating the
cable, and good video specifications (gain flatness if you are
designing for component video; differential gain and phase if
you are designing for composite video), and you’re in
business.
Of course, picking a current feedback amplifier adds a few
additional worries such as choosing the optimum feedback
resistor, and minimizing the capacitance on both the
summing node (-Input) and output. Still another problem is
achieving the desired performance at typical video loads
(≤75Ω if driving multiple back-terminated cables).
Choosing dual or quad amplifiers and/or SOIC packaging
complicates the equation even further. How does the
engineer find a way to optimally place eight gain-setting
resistors, not to mention termination resistors, around a
quad amplifier in an SOIC package? There is no easy
solution. Compromises must be made, which usually result
in inadequate terminations or long trace lengths.
3dB
0dB
-3dB
-6dB
1MHz
obviously is being limited somewhere in the system.
Invariably, substituting a shorter cable dramatically improves
the image quality, leading to the hypothesis that the cable
driver’s performance degrades when driving long cables.
This hypothesis requires some scrutiny.
It’s true that circuit performance changes when driving cables,
but is it really the cable driver that is at fault? Figure 1
illustrates the performance of Intersil’s EL8108 amplifier
driving 100 feet of back-terminated cable. It shows that the
amplifier’s 250MHz bandwidth decreases to 40MHz over the
measured range, lending credence to the previous
hypothesis. But what’s really happening?
Many engineers forget that all electrical elements have finite
bandwidth. Cables are usually taken for granted, but long
cables can limit system bandwidth to surprisingly low
frequencies. For example, a comparison of the frequency
response of the EL8108 driving the same 100 feet of cable
to the response of the cable alone shows that the problem
isn’t the cable driver, but rather the cable itself (see
Figure 2).
It is abundantly clear from Figure 2 that the cable
performance itself limits the system performance for most of
the frequency range. Throwing a higher bandwidth driver at
the cable will, in fact, gain the engineer designing the system
nothing, because you can’t get more bandwidth than the
cable allows.
GAIN AT CABLE OUTPUT
GAIN AT CABLE OUTPUT
A common complaint when working with long cables
involves a particular type of image degradation. The display
in question exhibits bright horizontal lines but gray vertical
lines. Since it is well known that narrow vertical lines require
higher bandwidth to be displayed properly, the bandwidth
10MHz
0dB
-3dB
-6dB
RESPONSE OF
EL8108 AND
CABLE
10MHz
100MHz
FREQUENCY
FREQUENCY
FIGURE 1. PERFORMANCE RESULTS INDICATE THAT THE
EL8108 AMPLIFIER’S 250MHz BANDWIDTH
DECREASES TO 40MHz WHEN DRIVING 100
FEET OF BACK-TERMINATED CABLE. THIS
SUPPORTS THE HYPOTHESIS THAT A CABLE
DRIVER’S PERFORMANCE DEGRADES WHEN
DRIVING LONG CABLES.
1
RESPONSE OF
CABLE ONLY
3dB
1MHz
100MHz
AN9507.2
FIGURE 2. ALTHOUGH USUALLY TAKEN FOR GRANTED,
LONG CABLES CAN LIMIT SYSTEM
BANDWIDTH TO LOW FREQUENCIES, AS IS
EVIDENT IN THIS COMPARISON BETWEEN THE
FREQUENCY RESPONSE OF THE EL8108
DRIVING THE CABLE AND THE RESPONSE OF
THE CABLE ALONE.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 9507
Upgrading to a higher performance cable, such as a Belden
8281 or equivalent, is one solution to boosting system
bandwidth. There are at least two downsides to this option,
however. The first is that it introduces significantly higher
cable costs. The second is problems presented to
technicians who have to work with more rigid cables.
A better solution may be to use a cable driving buffer such
as Intersil’s EL8108. The driver’s frequency response can be
tunable for a specific cable length via components
connected to the summing node (see Figure 3). By shunting
R1, RC acts to increase the amplifier’s gain while CC
controls the cut-in frequency of the compensation.
These three components peak the amplifier’s frequency
response to counteract the cable’s roll-off characteristic. By
squeezing more bandwidth out of a given cable, higherperformance cables aren’t needed.
R1
RF
1K
1K
CC
RC
+5
-
75Ω
VOUT
+
VIN
25Ω
75Ω
-5
FIGURE 3. INSTEAD OF UPGRADING TO A HIGHER
PERFORMANCE CABLE TO INCREASE SYSTEM
BANDWIDTH, A CABLE DRIVER LIKE THE
EL8108 CAN BE EMPLOYED. THE DRIVER’S
FREQUENCY RESPONSE IS TUNABLE FOR A
SPECIFIC CABLE LENGTH VIA THE
COMPONENTS CONNECTED TO THE SUMMING
NODE.
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Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
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reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
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