EL4501IU(2004) 데이터 시트보기 (PDF) - Intersil
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EL4501IU Datasheet PDF : 19 Pages
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EL4501
DATA SLICER
The data slicer is a fast comparator with the output of the
video amplifier connected to its inverting input and the
DS REF connected to its non-inverting input. The DS OUT is
logical inverse of the video output sliced at the DS REF
voltage. The propagation delay from the video amplifier
output to the DS OUT is about 18ns. There is about 10mV
hysteresis added internally in the comparator to prevent the
oscillation at the DS OUT when the voltages at the two
inputs are very close or equal. An adjustable DS REF
voltage may be used to extract data embedded within the
active video or video blanking interval regions of a video
signal. Logic low at the DS ENABLE pin enables the
comparator and logic low lets the DS OUT be three-state.
The DS MODE pin sets the mode of the DS comparator.
Logic low at the DS MODE pin selects a standard logic
output and a logic high selects an open drain/collector
output.
VIDEO AMPLIFIER
The EL4501 DC-restore block incorporates a wide
bandwidth, single supply, low power, rail-to-rail output,
voltage feedback operational amplifier. The amplifier is
internally compensated for closed loop feedback gains of +1
or greater. Larger gains are acceptable but bandwidth will be
reduced according to the familiar Gain-Bandwidth product.
Connected in a voltage follower mode and driving a high
impedance load, the amplifier has a -3dB bandwidth of
100MHz. Driving a 150Ω load, the -3dB bandwidth reduces
to 60MHz while maintaining a 200V/µs slew rate.
CHOICE OF FEEDBACK RESISTOR, RF
The video amplifier is optimized for applications that require
a gain of +1. Hence, no feedback resistor is required.
However, for gains greater than +1, the feedback resistor
forms a pole with the hold capacitance. As this pole
becomes larger, phase margin is reduced. This causes
ringing in the time domain and peaking in the frequency
domain. Therefore, RF has some maximum value that
should not be exceeded for optimum performance. If a large
value of RF must be used, a small capacitor in the few
picofarad range in parallel with RF can help to reduce ringing
and peaking at the expense of reducing the bandwidth. As
far as the output stage of the amplifier is concerned, RF +
RG appear in parallel with RL for gains other than +1. As this
combination gets smaller, the bandwidth falls off.
Consequently RF also has a minimum value that should not
be exceeded for optimum performance.
• For AV = +1, RF = 0Ω is optimum
• For AV = +2, RF between 300Ω and 1kΩ is optimum
VIDEO PERFORMANCE
For good video signal integrity, an amplifier is required to
maintain the same output impedance and frequency
response as DC levels are changed at the output. This can
be difficult when driving a standard video load of 150Ω
because of the change in output current with DC level. A
look at the Differential Gain and Differential Phase curves
will help to obtain optimal performance. Curves are provided
for AV = +1 and +2, and RL = 150Ω and 10kΩ. As with all
video amplifiers, there is a common mode sweet spot for
optimum differential gain/differential phase. For example,
with AV = +1 and RL = 150Ω and the video level kept
between 1V and 3V, the amplifier will provide dG/dP
performance of 0.17%/0.07°. This condition is representative
of using the amplifier as a buffer driving a DC coupled,
double terminated, 75Ω coaxial cable. Driving high
impedance loads, such as signals on computer video cards
gives much better dG/dP performance. For AV = 1,
RL = 10kΩ, and the video level kept between 1V and 3V, the
dG/dP are 0.03%/0.02°.
SHORT-CIRCUIT CURRENT LIMIT
The EL4501 video amplifier has no internal short circuit
protection circuitry. Short circuit current of 90mA sourcing
and 65mA sinking typically will flow if the output is shorted
midway between the rails. If the output is shorted indefinitely,
the power dissipated could easily increase the die
temperature such that the part will be destroyed. Maximum
reliability is maintained if the output current never exceeds
±50mA. This limit is set by internal metal interconnect
limitations. Obviously, short circuit conditions must not be
allowed to persist or internal metal connections will be
damaged or destroyed.
DRIVING CABLES AND CAPACITIVE LOADS
The EL4501 video amplifier can drive 39pF loads in parallel
with 150Ω with 5dB of peaking. For less peaking in theses
applications a small series resistor of between 5Ω and 50Ω
can be placed in series with the output. However, this will
obviously reduce the gain slightly. If your gain is greater than
1, the gain resistor RG can be adjusted to make up for any
lost gain caused by the additional output resistor. Peaking
may also be reducing by adding a “snubber” circuit at the
output. A snubber is a resistor in series with a capacitor,
150Ω and 100pF being typical values. The advantage of a
snubber is that it does not draw DC load current.
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, the back-termination series resistor decouples
the video amplifier from the cable and enables extensive
capacitive drive. However, other applications may have high
capacitive loads without a back-termination resistor. Again, a
small series resistor at the output can reduce peaking.
VIDEO SYNC SEPARATOR
The EL4501 includes an advanced sync separator, which is
used to generate the DC-restore control signal and seven
major sync outputs. The advanced sync separator operates
at a 5V DC (pin VSD) single-supply voltage. The input signal
source is composite video with levels of 0.5VP-P to 2.0VP-P.
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