Dec 2003 Video Signal Distribution Using Low Supply-Voltage Amplifiers

DESIGN IDEAS
Video Signal Distribution Using
Low Supply-Voltage Amplifiers
by Jon Munson
Introduction
Video designs are often pressed to
operate on the lowest possible rail
voltages—a simple result of the trend
towards lower voltage logic and the
advantages of sharing supply potentials wherever possible. Video designs
are further complicated by the need
to accommodate the dynamic offset
variation inherent in AC-coupled
designs as picture content varies.
Traditional op amps require relatively
large amounts of output-swing headroom and are therefore impractical for
AC coupling at even 5V. Linear Technology offers a new family of video op
amps which addresses these issues
and offers the ability to operate down
to 3.3V in most instances. This family
includes the LT6205 (single), LT6206
(dual), LT6207 (quad), LT6550 (triple,
fixed gain of 2), and LT6551 (quad,
fixed gain of 2).
Video Signal Characteristics
To determine the minimum video amplifier supply voltage, we must first
examine the nature of the signal. Composite video is the most commonly used
signal in broadcast-grade products.
Composite video combines luma (or
Figure 1. Typical 1Vp-p video
waveform (several lines overlaid)
luminance, the intensity information),
chroma (the colorimetry information)
and sync (vertical and horizontal raster
timing) elements into a single signal,
NTSC and PAL being the common
formats. The component video formats for entertainment systems use
separate signal(s) for the luma and
chroma (i.e. Y/C or YPbPr) with sync
generally applied to the luma channel
(Y signal). In some instances, native
RGB signals (separate intensity information for each primary color: red,
green, blue) have sync included as
well. All the signal types that include
sync are electrically similar from a volt-
3.3V
499Ω
1µF
499Ω
75Ω
VOUT1
8
75Ω
LT6206
2
VIN
–
3
+
5
+
1
75Ω
6
7
–
499Ω
VOUT2
75Ω
4
499Ω
75Ω
F3dB ≈ 50MHz
IS ≤ 25mA
Figure 2. DC-coupled dual composite-video driver powered from 3.3V.
36
age-swing standpoint, though various
timing and bandwidth relationships
exist depending on the applicable
standard.
The typical video waveforms that
include sync (including full composite)
are specified to have a nominal 1.0VP–P
amplitude, as shown in Figure 1. The
lower 0.3V is reserved for sync tips that
carry timing information. The black
level (zero intensity) of the waveform
is at (or setup very slightly above)
this upper limit of the sync information. Waveform content above the
black-level is intensity information,
with peak brightness represented
at the maximum signal level. The
sync potential represents blackerthan-black intensity, so scan retrace
activity is invisible on a CRT. In the
case of composite video, the modulated
color subcarrier is superimposed on
the waveform, but the dynamics generally remain inside the 1VP–P limit (a
notable exception is the chroma ramp
used for differential-gain and differential-phase measurements, which can
reach 1.15VP–P).
Amplifier Considerations
Most video amplifiers drive cables that
are series terminated (back-terminated) at the source and load-terminated
at the destination with resistances
equal to the cable characteristic
impedance, Z0 (usually 75Ω). This
configuration forms a 2:1 resistor
divider in the cabling that must be
corrected in the driver amplifier by
delivering 2.0VP–P output into an effective 2 • Z0 load (e.g. 150Ω). Driving
the cable can require in excess of 13mA
while the output is approaching the
saturation-limits of the amplifier output. The absolute minimum supply is
VMIN = 2.0 + VSATH + VSATL, where the
VSAT values represent the minimum
voltage drops that an amplifier can be
guaranteed to develop with respect to
the appropriate supply rail.
Linear Technology Magazine • December 2003
DESIGN IDEAS
BLACK
FIELD
WHITE
FIELD
Figure 3. Video offset-shift due to picture content with conventional AC coupling
For example, the LT6206 dual
operating on 3.3V in Figure 2, with
exceptionally low VSATH ≤ 0.5V and
VSATL ≤ 0.35V, provides a design margin of 0.45V, enough to cover supply
variations and DC bias accuracy for
the DC-coupled video input.
Handling AC-Coupled
Video Signals
Unfortunately, one cannot always be
assured that source video has the
appropriate DC content to satisfy the
amplifier involved, so other design
solutions are frequently required.
AC-coupled video inputs are intrinsically more difficult to handle than
those with DC-coupling because the
average signal voltage of the video
waveform is effected by the picture
content, meaning that the black-level
at the amplifier wanders with scene
brightness. By analyzing the worstcase wander, we can determine the
AC-coupled constraint.
Figure 3 shows two superimposed
AC-coupled waveforms, the raised
trace being black field and the lowered
trace being white field. The wander
is measured as 0.56V for the 1VP–P
NTSC waveform shown, so an additional 1.12V allowance must be made
in the amplifier supply (assuming gain
LTC1921, continued from page 35
Finally, the combined RTN reference
allows each supply monitor to properly
indicate status even if a fuse opens,
since power is supplied to both sets of
outputs as long as at least one supply
is functional.
Linear Technology Magazine • December 2003
of 2, so VMIN = 3.12 + VSATH + VSATL).
The amplifier output (for gain of 2)
must swing +1.47V to –1.65V around
the DC-operating point, so the biasing
circuitry needs to be designed accordingly for optimal fidelity. For example,
an LT6551 operating on 5.0V, with
excellent VSATH ≤ 0.8V and VSATL ≤
0.2V, has a healthy design margin of
0.88V for the Luminance (with sync)
signal. The chroma signal is a symmetric color subcarrier waveform that
is about 0.7VP–P max, so it works fine
with the same bias as the Luminance
channel.
A popular method of further minimizing supply requirements with
AC-coupling is to employ a simple
clamping scheme as shown in Figure 4.
In this circuit, the LT6205 (single version of LT6206) is able to operate from
3.3V by having the sync-tips control
the charge on the coupling capacitor,
thereby reducing the black-level input
wander to ≈ 0.07V. A minor drawback
to this circuit is the slight sync-tip
compression (≈ 0.025V at input) due to
the diode conduction current, though
the picture content remains full fidelity. This circuit has nearly the design
margin of its DC-coupled counterpart,
at 0.31V (for this circuit, VMIN = 2.14
+ VSATH +VSATL).
Conclusion
With the industry’s lowest VSAT output characteristics, the low voltage
video amplifiers, including the LT6205
(single), LT6206 (dual), LT6207 (quad),
LT6550 (triple, fixed gain of 2), and
LT6551 (quad, fixed gain of 2), offer
the video designer the ability to share
reduced supply voltages along with the
logic circuitry. This ability to share
supply voltages helps save space and
cost by reducing power dissipation and
power converter complexity.
3.3V
0.1µF
1k
75Ω
1k
2.4k
4
COMPOSITE
VIDEO
IN 1VP-P
4.7µF
3
75Ω
5
+
LT6205
–
BAT54
10k
4.7µF
VIDEO
OUT
470Ω
1
2
IS ≤ 19mA
IS ≈ 5mA WHEN NOT LOADED (IDLE)
Figure 4. AC-coupled clamped-video amplifier powered from 3.3V
Conclusion
The LTC1921 reduces the board space,
component costs and engineering time
devoted to telecom power supply
monitoring circuitry. It provides an
integrated precision monitoring sys-
tem contained entirely in an MSOP–8
or SO–8 package. Despite its remarkably simple setup requirements, the
LTC1921 is flexible enough to work in a
wide variety of complicated redundant
power supply systems.
37
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