LT6205CS5 LINER [Linear Technology], LT6205CS5 Datasheet - Page 11

• Manufacturer Part Number: LT6205CS5
• Description: Single/Dual/Quad Single Supply 3V, 100MHz Video Op Amps
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
invisible on a CRT. The “black” level of the waveform is at
(or “setup” very slightly above) the upper limit of the sync
information. Waveform content above the black-level is
intensity information, with peak brightness represented at
the maximum signal level. In the case of composite video,
the modulated color subcarrier is superimposed on the
waveform, but the dynamics remain inside the 1V
(a notable exception is the chroma ramp used for differen-
tial-gain and differential-phase measurements, which can
reach 1.15V
DC-Coupled Video Amplifier Considerations
Typically 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, Z
configuration forms a 2:1 resistor divider in the cabling
that must be accounted for in the driver amplifier by
delivering 2V
150 ). Driving the cable can require more than 13mA
while the output is approaching the saturation-limits of the
amplifier output. The absolute minimum supply is: V
2 + V
3.3V as shown on the front page of this datasheet, with
exceptionally low V
design margin of 0.45V. The design margin must be large
enough to include supply variations and DC bias accuracy
for the DC-coupled video input.
Handling AC-Coupled Video Signals
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 ampli-
fier “wanders” with scene brightness. The wander is
measured as 0.56V for a 1V
from black-field to white-field and vice-versa, so an addi-
tional 1.12V allowance must be made in the amplifier
supply (assuming gain of 2, so V
For example, an LT6205 operating on 5V has a conserva-
OH
+V
OL
P-P
. For example, the LT6206 dual operating on
P-P
).
output into an effective 2 • Z
OH
U
0.5V and V
U
P-P
NTSC waveform changing
MIN
0
W
OL
(usually 75 ). This
= 3.12 + V
0.35V, provides a
0
U
load (e.g.
OH
P-P
+V
MIN
limit
OL
).
=
tive design margin of 1.03V. 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.
Clamped AC-Input Cable Driver
A popular method of further minimizing supply require-
ments with AC-coupling is to employ a simple clamping
scheme as shown in Figure 2. In this circuit, the LT6205
operates from 3.3V by having the sync-tips control the
charge on the coupling capacitor C1, thereby reducing the
black-level input wander to
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 counter-
part, at 0.31V (for this circuit, V
clamp-diode anode bias is selected to set the sync-tip
output voltage at or slightly above V
YPbPr to RGB Component-Video Converter
The back-page application uses the LT6207 quad to imple-
ment a minimum amplifier count topology to transcode
consumer component-video into RGB. In this circuit,
signals only pass through one active stage from any input
to any output, with passive additions being performed by
the cable back-termination resistors. The compromise in
using passive output addition is that the amplifier outputs
must be twice as large as that of a conventional cable
driver. The Y-channel section also has the demanding
requirement that it single-handedly drives all three out-
puts to full brightness during times of white content, so a
helper current source is used to assure unclipped video
when operating from 5V supplies. This circuit maps
sync-on-Y to sync on all the RGB channels, and for best
results should have input black-levels at 0V nominal to
prevent clipping.
LT6205/LT6206/LT6207
MIN
0.07V. The only minor
= 2.14 + V
OL
.
OH
+V
OL
11
). The
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