LT6206 Linear Technology Corporation, LT6206 Datasheet - Page 13

• Manufacturer Part Number: LT6206
• Description: Lt6206 - Single/dual/quad Single Supply 3v, 100mhz Video Op Amps
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
be 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
limit (a notable exception is the chroma ramp used for
differential-gain and differential-phase measurements,
which can reach 1.15V
DC-Coupled Video Amplifi er Considerations
Typically video amplifi ers 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
confi guration forms a 2:1 resistor divider in the cabling
that must be accounted for in the driver amplifi er by
delivering 2V
150Ω). Driving the cable can require more than 13mA
while the output is approaching the saturation-limits of the
amplifi er 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 diffi cult
to handle than those with DC-coupling because the aver-
age signal voltage of the video waveform is effected by
the picture content, meaning that the black-level at the
amplifi er “wanders” with scene brightness. The wander
is measured as 0.56V for a 1V
ing from black-fi eld to white-fi eld and vice-versa, so an
additional 1.12V allowance must be made in the amplifi er
supply (assuming gain of 2, so V
For example, an LT6205 operating on 5V has a conservative
design margin of 1.03V. The amplifi er output (for gain of
2) must swing +1.47V to –1.65V around the DC-operat-
ing point, so the biasing circuitry needs to be designed
accordingly for optimal fi delity.
OH
+V
OL
. For example, the LT6206 dual operating on
P-P
output into an effective 2 • Z
OH
≤ 0.5V and V
P-P
).
P-P
MIN
NTSC waveform chang-
OL
0
= 3.12 + V
(usually 75Ω). This
≤ 0.35V, provides a
0
load (e.g.
OH
+V
MIN
OL
P-P
).
=
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 ≈ 0.07V. The only 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 fi delity. This circuit
has nearly the design margin of its DC-coupled counter-
part, at 0.31V (for this circuit, V
The 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 amplifi er count topology to transcode
consumer component-video into RGB. In this circuit, sig-
nals 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 amplifi er 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 outputs 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
OL
= 2.14 + V
.
OH
13
+V
620567fa
OL
).