AD8051/AD8052/AD8054
dynamic swing requirements are a signal that is mostly low but
goes high with a duty cycle that is a small fraction of a percent,
and the other extreme defined by the opposite condition.
SYNC STRIPPER
Synchronizing pulses are sometimes carried on video signals so
as not to require a separate channel to carry the synchronizing
information. However, for some functions, such as analog-to-
digital conversion, it is not desirable to have the sync pulses on
the video signal. These pulses reduce the dynamic range of the
video signal and do not provide any useful information for such
a function.
The worst case of composite video is not quite this demanding.
One bounding condition is a signal that is mostly black for an
entire frame but has a white (full amplitude) minimum width
spike at least once in a frame.
The other extreme is for a full white video signal. The blanking
intervals and sync tips of such a signal have negative-going
excursions in compliance with the composite video specifications.
The combination of horizontal and vertical blanking intervals
limit such a signal to being at the highest (white) level for a
maximum of about 75% of the time.
A sync stripper removes the synchronizing pulses from a video
signal while passing all the useful video information. Figure 51
shows a practical single-supply circuit that uses only a single
AD8051. It is capable of directly driving a reverse terminated
video line.
VIDEO WITHOUT SYNC
VIDEO WITH SYNC
As a result of the duty cycles between the two extremes
previously presented, a 1 V p-p composite video signal that is
multiplied by a gain of 2 requires about 3.2 V p-p of dynamic
voltage swing at the output for an op amp to pass a composite
video signal of arbitrarily varying duty cycle without distortion.
V
GROUND
0.4V
BLANK
GROUND
3V OR 5V
Some circuits use a sync tip clamp to hold the sync tips at a
relatively constant level to lower the amount of dynamic signal
swing required. However, these circuits can have artifacts, such
as sync tip compression, unless they are driven by a source with
a very low output impedance. The AD8051/AD8052/AD8054
have adequate signal swing when running on a single 5 V
supply to handle an ac-coupled composite video signal.
+
10µF
0.1µF
7
V
3
2
IN
TO A/D
6
AD8051
100Ω
4
R2
1kΩ
R1
1kΩ
The input to the circuit in Figure 52 is a standard composite
(1 V p-p) video signal that has the blanking level at ground. The
input network level shifts the video signal by means of ac coupling.
The noninverting input of the op amp is biased to half of the
supply voltage.
0.8V
(OR 2 × V
)
BLANK
Figure 51. Sync Stripper
The video signal plus sync is applied to the noninverting input
with the proper termination. The amplifier gain is set to 2 via
the two 1 kꢀ resistors in the feedback circuit. A bias voltage
must be applied to R1 so that the input signal has the sync
pulses stripped at the proper level.
The feedback circuit provides unity gain for the dc-biasing of
the input and provides a gain of 2 for any signals that are in the
video bandwidth. The output is ac-coupled and terminated to
drive the line.
The blanking level of the input video pulse is the desired place to
remove the sync information. This level is multiplied by 2 by the
amplifier. This level must be at ground at the output for the sync
stripping action to take place. Since the gain of the amplifier from
the input of R1 to the output is −1, a voltage equal to 2 × VBLANK
must be applied to make the blanking level come out at ground.
The capacitor values were selected for providing minimum tilt
or field time distortion of the video signal. These values would
be required for video that is considered to be studio or broadcast
quality. However, if a lower consumer grade of video, sometimes
referred to as consumer video, is all that is desired, the values
and the cost of the capacitors can be reduced by as much as a
factor of five with minimum visible degradation in the picture.
SINGLE-SUPPLY COMPOSITE VIDEO LINE DRIVER
5V
Many composite video signals have their blanking level at
ground and have video information that is both positive and
negative. Such signals require dual-supply amplifiers to pass
them. However, by ac level shifting, a single-supply amplifier
can be used to pass these signals. The following complications
can arise from such techniques.
4.99kΩ
+
10µF
4.99kΩ
+
0.1µF
10µF
COMPOSITE
VIDEO
IN
47µF
+
7
R
BT
75Ω
3
1000µF
+
R
T
V
6
10kΩ
OUT
AD8051
75Ω
R
L
2
75Ω
4
0.1µF
R
1kΩ
F
Signals of bounded peak-to-peak amplitude that vary in duty
cycle require larger dynamic swing capacity than their (bounded)
peak-to-peak amplitude after they are ac-coupled. As a worst
case, the dynamic signal swing will approach twice the peak-to-
peak value. The two conditions that define the maximum
R
1kΩ
G
220µF
Figure 52. Single-Supply Composite Video Line Driver
Rev. H | Page 20 of 24
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