AD8091/AD8092
pulses will 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.
A sync stripper will remove the synchronizing pulses from a video
signal while passing all the useful video information. Figure 8 shows
a practical single-supply circuit that uses only a single AD8091.
It is capable of directly driving a reverse terminated video line.
The other extreme is a full white video signal. The blanking intervals
and sync tips of such a signal will 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.
VIDEO WITHOUT SYNC
VIDEO WITH SYNC
As a result of the duty cycles between the two extremes presented
above, 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 arbitrary
varying duty cycle without distortion.
V
GROUND
+0.4V
BLANK
GROUND
+3V OR +5V
+
0.1F
10F
100⍀
V
Some circuits use a sync tip clamp to hold the sync tips at a rela-
tively constant level to lower the amount of dynamic signal swing
required. However, these circuits can have artifacts like sync tip
compression unless they are driven by a source with a very low
output impedance. The AD8091/AD8092 have adequate signal
swing when running on a single +5 V supply to handle an ac-coupled
composite video signal.
IN
TO A/D
AD8091
R2
1k⍀
R1
1k⍀
+0.8V
)
BLANK
The input to the circuit in Figure 9 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.
Figure 8. Sync Stripper
The video signal plus sync is applied to the noninverting input
with the proper termination. The amplifier gain is set equal to 2
via the two 1 kΩ resistors in the feedback circuit. A bias voltage
must be applied to R1 for the input signal to have the sync pulses
stripped at the proper level.
+5V
4.99k⍀
+
10F
4.99k⍀
+
0.1F
10F
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 in
order 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 ϫ VBLANK must be applied to make the blanking level
come out at ground.
COMPOSITE
47F
+
VIDEO
R
BT
IN
1000F
75⍀
R
+
T
V
OUT
10k⍀
AD8091
75⍀
R
L
75⍀
0.1F
R
F
1k⍀
R
G
1k⍀
Single-Supply Composite Video Line Driver
220F
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
may arise from such techniques.
Figure 9. Single-Supply Composite Video Line Driver
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.
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 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. The oppo-
site condition defines the other extreme.
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 5 with minimum visible degradation in the picture.
REV. A
–13–
ADI [ ADI ]
