AD8091/AD8092
Rev. C | Page 14 of 20
02859-033
2
1
6
5
7
2
3
6
AD8092
AD8091
C2
50pF
C1
50pF
R5
2kΩ
R4
2kΩ
R6
1kΩ
R3
2kΩ
R2
2kΩ
R1
3kΩ
V
OUT
3
AD8092
V
IN
Figure 33. 2 MHz Biquad Band-Pass Filter
The frequency response of the circuit is shown in Figure 34.
0
–10
–20
–30
–40
10k 100k 1M 10M 100M
02859-034
FREQUENCY (Hz)
GAIN (dB)
Figure 34. Frequency Response of 2 MHz Band-Pass Biquad Filter
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 A/D
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.
A sync stripper removes the synchronizing pulses from a video
signal while passing all the useful video information.
Figure 35
shows a practical single-supply circuit that uses only a single
AD8091. It is capable of directly driving a reverse terminated
video line.
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.
The blanking level of the input video pulse is the desired place
to remove the sync information. The amplifier multiplies this
level by 2. This level must be at ground at the output in order
for the sync stripping action to take place. Because the gain of
the amplifier from the input of R1 to the output is −1, a voltage
equal to 2 × V
BLANK
must be applied to make the blanking level
come out at ground.
02859-035
AD8091
+
R2
1kΩ
R1
1kΩ
+0.8V
(OR 2 × V
BLANK
)
100Ω
TO A/D
3V OR 5V
3
2
4
6
7
10µF0.1µF
V
BLANK
V
IN
GROUND
+0.4V
IDEO WITH SYNC
GROUND
IDEO WITHOUT SYN
Figure 35. Sync Stripper
SINGLE-SUPPLY COMPOSITE VIDEO LINE DRIVER
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.
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 approaches twice the
peak-to-peak value. One of two conditions that define the
maximum dynamic swing requirements is a signal that is
mostly low but goes high with a duty cycle that is a small
fraction of a percent. The opposite condition defines the second
condition.
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 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.
As a result of the duty cycles between the two extremes, 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.
