Data Sheet AD8061/AD8062/AD8063
Rev. J | Page 17 of 20
APPLICATIONS INFORMATION
SINGLE-SUPPLY SYNC STRIPPER
When a video signal contains synchronization pulses, it is
sometimes desirable to remove them prior to performing
certain operations. In the case of analog-to-digital conversion,
the sync pulses consume some of the dynamic range, so
removing them increases the converter’s available dynamic
range for the video information.
Figure 53 shows a basic circuit for creating a sync stripper using
the AD8061 powered by a single supply. When the negative
supply is at ground potential, the lowest potential to which the
output can go is ground. This feature is exploited to create a
waveform whose lowest amplitude is the black level of the video
and does not include the sync level.
75Ω
VIDEO OUT
75Ω
R
G
1kΩ
75Ω
R
F
1kΩ
10µF
3V
AD8061
0.1µF
3
2
4
6
7
VIDEO IN
PIN NUMBERS ARE
FOR 8-LEAD
PACKAGE
01065-053
Figure 53. Single 3 V Sync Stripper Using AD8061
In this case, the input video signal has its black level at ground,
so it comes out at ground at the input. Because the sync level is
below the black level, it does not show up at the output. However,
all of the active video portion of the waveform is amplified by a
gain of 2 and then normalized to unity gain by the back-
terminated transmission line. Figure 54 is an oscilloscope plot
of the input and output waveforms.
01065-054
500mV
INPUT
OUTPUT
1
2
10µs
Figure 54. Input and Output Waveforms for a Single-Supply
Video Sync Stripper Using an AD8061
Some video signals with sync are derived from single-supply
devices, such as video DACs. These signals can contain sync,
but the whole waveform is positive, and the black level is not
at ground but at a positive voltage.
The circuit can be modified to provide the sync stripping
function for such a waveform. Instead of connecting R
G
to
ground, connect it to a dc voltage that is two times the black
level of the input signal. The gain from the noninverting input
to the output is 2, which means the black level is amplified by 2
to the output. However, the gain through R
G
is −1 to the output.
It takes a dc level of twice the input black level to shift the black
level to ground at the output. When this occurs, the sync is
stripped, and the active video is passed as in the ground-
referenced case.
75Ω
10µF
0.1µF
3V
3
2
4
6
7
75Ω
MONITOR
#1
75Ω
75Ω
75Ω
1kΩ
1kΩ
1kΩ
1kΩ
3V
1kΩ
3
2
5
6
7
8
1
4
MONITOR
#2
GREEN
DAC
RED
GREEN
BLUE
RED
DAC
BLUE
DAC
75Ω
75Ω
75Ω
75Ω
AD8061
75Ω
10µF
0.1µF
1kΩ
AD8062
75Ω
AD8062
75Ω
01065-055
Figure 55. RGB Cable Driver Using AD8061 and AD8062
RGB AMPLIFIER
Most RGB graphics signals are created by video DAC outputs
that drive a current through a resistor to ground. At the video
black level, the current goes to zero, and the voltage of the video
is also zero. Before the availability of high speed rail-to-rail op
amps, it was essential that an amplifier have a negative supply
to amplify such a signal. Such an amplifier is necessary if one
wants to drive a second monitor from the same DAC outputs.
However, high speed, rail-to-rail output amplifiers like the
AD8061 and AD8062 accept ground-level input signals and
output ground-level signals. They are used as RGB signal
amplifiers. A combination of the AD8061 (single) and the
AD8062 (dual) amplifies the three video channels of an RGB
system. Figure 55 shows a circuit that performs this function.
