AD8042
Rev. E | Page 13 of 16
Single-Supply Composite Video Line Driver
The two op amps of an AD8042 can be configured as a single-
supply dual line driver for composite video. The wide signal
swing of the AD8042 enables this function to be performed
without using any type of clamping or dc restore circuit, which
can cause signal distortion.
Figure 38 shows a schematic for a circuit that is driven by a
single composite video source that is ac-coupled, level-shifted
and applied to both noninverting inputs of the two amplifiers.
Each op amp provides a separate 75 composite video output.
To obtain single-supply operation, ac coupling is used throughout.
The large capacitor values are required to ensure that there is
minimal tilting of the video signals due to their low frequency
(30 Hz) signal content. The circuit shown was measured to have
a differential gain of 0.06% and a differential phase of 0.06°.
The input is terminated in 75 and ac-coupled via C
IN
to a
voltage divider that provides the dc bias point to the input.
Setting the optimal bias point requires some understanding
of the nature of composite video signals and the video
performance of the AD8042.
+5V
0.1µF
8
3
2
V
OUT
4
6
5
1
7
10µF
4.99kΩ
4.99kΩ
75Ω
10kΩ
R
G
1kΩ
R
G
1kΩ
R
F
1kΩ
R
F
1kΩ
R
T
75Ω
R
T
75Ω
R
L
75Ω
75Ω
COAX
V
OUT
R
L
75Ω
220µF
220µF
1000µF
0.1µF
10µF
1000µF
0.1µF
COMPOSITE
VIDEO IN
01059-038
Figure 38. Single-Supply Composite Video Line Driver Using AD8042
Signals of bounded peak-to-peak amplitude that vary in duty
cycle require larger dynamic swing capability than their peak-
to-peak amplitude after ac coupling. As a worst case, the dynamic
signal swing required approaches twice the peak-to-peak value.
The two bounding cases are for a duty cycle that is mostly low,
but occasionally goes high at a fraction of a percent duty cycle,
and vice versa.
Composite video is not quite this demanding. One bounding
extreme is for a signal that is mostly black for an entire frame
but has a white (full intensity), minimum width spike at least
once per frame.
The other extreme is for a video signal that is full white
everywhere. The blanking intervals and sync tips of such a
signal have negative going excursions in compliance with
composite video specifications. The combination of horizontal
and vertical blanking intervals limit such a signal to being at its
highest level (white) for only about 75% of the time.
As a result of the duty cycle variations between the two extremes
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
arbitrary duty cycle without distortion.
Some circuits use a sync tip clamp along with ac coupling to
hold the sync tips at a relatively constant level, which lowers the
amount of dynamic signal swing required. However, these
circuits can have artifacts, such as sync tip compression, unless
they are driven by sources with very low output impedance.
The AD8042 not only has ample signal swing capability to handle
the dynamic range required without using a sync tip clamp but
also has good video specifications such as differential gain and
differential phase when buffering these signals in an ac-coupled
configuration.
To test the dynamic range, the differential gain and differential
phase were measured for the AD8042 while the supplies were
varied. As the lower supply is raised to approach the video
signal, the first effect observed is that the sync tips become
compressed before the differential gain and differential phase are
adversely affected. Therefore, there must be adequate swing in
the negative direction to pass the sync tips without compression.
As the upper supply is lowered to approach the video, the
differential gain and differential phase was not significantly
affected until the difference between the peak video output
and the supply reached 0.6 V. Therefore, the highest video level
should be kept at least 0.6 V below the positive supply rail.
Therefore, it was found that the optimal point to bias the
noninverting input is at 2.2 V dc. Operating at this point, the
worst-case differential gain is measured at 0.06% and the worst-
case differential phase is 0.06°.
The ac-coupling capacitors used in the circuit at first glance
appear quite large. A composite video signal has a lower frequency
band edge of 30 Hz. The resistances at the various ac coupling
points, especially at the output, are quite small. To minimize
phase shifts and baseline tilt, the large value capacitors are required.
For video system performance that is not to be of the highest
quality, the value of these capacitors can be reduced by a factor
of up to five with only a slightly observable change in the picture
quality.
