Figure 7. 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 will approach 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 every-
where. The blanking intervals and sync tips of such a signal will
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 ex-
tremes presented above, a 1 V p-p composite video signal that is
multiplied by a gain of two 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 in order to lower
the amount of dynamic signal swing required. However, these
circuits can have artifacts like 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 like differential
gain and differential phase when buffering these signals in an
To test this, 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 to be observed is that the sync tips become compressed
before the differential gain and differential phase are adversely
affected. Thus, 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 differ-
ential gain and differential phase were not significantly adversely
affected until the difference between the peak video output and
the supply reached 0.6 V. Thus, the highest video level should
be kept at least 0.6 V below the positive supply rail.
Taking the above into account, 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 fre-
quency band edge of 30 Hz. The resistances at the various ac
coupling points—especially at the output—are quite small. In
order 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 observ-
able change in the picture quality.
Using a cross-coupled single-ended-to-differential converter, the
AD8042 makes a good general purpose differential line driver.
This can be used for applications such as driving category 5 twisted
pair wire which is becoming common for data communications
in buildings. Figure 8 shows a configuration for a circuit that
performs this function that can be used for video transmission
over a differential pair or various data communication purposes.
2 AMP1 1 1k⍀
Figure 8. Single-Ended-to-Differential Twisted
Pair Line Driver