–13–

AD8091/AD8092

REV. A

pulses will reduce the dynamic range of the video signal and do

not provide any useful information for such a function.

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.

AD8091

0.1 F

10 F

+

R1

1k

R2

1k

100

TO A/D

+0.8V

(OR 2

+3V OR +5V

GROUND

+0.4V

VIDEO WITH SYNC

GROUND

VIDEO WITHOUT SYNC

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 k

Ω 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. 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

come out at ground.

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 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 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 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.

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.

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.

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.

AD8091

1k

1k

+5V

IN

+

10 F

4.99k

220 F

+

1000 F

0.1 F

75

10k

+

47 F

4.99k

75

75

COMPOSITE

VIDEO

0.1 F

10 F

+

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.

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.