AD8051ARTZ-R2 datasheet(21/25 Pages) AD

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AD8051/AD8052/AD8054

Rev. J | Page 20 of 24

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 analog-to-

digital 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 51

shows a practical single-supply circuit that uses only a single

AD8051. It is capable of directly driving a reverse terminated

video line.

AD8051

0.1µF

10µF

100Ω

TO A/D

3V OR 5V

VBLANK

GROUND

0.4V

VIDEO WITH SYNC

GROUND

VIDEO WITHOUT SYNC

1kΩ

VIN

0.8V

(OR 2 × VBLANK)

Figure 51. Sync Stripper

The video signal plus sync is applied to the noninverting input

with the proper termination. The amplifier gain is set to 2 via

the two 1 kΩ resistors in the feedback circuit. A bias voltage

must be applied to R1 so that the input signal has 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 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 × VBLANK

must be applied to make the blanking level 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

can 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,

and the other extreme defined by the opposite 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 for 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

previously 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 arbitrarily varying duty cycle without distortion.

Some circuits use a sync tip clamp to hold the sync tips at a

relatively constant level to lower the amount of dynamic signal

swing required. However, these circuits can have artifacts, such

as sync tip compression, unless they are driven by a source with

a very low output impedance. The AD8051/AD8052/AD8054

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 52 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.

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 five with minimum visible degradation in the picture.

AD8051

10µF

4.99kΩ

220µF

1000µF

0.1µF

10kΩ

47µF

4.99kΩ

0.1µF

10µF

COMPOSITE

VIDEO

1kΩ

75Ω

VOUT

RBT

75Ω

Figure 52. Single-Supply Composite Video Line Driver

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