Datasheet AD8051, AD8052, AD8054 (Analog Devices) - 20

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AD8051/AD8052/AD8054 SYNC STRIPPER
dynamic swing requirements are a signal that is mostly low but Synchronizing pulses are sometimes carried on video signals so goes high with a duty cycle that is a small fraction of a percent, as not to require a separate channel to carry the synchronizing and the other extreme defined by the opposite condition. information. However, for some functions, such as analog-to- The worst case of composite video is not quite this demanding. digital conversion, it is not desirable to have the sync pulses on One bounding condition is a signal that is mostly black for an the video signal. These pulses reduce the dynamic range of the entire frame but has a white (full amplitude) minimum width video signal and do not provide any useful information for such spike at least once in a frame. a function. The other extreme is for a full white video signal. The blanking A sync stripper removes the synchronizing pulses from a video intervals and sync tips of such a signal have negative-going signal while passing all the useful video information. Figure 51 excursions in compliance with the composite video specifications. shows a practical single-supply circuit that uses only a single The combination of horizontal and vertical blanking intervals AD8051. It is capable of directly driving a reverse terminated limit such a signal to being at the highest (white) level for a video line. maximum of about 75% of the time.
VIDEO WITH SYNC VIDEO WITHOUT SYNC
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
VBLANK 0.4V GROUND
video signal of arbitrarily varying duty cycle without distortion.
GROUND 3V OR 5V
Some circuits use a sync tip clamp to hold the sync tips at a
+
relatively constant level to lower the amount of dynamic signal
0.1µF 10µF V 7
swing required. However, these circuits can have artifacts, such
IN 3 TO A/D AD8051 6
as sync tip compression, unless they are driven by a source with
100Ω 2 4
a very low output impedance. The AD8051/AD8052/AD8054
R2
have adequate signal swing when running on a single 5 V
1kΩ
supply to handle an ac-coupled composite video signal.
R1 1kΩ
The input to the circuit in Figure 52 is a standard composite
0.8V
051 2- (1 V p-p) video signal that has the blanking level at ground. The
(OR 2 × VBLANK)
106 0 input network level shifts the video signal by means of ac coupling. Figure 51. Sync Stripper The noninverting input of the op amp is biased to half of the The video signal plus sync is applied to the noninverting input supply voltage. with the proper termination. The amplifier gain is set to 2 via The feedback circuit provides unity gain for the dc-biasing of the two 1 kΩ resistors in the feedback circuit. A bias voltage the input and provides a gain of 2 for any signals that are in the must be applied to R1 so that the input signal has the sync video bandwidth. The output is ac-coupled and terminated to pulses stripped at the proper level. drive the line. The blanking level of the input video pulse is the desired place to The capacitor values were selected for providing minimum tilt remove the sync information. This level is multiplied by 2 by the or field time distortion of the video signal. These values would amplifier. This level must be at ground at the output for the sync be required for video that is considered to be studio or broadcast stripping action to take place. Since the gain of the amplifier from quality. However, if a lower consumer grade of video, sometimes the input of R1 to the output is −1, a voltage equal to 2 × VBLANK referred to as consumer video, is all that is desired, the values must be applied to make the blanking level come out at ground. and the cost of the capacitors can be reduced by as much as a
SINGLE-SUPPLY COMPOSITE VIDEO LINE DRIVER
factor of five with minimum visible degradation in the picture. Many composite video signals have their blanking level at
5V 4.99kΩ
ground and have video information that is both positive and
+
negative. Such signals require dual-supply amplifiers to pass
4.99kΩ 10µF + 0.1µF 10µF COMPOSITE
them. However, by ac level shifting, a single-supply amplifier
47µF VIDEO + 7 R IN 3 1000µF BT
can be used to pass these signals. The following complications
R 75Ω T 10kΩ 75Ω AD8051 + 6 VOUT
can arise from such techniques.
RL 2 4 75Ω 0.1µF
Signals of bounded peak-to-peak amplitude that vary in duty
RF 1kΩ
cycle require larger dynamic swing capacity than their (bounded)
RG 1kΩ
peak-to-peak amplitude after they are ac-coupled. As a worst 2
220µF
-05 case, the dynamic signal swing will approach twice the peak-to- 62 10 0 peak value. The two conditions that define the maximum Figure 52. Single-Supply Composite Video Line Driver Rev. J | Page 20 of 24 Document Outline Features Applications Pin Connections (Top Views) General Description Revision History Specifications Absolute Maximum Ratings Thermal Resistance Maximum Power Dissipation ESD Caution Typical Performance Characteristics Theory of Operation Circuit Description Application Information Overdrive Recovery Driving Capacitive Loads Layout Considerations Active Filters Analog-to-Digital and Digital-to-Analog Applications Sync Stripper Single-Supply Composite Video Line Driver Outline Dimensions Ordering Guide