Datasheet AD8042 (Analog Devices) - 15

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AD8042 Single-Supply Composite Video Line Driver
The other extreme is for a video signal that is full white The two op amps of an AD8042 can be configured as a single- everywhere. The blanking intervals and sync tips of such a supply dual line driver for composite video. The wide signal signal have negative going excursions in compliance with swing of the AD8042 enables this function to be performed composite video specifications. The combination of horizontal without using any type of clamping or dc restore circuit, which and vertical blanking intervals limit such a signal to being at its can cause signal distortion. highest level (white) for only about 75% of the time. Figure 38 shows a schematic for a circuit that is driven by a As a result of the duty cycle variations between the two extremes single composite video source that is ac-coupled, level-shifted presented, a 1 V p-p composite video signal that is multiplied by and applied to both noninverting inputs of the two amplifiers. a gain of 2 requires about 3.2 V p-p of dynamic voltage swing at Each op amp provides a separate 75 Ω composite video output. the output for an op amp to pass a composite video signal of To obtain single-supply operation, ac coupling is used throughout. arbitrary duty cycle without distortion. The large capacitor values are required to ensure that there is minimal tilting of the video signals due to their low frequency Some circuits use a sync tip clamp along with ac coupling to (30 Hz) signal content. The circuit shown was measured to have hold the sync tips at a relatively constant level, which lowers the a differential gain of 0.06% and a differential phase of 0.06°. amount of dynamic signal swing required. However, these circuits can have artifacts, such as sync tip compression, unless The input is terminated in 75 Ω and ac-coupled via CIN to a they are driven by sources with very low output impedance. voltage divider that provides the dc bias point to the input. Setting the optimal bias point requires some understanding The AD8042 not only has ample signal swing capability to handle of the nature of composite video signals and the video the dynamic range required without using a sync tip clamp but performance of the AD8042. also has good video specifications such as differential gain and differential phase when buffering these signals in an ac-coupled
+5V
configuration.
4.99kΩ 0.1µF 10µF 10µF
To test the dynamic range, the differential gain and differential
4.99kΩ 3 75Ω 8 1000µF COAX 1
phase were measured for the AD8042 while the supplies were
2 VOUT COMPOSITE
varied. As the lower supply is raised to approach the video
R R R F T L VIDEO IN 1kΩ 75Ω 75Ω
signal, the first effect observed is that the sync tips become
75Ω 0.1µF R 10kΩ G
compressed before the differential gain and differential phase are
1kΩ 220µF
adversely affected. Therefore, there must be adequate swing in the negative direction to pass the sync tips without compression.
5 1000µF 7 VOUT 6
As the upper supply is lowered to approach the video, the
R 4 T RL 75Ω 75Ω
differential gain and differential phase was not significantly
0.1µF
affected until the difference between the peak video output
RG RF 1kΩ 1kΩ
and the supply reached 0.6 V. Therefore, the highest video level 8
220µF
03 should be kept at least 0.6 V below the positive supply rail. 9- 05 01 Figure 38. Single-Supply Composite Video Line Driver Using AD8042 Therefore, it was found that the optimal point to bias the noninverting input is at 2.2 V dc. Operating at this point, the Signals of bounded peak-to-peak amplitude that vary in duty worst-case differential gain is measured at 0.06% and the worst- cycle require larger dynamic swing capability than their peak- case differential phase is 0.06°. to-peak amplitude after ac coupling. As a worst case, the dynamic signal swing required approaches twice the peak-to-peak value. The ac-coupling capacitors used in the circuit at first glance The two bounding cases are for a duty cycle that is mostly low, appear quite large. A composite video signal has a lower frequency but occasionally goes high at a fraction of a percent duty cycle, band edge of 30 Hz. The resistances at the various ac coupling and vice versa. points, especially at the output, are quite small. To minimize phase shifts and baseline tilt, the large value capacitors are required. Composite video is not quite this demanding. One bounding For video system performance that is not to be of the highest extreme is for a signal that is mostly black for an entire frame quality, the value of these capacitors can be reduced by a factor but has a white (full intensity), minimum width spike at least of up to five with only a slightly observable change in the picture once per frame. quality. Rev. E | Page 13 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION CONNECTION DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION CIRCUIT DESCRIPTION DRIVING CAPACITIVE LOADS OVERDRIVE RECOVERY Single-Supply Composite Video Line Driver Single-Ended-to-Differential Driver Single-Supply Differential A/D Driver HDSL Line Driver LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE