AD8146ACPZ-R2 datasheet(16/24 Pages) AD

16 / 24 page

AD8146/AD8147/AD8148

Rev. 0 | Page 16 of 24

APPLICATIONS

DRIVING RGB VIDEO SIGNALS OVER CATEGORY-5

UTP CABLE

The foremost application of the drivers is the transmission of

RGB video signals over UTP cable in KVM networks. The

excellent balance of the differential outputs ensures low radiated

energy from each of the twisted pairs. Single-ended video signals

are easily converted to differential signals for transmission over

the cable, and the internally fixed gain of 2 or 4 automatically

compensates for the losses incurred by the source and load

terminations. The common topologies used in KVM networks,

such as daisy-chained, star, and point-to-point, are supported

by the drivers. Figure 29 shows the AD8146 in a triple single-

ended-to-differential application when driven from a 75 Ω

source, which is typical of how RGB video is driven over an

UTP cable.

VOCM

+2.5V

–

OUT A

1kΩ

VIDEO

SOURCE A

1kΩ

AD8146

500Ω

82.5Ω

39.2Ω

500Ω

49.9Ω

75Ω

VOCM

+2.5V

–

OUT B

1kΩ

VIDEO

SOURCE B

1kΩ

500Ω

82.5Ω

OUTPUT

PULLDOWN

500Ω

49.9Ω

75Ω

VOCM

+2.5V

–

OUT C

1kΩ

VIDEO

SOURCE C

1kΩ

500Ω

82.5Ω

500Ω

OPD

49.9Ω

75Ω

+5V

VS+

0.1µF ON ALL VS+ PINS

VS–

Figure 29. AD8146 in Single-Ended-to-Differential Application

VIDEO SYNC-ON COMMON-MODE

In computer video applications, the horizontal and vertical sync

signals are often separate from the video information signals.

For example, in typical computer monitor applications, the red,

green, and blue (RGB) color signals are transmitted over separate

cables, as are the vertical and horizontal sync signals. When

transmitting these types of video signals over long distances on

UTP cable, it is desirable to reduce the required number of

physical channels. One way to do this is to encode the vertical

and horizontal sync signals as weighted sums and differences of

the output common-mode signals. The RGB color signals are

each transmitted differentially over separate physical channels.

The fact that the differential and common-mode signals are

orthogonal allows the RGB color and sync signals to be

separated at the channel’s receiver.

Cat-5 cable contains four balanced twisted-pair physical

channels that can support both differential and common-mode

signals. Transmitting typical computer monitor video over this

cable can be accomplished by using three of the twisted pairs for

the RGB and sync signals and one wire of the fourth pair as a

return path for the Schottky diode bias currents. Each color is

transmitted differentially, one on each of the three pairs, and the

encoded sync signals are transmitted among the common-mode

signals of each of the three pairs. To minimize EMI from the

sync signals, the common-mode signals on each of the three

pairs produced by the sync encoding scheme induce electric

and magnetic fields that for the most part cancel each other. A

conceptual block diagram of the sync encoding scheme is

presented in Figure 30. Because the AD8147/AD8148 have the

sync encoding scheme implemented internally, the user simply

applies the horizontal and vertical sync signals to the appropriate

inputs. (See the Specifications tables for the high and low levels

of the horizontal and vertical sync pulse voltages).

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