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4543ILZ

Triple Differential Twisted-Pair Driver with Common-Mode Sync Encoding

Intersil 

EL4543

Operational Description and Application

Information

Introduction

The EL4543 is designed to differentially drive composite

RGB video signals onto twisted pair lines, while

simultaneously encoding horizontal and vertical sync signals

as common mode output. The entire video signal plus sync

can therefore be transmitted on 3 twisted pairs of wire. When

utilizing CAT-5 cable, the 4th available twisted pair can be

used for transmission of audio, data or control information.

The distribution of composite video over standard CAT-5

cable enables enormous cost and labor savings compared

with traditional coaxial cable, when considering both the

relative low price and ease of pulling CAT-5 cable.

Functional Description

The EL4543 provides three fully differential high-speed

amplifiers, suitable for driving high-resolution composite

video signals onto twisted pair or standard coaxial cable.

The input common-mode range extends to the negative rail,

allowing simple ground-referenced input termination to be

used with a single supply. The amplifiers provide a fixed gain

of +2 to compensate for standard video cable termination

schemes. Horizontal and Vertical sync signals (HSYNC and

VSYNC) are passed to an internal Logic Encoding Block to

encode the sync information as three discrete signals of

different voltage levels. Generally, in differential amplifiers an

external VREF pin is used to control the common mode level

of the differential output; in the case of the EL4543 the VREF

of each of the three internal amplifier channels receives a

signal from the Logic Encoding Block with encoded HSYNC

and VSYNC information. The final output consists of three

fully differential video signals, with sync encoded on the

common mode of each of the three RGB differential signals.

HSYNC and VSYNC can easily be separated from the

differential output signals, decoded and transmitted along

with the RGB video signals to the video monitor.

EN

+

INA

-

+

-

+

OUTA

-

VREF

VSYNC

HSYNC

EN

LOGIC

DECODING

RCM

GCM

BCM

EN

+

INB

-

+

-

+

OUTB

-

VREF

EN

+

INC

-

+

-

+

OUTC

-

VREF

FIGURE 37. BLOCK DIAGRAM EL4543

Sync Transmission

The EL4543 encodes HSYNC and VSYNC signals on the

common mode output of the differential video signals; Red,

Green and Blue respectively. Data Sheet Figure 16, 17 and 18

clearly illustrate that the sum of the common mode voltages

results in a fixed average DC level with no AC content and

illustrates the logic levels. This eliminates EMI radiation into

any common mode signal along the twisted pairs of CAT 5

cable.

Extract Common Mode Sync and Decode HSYNC &

VSYNC

HSYNC and VSYNC can be regenerated from the Common

Mode sync output voltages. The relationships between

HSYNC, VSYNC and the 3 common mode levels are given by

Table 1. The common mode levels are easily separated from

the differential outputs of the EL4543 using this simple

resistor network at the cable receiver input of each

differential channel; see Figure 38.

Twisted Pair Termination

The schematic in Figure 38 illustrates a termination scheme

for 50Ω series termination and a 100Ω twisted pair cable.

Note RCM is the common mode termination to allow

measurement of VCM and should not be too small since it

loads the EL4543; a little over a 100Ω is recommended for

RCM.

TYPICAL EL4543 TERMINATION DRIVER

+

-

VREF

50Ω TWISTED

PAIR

50Ω ZO=100Ω

+

50Ω

50Ω

-

VCM

120Ω

(RCM: SHOULD BE >100Ω)

(FOR LOADING

CONSIDERATIONS)

FIGURE 38. TWISTED PAIR TERMINATION EL4543

Video Transmission

The EL4543 is a twisted pair differential line driver directed

at the transmission of Video Signals through cables up to

100 feet; however, as signal losses increase with

transmission line length the EL4543 will need additional

support to equalize video signals along longer twisted pair

transmission lines. A full solution to accomplish this is the

SXGA Video Transmission System presented in the EL4543

Data Sheet. Note the inclusion of the EL9110 for signal

equalization of up to 1000ft of CAT-5 cable and common

mode extraction; see Data Sheet for additional information

on the EL9110.

Long Distance Video Transmission

The SXGA Video Transmission System makes it possible to

transmit Red, Green and Blue (RGB) video plus sync up to

1000 feet through CAT-5 cable. The input to the SXGA Video

Transmission System is the output of a video source

transmitting RGB video signals plus sync. The signals are

received initially by the EL4543; which converts the single

11

FN7325.5

October 26, 2005

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