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LT1675CMS8-1

250MHz, Triple and Single RGB Multiplexer with Amplifiers

Linear Technology 

LT1675/LT1675-1

TYPICAL APPLICATIO S

RED

VIDEO IN GREEN

BLUE

75Ω

75Ω

75Ω

SELECT A

0

0

1

1

SELECT B

0

1

0

1

OUTPUT

NO VIDEO, 100% WHITE

VIDEO PLUS 66% WHITE

VIDEO PLUS 33% WHITE

VIDEO, NO WHITE

SELECT A

SELECT B

Logo or “Bug” Inserter

LT1675

+1

+1

+1

V+

113Ω

+2

113Ω

+2

113Ω

+1

+2

+1

V–

SELECT

+1

ENABLE

A

B

LT1675

+1

V+

226Ω

+2

+1

226Ω

+1

+2

CABLE

CABLE

CABLE

VOUT RED

75Ω

VOUT GREEN

75Ω

VOUT BLUE

75Ω

+1

5V

10k

+1

1.25V

0.714V

LT1634

10k

+1

226Ω

+2

V–

SELECT

ENABLE

1675 TA05

This circuit highlights a section of the picture under

control of a synchronous key signal. It can be used for

adding the logo (also called a “bug”) you see in the bottom

corner of commercial television pictures or any sort of

overlay signal, such as a crosshair or a reticule. The key

signal has 2 bits of control so there can be four levels of

highlighting: unmodified video, video plus 33% white,

video plus 66% white and 100% white. The two LT1675s

are configured as a 2-bit DAC. The resistors on the outputs

set the relative bit weights. The output of the LT1675

labeled B in the schematic is one half the weight of the A

device. To properly match the 75Ω video cable, the output

resistors are selected so the parallel combination of the

two is 75 ohms. The output will never exceed peak white,

which is 0.714V for this NTSC-related RGB video. The

reference white signal is adjustable to lower than peak

white to make the effect less intrusive, if desired.

16751fb

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