LT1675 datasheet(13/16 Pages) LINER

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LT1675/LT1675-1

TYPICAL APPLICATIO S

Logo or “Bug” Inserter

RED

VIDEO IN

GREEN

BLUE

113

Ω

CABLE

V+

V–

SELECT

SELECT A

SELECT B

1675 TA05

ENABLE

Ω

VOUT RED

113

Ω

LT1675

113

Ω

VOUT GREEN

VOUT BLUE

226

Ω

V+

V–

SELECT

ENABLE

226

Ω

LT1675

226

Ω

10k

LT1634

0.714V

1.25V

SELECT A

SELECT B

OUTPUT

NO VIDEO, 100% WHITE

VIDEO PLUS 66% WHITE

VIDEO PLUS 33% WHITE

VIDEO, NO WHITE

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 75

Ω 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.

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

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