LT1675-1 Linear Technology, LT1675-1 Datasheet - Page 13

• Manufacturer Part Number: LT1675-1
• Description: 250MHz/ Triple and Single RGB Multiplexer with Current Feedback Amplifiers
• Manufacturer: Linear Technology

TYPICAL APPLICATIO S
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
SELECT A
0
0
1
1
VIDEO IN
SELECT B
0
1
0
1
SELECT A
SELECT B
GREEN
BLUE
OUTPUT
NO VIDEO, 100% WHITE
VIDEO PLUS 66% WHITE
VIDEO PLUS 33% WHITE
VIDEO, NO WHITE
RED
LT1634
10k
5V
1.25V
U
75
75
10k
75
0.714V
Logo or “Bug” Inserter
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
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 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.
LT1675
LT1675
A
B
+2
+2
+2
+2
+2
+2
113
113
113
226
226
226
V
V
ENABLE
ENABLE
SELECT
SELECT
+
+
V
V
–
–
LT1675/LT1675-1
1675 TA05
CABLE
CABLE
CABLE
75
75
75
V
V
V
OUT RED
OUT GREEN
OUT BLUE
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