Datasheet

13
LT1675/LT1675-1
16751fb
TYPICAL APPLICATIO S
U
Logo or “Bug” Inserter
RED
VIDEO IN GREEN
BLUE
113Ω
75Ω
75Ω
75Ω
CABLE
CABLE
CABLE
V
+
V
–
SELECT
SELECT A
SELECT B
1675 TA05
ENABLE
75Ω
75Ω
75Ω
V
OUT RED
113Ω
LT1675
113Ω
V
OUT GREEN
V
OUT BLUE
+1
+1
+2
+2
+2
+1
+1
+1
+1
226Ω
V
+
V
–
SELECT
ENABLE
226Ω
LT1675
B
A
226Ω
+1
+1
+2
+2
+2
+1
+1
+1
+1
10k
10k
LT1634
5V
0.714V
1.25V
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
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.
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