Datasheet
AD8122 Data Sheet
Rev. 0 | Page 14 of 20
SYNC PULSE EXTRACTION USING COMPARATORS
The AD8122 is useful in many systems that transport computer
video signals, which typically comprise red, green, and blue video
signals, as well as separate horizontal and vertical sync signals
(RGBHV). Because the sync signals are separate and not embedded
in the color signals, it is advantageous to transmit them using a
simple scheme that encodes them on the three common-mode
voltages of the RGB signals. The AD8134, AD8142, AD8147,
and AD8148 triple differential drivers are natural complements
to the AD8122 because they perform the sync pulse encoding
with the necessary circuitry on chip.
The sync encoding equations are as follows:
[ ]
HV
K
VRed
CM
−=
2
(1)
[ ]
V2
2
−=
K
VGreen
CM
(2)
[ ]
HV
K
VBlue
CM
+=
2
(3)
where:
Red V
CM
, Green V
CM
, and Blue V
CM
are the transmitted common-
mode voltages of the respective color signals.
K is an adjustable gain constant that is set by the driver.
V and H are the vertical and horizontal sync pulses, respectively,
defined with a weight of −1 when the pulses are in their low states
and a weight of +1 when the pulses are in their high states.
For more information about the encoding scheme, see the data
sheets for the AD8134, AD8142, AD8147, and AD8148 drivers.
Figure 30 shows how the AD8122 comparators can be used to
extract the horizontal and vertical sync pulses that are encoded
on the RGB common-mode voltages by the drivers.
RED VIDEO OUTPUT
RED GAIN
RED
GREEN
BLUE
GREEN VIDEO OUTPUT
GREEN GAIN
BLUE VIDEO OUTPUT
BLUE GAIN
HSYNC OUTPUT
VSYNC OUTPUT
AD8122
4
11
12
15
16
19
20
7
49.9Ω
POWER-DOWN CONTROL
CABLE SELECT CONTROL
ANALOG
CONTROL
INPUTS
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
1kΩ
BLUE V
CM
RECEIVED
RED VIDEO
RECEIVED
GREEN VIDEO
RECEIVED
BLUE VIDEO
RED V
CM
GREEN V
CM
1kΩ
475Ω
47pF 47pF
31
30
28
23
27
25
26
32
34
35
37
38
2
3
9
8
10780-024
V
OFFSET
V
GAIN
V
PEAK
V
FILTER
PD
COAX/UTP
1
2
Figure 30. Basic Application Circuit with Common-Mode Sync Pulse Extraction (Supplies and Input Protection Not Shown)