AD8123ACPZ Analog Devices Inc, AD8123ACPZ Datasheet - Page 12

• Manufacturer Part Number: AD8123ACPZ
• Description: Triple Equalizer
• Manufacturer: Analog Devices Inc

AD8123
SYNC PULSE EXTRACTION USING COMPARATORS
The AD8123 is useful in many systems that transport computer
video signals, which are typically comprised of red, green, and
blue (RGB) video signals and separate horizontal and vertical
sync signals. 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 among the three
common-mode voltages of the RGB signals. The AD8134,
AD8147, and
complements to the AD8123 because they perform the sync
pulse encoding with the necessary circuitry on-chip.
The sync encoding equations follow:
where:
Red V
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, defined
with a weight of −1 when the pulses are in their low states, and a
weight of +1 when they are in their high states.
The
further details regarding the encoding scheme. Figure 20 illustrates
how the AD8123 comparators can be used to extract the horizontal
and vertical sync pulses that are encoded on the RGB common-
mode voltages by the aforementioned drivers.
USING THE V
The V
compensate for the low-pass roll-off in the cable response. The
V
that shifts the positions of the equalizer poles. The V
controls the wideband flat gain and is used to compensate for
the low frequency cable loss that is nominally flat. The V
input is used to produce an offset at the AD8123 output. The
output offset is equal to the voltage applied to the V
limited by the output swing limits.
The V
can be coupled to form a single peaking control. While Figure 17
and Figure 18 show recommended settings vs. cable length,
designers may find other combinations that they prefer. These
two controls give designers extra freedom, as well as the ability
to compensate for different cable types (such as UTP and coaxial
cable), as opposed to having only a single frequency shaping
control.
POLE
AD8134
Green
Red
Blue
input is a secondary frequency response shaping control
CM
PEAK
PEAK
, Green V
V
V
input is the main peaking control and is used to
and V
CM
V
CM
CM
and
AD8148
=
=
=
K
PEAK
POLE
2
K
2
CM
AD8146/AD8147/AD8148
K
[
2
V
[
, and Blue V
V
, V
controls can be used independently or they
[
−
−
+
triple differential drivers are natural
2
H
POLE
H
V
]
]
]
, V
GAIN
CM
are the transmitted common-
, AND V
OFFSET
data sheets contain
INPUTS
OFFSET
GAIN
OFFSET
input
input,
Rev. A | Page 12 of 16
(1)
(2)
(3)
In some cases, as would likely be with automatic control, the
V
an op amp. Figure 21 shows how to derive V
UTP application according to the recommended curves shown
in Figure 17, when V
source. Clearly, the 5 V supply must be clean to provide a clean
V
The 20 Ω series resistor in the V
load buffering for the op amp. This value can be modified,
depending on the actual capacitive load.
In automatic equalization circuits that place the control voltages
inside feedback loops, attention must be paid to the poles
produced by the summing resistors and load capacitances.
The peaking can also be adjusted by a mechanical or digitally
controlled potentiometer. In these cases, if the resistance of the
potentiometer is a couple of orders of magnitude lower than the
values of the resistors used to develop V
ignored. Figure 22 shows how to use a 500 Ω potentiometer with
the resistor values shown in Figure 21 scaled up by a factor of 10.
Many potentiometers have wide tolerances. If a wide tolerance
potentiometer is used, it may be necessary to change the value
of the 750 Ω resistor to obtain a full swing for V
The V
by adjusting it to produce the correct amplitude of a known test
signal (such as a white screen) at the AD8123 output.
V
relationships shown in Figure 17 and Figure 18. Figure 23 shows
how to derive V
that originates from a low-Z source.
PEAK
POLE
GAIN
Figure 23. Deriving V
Figure 22. Deriving V
Figure 21. Deriving V
voltage.
control is derived from a low impedance source, such as
can also be derived from V
GAIN
750Ω
500Ω
input is essentially a contrast control and can be set
V
PEAK
5V
V
PEAK
POLE
POLE
51.1kΩ
and V
5.11kΩ
5.11kΩ
POLE
PEAK
and V
POLE
5.11kΩ
from V
from V
20Ω
originates from a low impedance
GAIN
GAIN
20Ω
82.5kΩ
5V
8.25kΩ
133kΩ
from V
5V
5V
140kΩ
PEAK
PEAK
8.25kΩ
from V
5V
14kΩ
60.4kΩ
V
PEAK
PEAK
14kΩ
with Potentiometer for UTP Cable
PEAK
with Low-Z Source for UTP Cable
PEAK
V
PEAK
V
path provides capacitive
POLE
V
V
with Low-Z Source for UTP Cable
V
according to the linear
PEAK
POLE
GAIN
PEAK
V
POLE
POLE
≈
≈ 0.89 × V
in a UTP application
≈
V
≈
, its resistance can be
PEAK
V
POLE
2
PEAK
V
2
PEAK
2
PEAK
from V
+ 0.9V
PEAK
+ 0.9V
+ 0.9V
.
+ 0.38V
PEAK
in a