AD8123

Rev. 0 | Page 12 of 16

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 AD8148 triple differential drivers are natural

complements to the AD8123 seeing that they perform the sync

pulse encoding with the necessary circuitry on-chip.

The sync encoding equations follow:

V

K

V

Red

CM

−

=

2

]

(1)

2

2

−

= K

V

Green

CM

]

(2)

V

K

V

Blue

CM

+

=

2

]

(3)

where:

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 AD8134 and AD8146/AD8147/AD8148 data sheets contain

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.

compensate for the low-pass roll-off in the cable response. The

controls the wideband flat gain and is used to compensate for

input is used to produce an offset at the AD8123 output. The

limited by the output swing limits.

can be coupled together 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.

In some cases, as would likely be with automatic control, the

UTP application according to the recommended curves shown

source. Clearly, the 5 V supply must be clean to provide a clean

2

+ 0.9V

20Ω

5.11kΩ

5V

14kΩ

8.25kΩ

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

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.

2

+ 0.9V

51.1kΩ

5V

5V

140kΩ

82.5kΩ

750Ω

500Ω

Many potentiometers have wide tolerances. If a wide tolerance

potentiometer is used, it may be necessary to change the value

by adjusting it to produce the correct amplitude of a known test

signal (such as a white screen) at the AD8123 output.

relationships shown in Figure 17 and Figure 18. Figure 23 shows

that originates from a low-Z source.

2

+ 0.9V

20Ω

5.11kΩ

5V

14kΩ

8.25kΩ

5.11kΩ

5V

60.4kΩ

133kΩ