AD8123

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THEORY OF OPERATION

The AD8123 is a unity-gain, triple, wideband, low noise analog

line equalizer that compensates for losses in UTP and coaxial

cables up to 300 meters in length. The 3-channel architecture is

targeted at high resolution RGB applications but can be used in

HD YPbPr applications as well.

Three continuously adjustable control voltages, common

to the RGB channels, are available to the designer to provide

compensation for various cable lengths as well as for variations

used to compensate for frequency and cable-length dependent,

high frequency losses that are present due to the skin effect of

gain to compensate for low frequency flat losses present in the

in the Typical Performance Characteristics and Applications

Information sections, for UTP and coaxial cables. Finally, an

the output dc level.

The AD8123 has a high impedance differential input that makes

termination simple and allows dc-coupled signals to be received

directly from the cable. The AD8123 input can also be used in a

single-ended fashion in coaxial cable applications. For differential

systems that require very high CMRR, a triple differential

receiver, such as the AD8143 or AD8145, can be placed in

front of the AD8123.

The AD8123 has a low impedance output that is capable of

driving a 150 Ω load. For systems where the AD8123 has to

drive a high impedance capacitive load, it is recommended that

a small series resistor be placed between the output and load to

buffer the capacitance. The resistor should not be so large as to

reduce the overall bandwidth to an unacceptable level.

The AD8123 is designed such that systems that use short-to-

medium-length cables do not pay a noise penalty for excess gain

that they do not require. The high gain is only available for

longer length systems where it is required. This feature is built

Two comparators are provided on-chip that can be used for

sync pulse extraction in systems that use sync-on-common

mode encoding. Each comparator has very low output impedance

and can therefore be used in a source-only cable termination

scheme by placing a series resistor equal to the cable characteristic

impedance directly on the comparator output. Additional

details are provided in the Applications Information section.

INPUT COMMON-MODE VOLTAGE RANGE

CONSIDERATIONS

When using the AD8123 as a receiver, it is important to ensure

that its input common-mode voltage stays within the specified

range. The received common-mode level is calculated by adding

the common-mode level of the driver, the single-ended peak

amplitude of the received signal, the amplitude of any sync

pulses, and the other induced common-mode signals, such as

ground shifts between the driver and the AD8123 and pickup

from external sources, such as power lines and fluorescent

lights. See the Applications Information section for more details.