AD8124 データシートの表示（PDF） - Analog Devices

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AD8124

Triple Differential Receiver with 200 Meter Adjustable Cable Equalization

Analog Devices 

AD8124

THEORY OF OPERATION

The AD8124 is a unity-gain, triple, wideband, low noise analog

line equalizer that compensates for losses in UTP and coaxial

cables up to 200 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

in the cable itself. The VPEAK input is used to control the amount

of high frequency peaking. VPEAK is the primary control that is

used to compensate for frequency and cable-length dependent,

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

the cable. A second control pin, VGAIN, is used to adjust broadband

gain to compensate for low frequency flat losses present in the

cable. A third control, VPOLE, is used to move the positions of the

equalizer poles and can be linearly derived from VPEAK, as illustrated

in the Typical Performance Characteristics section and

Applications Information section, for UTP and coaxial cables.

Finally, an output offset adjust control, VOFFSET, allows the designer

to shift the output dc level.

The AD8124 has a high impedance differential input that makes

termination simple and allows dc-coupled signals to be received

directly from the cable. The AD8124 input can also be used in a

single-ended fashion in coaxial cable applications.

The AD8124 has a low impedance output that is capable of driving

a 150 Ω load. For systems where the AD8124 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.

Data Sheet

The AD8124 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

into the VPEAK control and is transparent to the user.

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 AD8124 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 AD8124 and pickup

from external sources, such as power lines and fluorescent lights.

See the Applications Information section for more details.

Rev. A | Page 10 of 15

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