AD8122 Datasheet, PDF (9/13 Pages) Analog Devices – Triple Differential Receiver

Preliminary Technical Data
THEORY OF OPERATION
The AD8122 is a 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. The transfer function of
theAD8122 can be pin-selected for UTP or coaxial cable, and
the gain of each channel can be set to 1 or 2.
Four 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 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 pin, VFILTER, is used to adjust the cutoff
frequency of the output lowpass filters. Finally, an output offset
adjust control, VOFFSET, allows the designer to shift the output dc
level.
The AD8122 has a high impedance differential input that makes
termination simple and allows dc-coupled signals to be received
directly from the cable. The AD8122 input can also be used in a
single-ended fashion in coaxial cable applications. For differential
systems that require very wide input common-mode range, the
AD8143 high-voltage triple differential receiver can be placed
in front of the AD8122.
The AD8122 has a low impedance output that is capable of
driving a 150 Ω load. For systems where the AD8122 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.
AD8122
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 SINGLE-ENDED VOLTAGE RANGE
CONSIDERATIONS
When using the AD8122 as a receiver, it is important to ensure
that its single-ended input voltages stay within their specified
ranges. The received single-ended level for each input 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
AD8122 and pickup from external sources, such as power lines
and fluorescent lights. See the Applications Information section
for more details.
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