AD8115AST Analog Devices Inc, AD8115AST Datasheet - Page 22

• Manufacturer Part Number: AD8115AST
• Description: IC VIDEO CROSSPOINT SWIT 100LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD8115AST

• Rohs Status: RoHS non-compliant
• Function: Video Crosspoint Switch
• Circuit: 1 x 16:16
• Voltage Supply Source: Dual Supply
• Voltage - Supply, Single/dual (±): ±4.5 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 100-LQFP
• Array Configuration: 16x16
• Number Of Arrays: 1
• Screening Level: Industrial
• Pin Count: 100
• Package Type: LQFP
• Power Supply Requirement: Dual
• Lead Free Status / RoHS Status: Not Compliant

AD8114/AD8115
signals that make up the video channel. Thus, one differential
video channel is assigned to a pair of crosspoint channels, both
input and output. For a single AD8114/AD8115, eight
differential video channels can be assigned to the 16 inputs and
16 outputs. This will effectively form an 8 × 8 differential
crosspoint switch.
Programming such a device will require that inputs and outputs
be programmed in pairs. This information can be deduced by
inspection of the programming format of the AD8114/AD8115
and the requirements of the system.
There are other analog video formats requiring more than one
analog circuit per video channel. One 2-circuit format that is
commonly being used in systems such as satellite TV, digital
cable boxes, and higher quality VCRs is called S-video or Y/C
video. This format carries the brightness (luminance or Y)
portion of the video signal on one channel and the color
(chrominance, chroma, or C) on a second channel.
Since S-video also uses two separate circuits for one video
channel, creating a crosspoint system requires assigning one
video channel to two crosspoint channels, as in the case of a
differential video system. Aside from the nature of the video
format, other aspects of these two systems will be the same.
There are yet other video formats using three channels to carry
the video information. Video cameras produce RGB (red, green,
blue) directly from the image sensors. RGB is also the usual
format used by computers internally for graphics. RGB can be
converted to Y, R-Y, B-Y format, sometimes called YUV format.
These 3-circuit video standards are referred to as component
analog video.
The component video standards require three crosspoint
channels per video channel to handle the switching function. In
a fashion similar to the 2-circuit video formats, the inputs and
outputs are assigned in groups of three, and the appropriate
logic programming is performed to route the video signals.
CROSSTALK
Many systems, such as broadcast video, that handle numerous
analog signal channels have strict requirements for keeping the
various signals from influencing any of the others in the system.
Crosstalk is the term used to describe the coupling of the
signals of other nearby channels to a given channel.
When there are many signals in close proximity in a system, as
will undoubtedly be the case in a system that uses the
AD8114/AD8115, the crosstalk issues can be quite complex. A
good understanding of the nature of crosstalk and some
definition of terms is required to specify a system that uses one
or more AD8114/AD8115s.
Rev. B | Page 22 of 32
Types of Crosstalk
Crosstalk can be propagated by means of any of three methods.
These fall into the categories of electric field, magnetic field,
and sharing of common impedances. This section will explain
these effects.
Every conductor can be both a radiator of electric fields and a
receiver of electric fields. The electric field crosstalk mechanism
occurs when the electric field created by the transmitter
propagates across a stray capacitance (e.g., free space) and
couples with the receiver and induces a voltage. This voltage is
an unwanted crosstalk signal in any channel that receives it.
Currents flowing in conductors create magnetic fields that
circulate around the currents. These magnetic fields will then
generate voltages in any other conductors whose paths they
link. The undesired induced voltages in these other channels are
crosstalk signals. The channels that crosstalk can be said to have
a mutual inductance that couples signals from one channel to
another.
The power supplies, grounds, and other signal return paths of a
multichannel system are generally shared by the various
channels. When a current from one channel flows in one of
these paths, a voltage that is developed across the impedance
becomes an input crosstalk signal for other channels that share
the common impedance.
All these sources of crosstalk are vector quantities, so the
magnitudes cannot simply be added together to obtain the total
crosstalk. In fact, there are conditions where driving additional
circuits in parallel in a given configuration can actually reduce
the crosstalk.
Areas of Crosstalk
For a practical AD8114/AD8115 circuit, it is required that it be
mounted to some sort of circuit board to connect it to power
supplies and measurement equipment. Great care has been
taken to create a characterization board (also available as an
evaluation board) that adds minimum crosstalk to the intrinsic
device. This, however, raises the issue that a system’s crosstalk is
a combination of the intrinsic crosstalk of the devices in
addition to the circuit board to which they are mounted. It is
important to try to separate these two areas of crosstalk when
attempting to minimize its effect.
In addition, crosstalk can occur among the inputs to a
crosspoint and among the output. It can also occur from input
to output. Techniques will be discussed for diagnosing which
part of a system is contributing to crosstalk.