AD8110AST Analog Devices Inc, AD8110AST Datasheet - Page 18

• Manufacturer Part Number: AD8110AST
• Description: IC VIDEO CROSSPOINT SWIT 80LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD8110AST

• Rohs Status: RoHS non-compliant
• Function: Video Crosspoint Switch
• Circuit: 1 x 16:8
• 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: 80-LQFP
• Array Configuration: 16x8
• Number Of Arrays: 1
• Screening Level: Industrial
• Pin Count: 80
• Package Type: LQFP
• Power Supply Requirement: Dual
• Lead Free Status / RoHS Status: Not Compliant

AD8110/AD8111
Multichannel Video
The excellent video specifications of the AD8110/AD8111 make
them ideal candidates for creating composite video crosspoint
switches. These can be made quite dense by taking advantage
of the AD8110/AD8111’s high level of integration and the fact
that composite video requires only one crosspoint channel per
system video channel. There are, however, other video formats
that can be routed with the AD8110/AD8111 requiring more
than one crosspoint channel per video channel.
Some systems use twisted-pair wiring to carry video signals.
These systems utilize differential signals and can lower costs
because they use lower cost cables, connectors and termination
methods. They also have the ability to lower crosstalk and reject
common-mode signals, which can be important for equipment
that operates in noisy environments or where common-mode volt-
ages are present between transmitting and receiving equipment.
In such systems, the video signals are differential; there is a
positive and negative (or inverted) version of the signals. These
complementary signals are transmitted onto each of the two
wires of the twisted pair, yielding a first order zero common-
mode signal. At the receive end, the signals are differentially
received and converted back into a single-ended signal.
When switching these differential signals, two channels are
required in the switching element to handle the two differential
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 AD8110/AD8111, eight differen-
tial video channels can be assigned to the 16 inputs and four to
the outputs. This will effectively form an 8 × 4 differential cross-
point 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 AD8110/AD8111
and the requirements of the system.
There are other analog video formats requiring more than one
analog circuit per video channel. One two-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 (chromi-
nance, chroma or C) on a second channel.
Since S-video also uses two separate circuits for one video chan-
nel, 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 also
be converted to Y, R-Y, B-Y format, sometimes called YUV
format. These three-circuit, video standards are referred to as
component analog video.
The component video standards require three crosspoint chan-
nels per video channel to handle the switching function. In a
fashion similar to the two-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 proximity in a system, as will
undoubtedly be the case in a system that uses the AD8110/
AD8111, the crosstalk issues can be quite complex. A good
understanding of the nature of crosstalk and some definition of
terms is required in order to specify a system that uses one or
more AD8110/AD8111s.
Types of Crosstalk
Crosstalk can be propagated by means of any of three meth-
ods. 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 propa-
gates 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 undes-
ired 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 chan-
nels. 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 com-
mon impedance.
All these sources of crosstalk are vector quantities, so the magnitudes
cannot be simply added together to obtain the total crosstalk. In
fact, there are conditions where driving additional circuits in paral-
lel in a given configuration can actually reduce the crosstalk.
Areas of Crosstalk
For a practical AD8110/AD8111 circuit, it is required that it be
mounted to some sort of circuit board in order to connect it to
power supplies and measurement equipment. Great care has been
taken to create a characterization board (also available as an evalu-
ation 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 cross-
point and among the outputs. It can also occur from input to
output. Techniques will be discussed for diagnosing which part
of a system is contributing to crosstalk.