AD8108 Analog Devices, AD8108 Datasheet - Page 18

• Manufacturer Part Number: AD8108
• Description: 325 Mhz, 8x8 Buffered Video Crosspoint Switch (Gain=1)
• Manufacturer: Analog Devices

AD8108/AD8109
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 comple-
mentary 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 AD8108/AD8109, four differential video
channels can be assigned to the eight inputs and eight outputs.
This will effectively form a 4 × 4 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 AD8108/AD8109
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 (chrominance, 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 differen-
tial 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 AD8108/
AD8109, 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 AD8108/AD8109s.
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 undes-
ired induced voltages in these other channels are crosstalk signals.
The channels that crosstalk can be said to have a mutual induc-
tance that couples signals from one channel to another.
The power supplies, grounds and other signal return paths of a multi-
channel 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 mag-
nitudes 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 AD8108/AD8109 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 combi-
nation 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 outputs. It can also occur from input to output.
Techniques will be discussed for diagnosing which part of a system
is contributing to crosstalk.
Measuring Crosstalk
Crosstalk is measured by applying a signal to one or more chan-
nels and measuring the relative strength of that signal on a desired
selected channel. The measurement is usually expressed as dB
down from the magnitude of the test signal. The crosstalk is
expressed by:
where s = jω is the Laplace transform variable, Asel(s) is the
amplitude of the crosstalk-induced signal in the selected channel
and Atest(s) is the amplitude of the test signal. It can be seen
that crosstalk is a function of frequency, but not a function of
the magnitude of the test signal (to first order). In addition, the
crosstalk signal will have a phase relative to the test signal asso-
ciated with it.
|XT| = 20 log
10
(Asel(s)/Atest(s))