AD8116 Datasheet, PDF (13/26 Pages) Analog Devices – 200 MHz, 16 x 16 Buffered Video Crosspoint Switch

AD8116
IN 0–15
16
IN 16–31
16
IN 32–47
16
IN 48–63 ₁₆
IN 64–79
16
IN 80–95
16
IN 96–111
16
IN 112–127
16
RANK 1
(128:32)
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
RANK 2
32:16 NONBLOCKING
(32:32 BLOCKING)
NONBLOCKING
8 OUTPUTS
OUT 0–16
8
8
ADDITIONAL
16 OUTPUTS
8
FOUR AD8116 OUTPUTS
WIRE-ORED TOGETHER
Figure 26. Nonblocking 128 × 16 Array (128 × 32 Blocking)
Logic Operation
There are two basic options for controlling the logic in multi-
crosspoint arrays. One is to serially connect the data paths
(DATA OUT to DATA IN) of all the devices and tie all the
CLK and UPDATE signals in parallel. CE can be tied low for
all the devices. A long serial sequence with the desired pro-
gramming data consisting of 80 bits times the number of
AD8116 devices can then be shifted through all the parallel
devices by using the DATA IN of the first device and the CLK.
When finished clocking in the data, UPDATE can be pulled low
to program all the device crosspoint matrices.
This technique has an advantage in that a separate CE signal
is not required for each chip, but has a disadvantage in that
several chips’ data cannot be shifted in parallel. In addition, if
another device is added into the system between already existing
devices, the programming sequence will have to be lengthened
at some midpoint to allow for programming of the added device.
The second programming method is to connect all the CLK
and the DATA IN pins in parallel and use the CE pins in se-
quence to program each device. If a byte or 16-bit word of data
is available for providing the programming data, then multiple
AD8116s can be programmed in parallel with just 80 clock
cycles. This method can be used to speed up the programming
of large arrays. Of course, in a practical system, various combi-
nations of these basic methods can be used.
Power-On Reset
Most systems will want all the AD8116s to be in the reset state
(all outputs disabled) when power is applied to the system. This
ensures that two outputs that are wire-ORed together will not
fight each other at power up.
The power-on reset function can be implemented by adding a
0.1 µF capacitor from the RESET pin to ground. This will hold
this signal low after the power is applied to reset the device. An
on-chip 20 kΩ resistor from RESET to DVCC will charge the
capacitor to the logical high state. If several AD8116s are used,
the pull-up resistors will be in parallel, so a larger value capaci-
tance should be used.
If the system requires the ability to be reset while power is still
applied, the RESET driver will have to be able to charge and
discharge this capacitance in the required time. With too many
devices in parallel, this might become more difficult; if this
occurs, the reset circuits should be broken up into smaller sub-
sets with each controlled by a separate driver.
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 AD8116,
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 AD8116s.
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 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 circu-
late around the currents. These magnetic fields will then gener-
ate 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 be simply 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 AD8116 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
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 and the
circuit board to which they are mounted. It is important to try
REV. A
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