AD8116 Analog Devices, AD8116 Datasheet - Page 13

• Manufacturer Part Number: AD8116
• Description: 200 Mhz, 16 3 16 Buffered Video Crosspoint Switch
• Manufacturer: Analog Devices

IN 112–127
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 programming
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 pro-
gramming 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 sequence 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 combinations 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
IN 96–111
IN 16–31
IN 32–47
IN 48–63
IN 64–79
IN 80–95
IN 0–15
16
16
16
16
16
16
16
16
RANK 1
(128:32)
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
×
32:16 NONBLOCKING
(32:32 BLOCKING)
FOUR AD8116 OUTPUTS
WIRE-ORED TOGETHER
RANK 2
×
8
8
8
8
NONBLOCKING
OUTPUTS
OUT 0–16
ADDITIONAL
16 OUTPUTS
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 mecha-
nism 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
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 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
AD8116