AD8109-EB Analog Devices, AD8109-EB Datasheet - Page 16

• Manufacturer Part Number: AD8109-EB
• Description: 250MHz, 8x8 Buffered Video Crosspoint Switch(Gain=2)
• Manufacturer: Analog Devices

AD8108/AD8109
In a similar fashion, if both CE and UPDATE are taken LOW after
initial power-up, the random power-up data in the shift register will
be programmed into the matrix. Therefore, in order to prevent the
crosspoint from being programmed into an unknown state do
not apply low logic levels to both CE and UPDATE after power is
initially applied. Programming the full shift register one time to a
desired state by either serial or parallel programming after initial
power-up will eliminate the possibility of programming the
matrix to an unknown state.
To change an output’s programming via parallel programming,
SER/PAR and UPDATE should be taken HIGH and CE should
be taken LOW. The CLK signal should be in the HIGH state.
The address of the output that is to be programmed should be
put on A0–A2. The first three data bits (D0–D2) should contain
the information that identifies the input that is programmed to the
output that is addressed. The fourth data bit (D3) will determine
the enabled state of the output. If D3 is LOW (output disabled)
the data on D0–D2 does not matter.
After the desired address and data signals have been established,
they can be latched into the shift register by a HIGH to LOW
transition of the CLK signal. The matrix will not be programmed,
however, until the UPDATE signal is taken low. Thus, it is pos-
sible to latch in new data for several or all of the outputs first via
successive negative transitions of CLK while UPDATE is held
high, and then have all the new data take effect when UPDATE
goes LOW. This is the technique that should be used when pro-
gramming the device for the first time after power-up when
using parallel programming.
POWER-ON RESET
When powering up the AD8108/AD8109 it is usually desirable
to have the outputs come up in the disabled state. The RESET
pin, when taken LOW will cause all outputs to be in the dis-
abled state. However, the RESET signal does not reset all registers
in the AD8108/AD8109. This is important when operating in
the parallel programming mode. Please refer to that section for
information about programming internal registers after power-
up. Serial programming will program the entire matrix each
time, so no special considerations apply.
Since the data in the shift register is random after power-up,
they should not be used to program the matrix or else the matrix
can enter unknown states. To prevent this, do not apply logic low
signals to both CE and UPDATE initially after power-up. The shift
register should first be loaded with the desired data, and then
UPDATE can be taken LOW to program the device.
The RESET pin has a 20 kΩ pull-up resistor to DVDD that can
be used to create a simple power-up reset circuit. A capacitor
from RESET to ground will hold RESET LOW for some time
while the rest of the device stabilizes. The LOW condition will
cause all the outputs to be disabled. The capacitor will then
charge through the pull-up resistor to the HIGH state, thus
allowing full programming capability of the device.
Gain Selection
The 8 × 8 crosspoints come in two versions depending on the
desired gain of the analog circuit paths. The AD8108 device is
unity gain and can be used for analog logic switching and other
applications where unity gain is desired. The AD8108 can also
be used for the input and interior sections of larger crosspoint
arrays where termination of output signals is not usually used.
The AD8108 outputs have a very high impedance when their
outputs are disabled.
For devices that will be used to drive a terminated cable with its
outputs, the AD8109 can be used. This device has a built-in gain
of two that eliminates the need for a gain-of-two buffer to drive
a video line. Because of the presence of the feedback network in
these devices, the disabled output impedance is about 1 kΩ.
If external amplifiers will be used to provide a G = +2, Analog
Devices’ AD8079 is a fixed gain of +2 buffer.
Creating Larger Crosspoint Arrays
The AD8108/AD8109 are high density building blocks for cre-
ating crosspoint arrays of dimensions larger than 8 × 8. Various
features such as output disable, chip enable, and gain-of-one
and-two options are useful for creating larger arrays. For very
large arrays, they can be used along with the AD8116, a 16 × 16
video crosspoint device. In addition, systems that require more
inputs than outputs can use the AD8110 and/or the AD8111,
which are (gain-of-one and gain-of-two) 16 × 8 crosspoint
switches.
The first consideration in constructing a larger crosspoint is to
determine the minimum number of devices required. The 8 × 8
architecture of the AD8108/AD8109 contains 64 “points,” which
is a factor of 16 greater than a 4 × 1 crosspoint. The PC board
area and power consumption savings are readily apparent when
compared to using these smaller devices.
For a nonblocking crosspoint, the number of points required is
the product of the number of inputs multiplied by the number
of outputs. Nonblocking requires that the programming of a given
input to one or more outputs does not restrict the availability of
that input to be a source for any other outputs.
Some nonblocking crosspoint architectures will require more than
this minimum as calculated above. Also, there are blocking
architectures that can be constructed with fewer devices than this
minimum. These systems have connectivity available on a statisti-
cal basis that is determined when designing the overall system.
The basic concept in constructing larger crosspoint arrays is to
connect inputs in parallel in a horizontal direction and to “wire-OR”
the outputs together in the vertical direction. The meaning of
horizontal and vertical can best be understood by looking at a diagram.
An 8 input by 16 output crosspoint array can be constructed as
shown in Figure 6. This configuration parallels two inputs per
channel and does not require paralleling of any outputs. Inputs are
easier to parallel than outputs, because there are lower parasitics
involved. For a 16 × 8 crosspoint, the AD8110 (gain of one) or
AD8111 (gain of two) device can be used. These devices are already
configured into a 16 × 8 crosspoint in a single device.