AD8112-EVALZ Analog Devices Inc, AD8112-EVALZ Datasheet - Page 21

• Manufacturer Part Number: AD8112-EVALZ
• Manufacturer: Analog Devices Inc

Specifications of AD8112-EVALZ

• Lead Free Status / Rohs Status: Compliant

Video signals usually use 75 Ω transmission lines that need to
be terminated with this value of resistance at each end. When
such a source is delivered to one of the AD8112 inputs, the high
input impedance does not properly terminate these signals.
Therefore, the line should be terminated with a 75 Ω shunt
resistor to ground. Because video signals are limited in their
peak-to-peak amplitude, there is no need to attenuate video
signals before they pass through the AD8112.
The AD8112 outputs are very low impedance and do not prop-
erly terminate the source end of a 75 Ω transmission line. In
these cases, a series 75 Ω resistor should be inserted at an output
that drives a video signal. Then the transmission line should be
terminated with 75 Ω at its far end. This overall termination
scheme divides the amplitude of the AD8112 output by 2. An
overall unity gain channel is produced as a result of the
AD8112’s channel gain of +2.
CREATING LARGER CROSSPOINT ARRAYS
The AD8112 is a high density building block for creating
crosspoint arrays of dimensions larger than 16 × 8. Various
features, such as output disable and chip enable, are useful
for creating larger arrays.
The first consideration in constructing a larger crosspoint is
to determine the minimum number of devices required. The
16 × 8 architecture of the AD8112 contains 128 points, which
is a factor of 32 greater than a 4 × 1 crosspoint (or multiplexer).
The PC board area, power consumption, and design effort
savings are readily apparent when compared with 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 avail-
ability of that input to be a source for any other outputs.
Some nonblocking crosspoint architectures require more than
this minimum as previously calculated. Also, there are blocking
architectures that can be constructed with fewer devices than
this minimum. These systems have connectivity available on a
statistical basis that is determined when designing the overall
system.
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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 Figure 46, which illustrates this concept for a 32 × 16
crosspoint array that uses four AD8112s.
The inputs are individually assigned to each of the 32 inputs of
the two devices and a divider is used to normalize the channel
gain. The outputs are wire-OR’ e d together in pairs. The output
from only one wire-OR’ e d pair should be enabled at any given
time. The device programming software must be properly
written to for this to happen.
Using additional crosspoint devices in the design can lower the
number of outputs that must be wire-OR’ e d together. Figure 47
shows a block diagram of a system using ten AD8112s to create
a nonblocking, gain of +2, 128 × 8 crosspoint that restricts the
wire-OR’ing at the output to only four outputs.
Additionally, by using the lower eight outputs from each of the
two Rank 2 AD8112s, a blocking 128 × 16 crosspoint array can
be realized. There are, however, some drawbacks to this technique.
The offset voltages of the various cascaded devices accumulate,
and the bandwidth limitations of the devices compound. In
addition, the extra devices consume more current and take up
more board space. Consider the overall system design speci-
fications when using the various trade-offs.
IN00 TO IN15
IN16 TO IN31
Figure 46. 32 x 16 Audio Crosspoint Array Using Four AD8112s
16
16
1kΩ
1kΩ
1kΩ
1kΩ
AD8112
AD8112
8
8
8
16
16
AD8112
AD8112
AD8112
8
8
8