AD8112 datasheet(21/28 Pages) AD

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AD8112

Rev. 0 | Page 21 of 28

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 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.

1kΩ

AD8112

IN00 TO IN15

IN16 TO IN31

AD8112

1kΩ

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

Figure 46. 32 x 16 Audio Crosspoint Array Using Four AD8112s

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 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’ed together in pairs. The output

from only one wire-OR’ed pair should be enabled at any given

time. The device programming software must be properly

written to for this to happen.

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.

Using additional crosspoint devices in the design can lower the

number of outputs that must be wire-OR’ed 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.

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.

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.

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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