AD8113JSTZ datasheet(18/28 Pages) AD

18 / 28 page

REV. A

AD8113

–18–

Inputs and outputs should be preassigned to be either audio or

video. As described above, audio and video signals are treated

differently, so it is difficult to have the same AD8113 inputs or

outputs route audio or video signals in the same system at

different times. The various audio and video channels should

be configured as described in the above sections.

Video outputs that drive a terminated 75

Ω transmission line

(150

Ω equivalent load) will dissipate significantly more power

with

±12 V supplies. An upper bound on power dissipation can

be approximated by the following method.

A video signal at the AD8113 output can have a maximum

value of 2 V. This is quite conservative, because most video

signals are about 700 mV peak at unity gain or 1.4 V peak after

a gain-of-two. A video signal only reaches this level when the video

content is at peak white, so this value is even more pessimistic.

Finally, a video signal will generally have some kind of sync and

blanking interval where its amplitude will be either 0 V (or black)

or very close to this level. The power dissipation will be much

lower during this period and this will occur at a very regular

duty cycle.

If the full 2 V signal is assumed to be present at 100% duty

cycle at the output, then the current in the output is 2 V/150

Ω

= 13.3 mA. If the positive supply is at 12 V, there will be a

10 V drop in the AD8113 output stage from the supply to the

output. This yields a power dissipated in the output of 133 mW

from one video load when running on supplies of

±12 V. This

is by far a worst-case situation, and this power dissipation fac-

tor can be adjusted lower by adjusting for actual video levels,

sync-interval duty cycle, and average picture level considerations.

If too much power will be dissipated in this type of configuration,

it is possible to lower it by buffering the output. An AD8113

video output drives a divide-by-two resistive divider that is

made up of two 1 k

Ω resistors. This presents a total load of

Ω to the AD8113 outputs, which significantly reduces the

power dissipation. Refer to Figure 10.

AD8057

+5V

–5V

0.1 F

10 F

TRANSMISSION

LINE

TYPICAL

INPUT

AD8113

G = 2

+12V

–12V

0.1 F

10 F

TYPICAL

OUTPUT

–

Figure 10. Video Buffer Circuit

After this divider, the signal is now at a unity level because of

the channel gain of the AD8113 and the attenuation of the

divider. An AD8057 is configured as a gain-of-two buffer to

drive the terminated transmission line. The AD8058 is a dual

version of the AD8057.

The maximum supply voltage of the AD8057 is only about

±6 V. If the only system supplies that are available are ±12 V, a

higher voltage video op amp can be substituted for the AD8057.

Good candidates are the AD817 and AD818 or, if dual op amps

are needed, the AD826 and AD828.

CREATING LARGER CROSSPOINT ARRAYS

The AD8113 is a high density building block for creating cross-

point arrays of dimensions larger than 16

× 16. 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

× 16

architecture of the AD8113 contains 256 points, which is a

factor of 64 greater than a 4

× 1 crosspoint (or multiplexer). The

PC board area, power consumption, and design effort 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 archi-

tectures that can be constructed with fewer devices than this

minimum. These systems have connectivity available on a statis-

tical 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. Figure 11 illustrates this concept for a

× 32 crosspoint array that uses four AD8113s.

AD8113

IN 00 –15

IN 16 –31

AD8113

Figure 11. 32

× 32 Audio Crosspoint Array Using Four

AD8113s

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-ORed together in pairs. The output

from only one of a wire-ORed pair should be enabled at any

given time. The device programming software must be properly

written to cause this to happen.

Using additional crosspoint devices in the design can lower the

number of outputs that have to be wire-ORed together. Figure 12

shows a block diagram of a system using ten AD8113s to create

a nonblocking, gain-of-two, 128

× 16 crosspoint that restricts

the wire-ORing at the output to only four outputs.

Additionally, by using the lower eight outputs from each of the

two Rank 2 AD8113s, a blocking 128

× 32 crosspoint array can be

realized. There are, however, some drawbacks to this technique.

The offset voltages of the various cascaded devices will accumu-

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