AD8109AST datasheet(21/33 Pages) AD

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AD8108/AD8109

Rev. B | Page 20 of 32

CREATING LARGER CROSSPOINT ARRAYS

The AD8108/AD8109 are high density building blocks for creating

crosspoint arrays of dimensions larger than 8 × 8. Various features,

such as output disable, chip enable, and gain-of-1 and-2 options,

are useful for creating larger arrays. For very large arrays, they

can be used along with the AD8116, a 16 × 16 video cross-point

device. In addition, systems that require more inputs than

outputs can use the AD8110 and/or the AD8111, which are

(gain-of-1 and gain-of-2) 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 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.

An 8 input by 16 output crosspoint array can be constructed as

shown in Figure 48. 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 1) or

AD8111 (gain of 2) device can be used. These devices are

already configured into a 16 × 8 crosspoint in a single device.

8 INPUTS

IN 00–07

16 OUTPUTS

OUT 00–15

ONE

TERMINATION

PER INPUT

AD8108

AD8109

AD8108

AD8109

Figure 48. 8 × 16 Crosspoint Array Using Two AD8108s (Unity Gain) or Two

AD8109s (Gain of 2)

Figure 49 illustrates a 16 × 16 crosspoint array, while a 24 × 24

crosspoint is illustrated in Figure 50. The 16 × 16 crosspoint

requires that each input driver drive two inputs in parallel and

each output be wire-OR’ed with one other output. The 24 × 24

crosspoint requires driving three inputs in parallel and having

the outputs wire-OR’ed in groups of three. It is required of the

system programming that only one output of a wired-OR node

be active at a time.

IN 00–07

00–07

08–15

IN 08–15

OUT 00–07

OUT 08–15

RTERM

× 8

Figure 49. 16 × 16 Crosspoint Array Using Four AD8108s or AD8109s

IN 00–07

IN 08–15

IN 16–23

OUT 16–23

OUT 08–15

OUT 00–07

RTERM

× 8

RTERM

Figure 50. 24 × 24 Crosspoint Array Using Nine AD8108s or AD8109s

At some point, the number of outputs that are wire-OR’ed

becomes too great to maintain system performance. This will

vary according to which system specifications are most

important. For example, a 64 × 8 crosspoint can be created with

eight AD8108/AD8109s. This design will have 64 separate

inputs and have the corresponding outputs of each device wire-

OR’ed together in groups of eight.

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