AD8108AST datasheet(15/28 Pages) AD

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AD8108/AD8109

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THEORY OF OPERATION:

The AD8108 (G = +1) and AD8109 (G = +2) share a common

core architecture consisting of an array of 64 transconductance

(gm) input stages organized as eight 8:1 multiplexers with a

common, 8-line analog input bus. Each multiplexer is basically

a folded-cascode high impedance voltage feedback amplifier

with eight input stages. The input stages are NPN differential

pairs whose differential current outputs are combined at the

output stage, which contains the high impedance node, com-

pensation and a complementary emitter follower output buffer.

In the AD8108, the output of each multiplexer is fed back di-

rectly to the inverting inputs of its eight gm stages. In the

AD8109, the feedback network is a voltage divider consisting of

a two equal resistors.

This switched-gm architecture results in a low power crosspoint

switch that is able to directly drive a back terminated video load

(150

Ω) with low distortion (differential gain and differential

phase errors are better than 0.02% and 0.02

°, respectively).

This design also achieves high input resistance and low input

capacitance without the signal degradation and power dissipa-

tion of additional input buffers. However, the small input bias

current at any input will increase almost linearly with the num-

ber of outputs programmed to that input.

The output disable feature of these crosspoints allows larger

switch matrices to be built by simply busing together the out-

puts of multiple 8

× 8 ICs. However, while the disabled output

impedance of the AD8108 is very high (10 M

Ω), that of the

AD8109 is limited by the resistive feedback network (which has

a nominal total resistance of 1 k

Ω that appears in parallel with

the disabled output. If the outputs of multiple AD8109s are

connected through separate back termination resistors, the

loading due to these finite output impedances will lower the

effective back termination impedance of the overall matrix. This

problem is eliminated if the outputs of multiple AD8109s are

connected directly and share a single back termination resistor

for each output of the overall matrix. This configuration in-

creases the capacitive loading of the disabled AD8109s on the

output of the enabled AD8109.

APPLICATIONS

The AD8108/AD8109 have two options for changing the pro-

gramming of the crosspoint matrix. In the first, a serial word of

32 bits can be provided that will update the entire matrix each

time. The second option allows for changing a single output’s

programming via a parallel interface. The serial option requires

fewer signals, but requires more time (clock cycles) for changing

the programming, while the parallel programming technique re-

quires more signals, but can change a single output at a time and

requires fewer clock cycles to complete programming.

Serial Programming

The serial programming mode uses the device pins

CE, CLK,

DATA IN,

UPDATE, and SER/PAR. The first step is to assert

a LOW on

SER/PAR in order to enable the serial program-

ming mode.

CE for the chip must be LOW to allow data to be

clocked into the device. The

CE signal can be used to address

an individual device when devices are connected in parallel.

The

UPDATE signal should be HIGH during the time that data

is shifted into the device’s serial port. Although the data will still

shift in when

UPDATE is LOW, the transparent, asynchronous

latches will allow the shifting data to reach the matrix. This will

cause the matrix to try to update to every intermediate state as

defined by the shifting data.

The data at DATA IN is clocked in at every down edge of CLK.

A total of 32 data bits must be shifted in to complete the pro-

gramming. For each of the eight outputs, there are three bits

(D0–D2) that determine the source of its input followed by one

bit (D3) that determines the enabled state of the output. If D3

is LOW (output disabled), the three associated bits (D0–D2) do

not matter because no input will be switched to that output.

The most-significant-output-address data is shifted in first, then

following in sequence until the least-significant-output-address

data is shifted in. At this point

UPDATE can be taken LOW,

which will cause the programming of the device according to the

data that was just shifted in. The

UPDATE registers are asyn-

chronous and when

UPDATE is LOW, they are transparent.

If more than one AD8108/AD8109 device is to be serially pro-

grammed in a system, the DATA OUT signal from one device

can be connected to the DATA IN of the next device to form a

serial chain. All of the CLK,

CE, UPDATE and SER/PAR

pins should be connected in parallel and operated as de-

scribed above. The serial data is input to the DATA IN pin of

the first device of the chain, and it will ripple on through to the

last. Therefore, the data for the last device in the chain should

come at the beginning of the programming sequence. The length

of the programming sequence will be 32 times the number of

devices in the chain.

PARALLEL PROGRAMMING

When using the parallel programming mode, it is not neces-

sary to reprogram the entire device when making changes to

the matrix. In fact, parallel programming allows the modifica-

tion of a single output at a time. Since this takes only one CLK/

UPDATE cycle, significant time savings can be realized by

using parallel programming.

One important consideration in using parallel programming is

that the

RESET signal DOES NOT RESET ALL REGISTERS

in the AD8108/AD8109. When taken low, the

RESET signal

will only set each output to the disabled state. This is helpful

during power-up to ensure that two parallel outputs will not be

active at the same time.

After initial power-up, the internal registers in the device will

generally have random data, even though the

RESET signal was

asserted. If parallel programming is used to program one out-

put, that output will be properly programmed but the rest of the

device will have a random program state depending on the inter-

nal register content at power-up. Therefore, when using parallel

programming, it is essential that ALL OUTPUTS BE PRO-

GRAMMED TO A DESIRED STATE AFTER POWER-UP.

This will ensure that the programming matrix is always in a

known state. From then on, parallel programming can be used

to modify a single, or more, output at a time.

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