AD8116 datasheet(13/26 Pages) AD

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AD8116

–13–

REV. A

IN 16–31

IN 0–15

OUT 0–16

IN 32–47

IN 48–63

IN 64–79

IN 80–95

IN 96–111

IN 112–127

NONBLOCKING

OUTPUTS

ADDITIONAL

16 OUTPUTS

RANK 2

32:16 NONBLOCKING

(32:32 BLOCKING)

RANK 1

(128:32)

FOUR AD8116 OUTPUTS

WIRE-ORED TOGETHER

Figure 26. Nonblocking 128

× 16 Array (128 × 32 Blocking)

Logic Operation

There are two basic options for controlling the logic in multi-

crosspoint arrays. One is to serially connect the data paths

(DATA OUT to DATA IN) of all the devices and tie all the

CLK and

UPDATE signals in parallel. CE can be tied low for

all the devices. A long serial sequence with the desired pro-

gramming data consisting of 80 bits times the number of

AD8116 devices can then be shifted through all the parallel

devices by using the DATA IN of the first device and the CLK.

When finished clocking in the data,

UPDATE can be pulled low

to program all the device crosspoint matrices.

This technique has an advantage in that a separate

CE signal

is not required for each chip, but has a disadvantage in that

several chips’ data cannot be shifted in parallel. In addition, if

another device is added into the system between already existing

devices, the programming sequence will have to be lengthened

at some midpoint to allow for programming of the added device.

The second programming method is to connect all the CLK

and the DATA IN pins in parallel and use the

CE pins in se-

quence to program each device. If a byte or 16-bit word of data

is available for providing the programming data, then multiple

AD8116s can be programmed in parallel with just 80 clock

cycles. This method can be used to speed up the programming

of large arrays. Of course, in a practical system, various combi-

nations of these basic methods can be used.

Power-On Reset

Most systems will want all the AD8116s to be in the reset state

(all outputs disabled) when power is applied to the system. This

ensures that two outputs that are wire-ORed together will not

fight each other at power up.

The power-on reset function can be implemented by adding a

0.1

µF capacitor from the RESET pin to ground. This will hold

this signal low after the power is applied to reset the device. An

on-chip 20 k

Ω resistor from RESET to DVCC will charge the

capacitor to the logical high state. If several AD8116s are used,

the pull-up resistors will be in parallel, so a larger value capaci-

tance should be used.

If the system requires the ability to be reset while power is still

applied, the

RESET driver will have to be able to charge and

discharge this capacitance in the required time. With too many

devices in parallel, this might become more difficult; if this

occurs, the reset circuits should be broken up into smaller sub-

sets with each controlled by a separate driver.

CROSSTALK

Many systems, such as broadcast video, that handle numerous

analog signal channels have strict requirements for keeping the

various signals from influencing any of the others in the system.

Crosstalk is the term used to describe the coupling of the signals

of other nearby channels to a given channel.

When there are many signals in close proximity in a system, as

will undoubtedly be the case in a system that uses the AD8116,

the crosstalk issues can be quite complex. A good understanding

of the nature of crosstalk and some definition of terms is required

in order to specify a system that uses one or more AD8116s.

Types of Crosstalk

Crosstalk can be propagated by means of any of three methods.

These fall into the categories of electric field, magnetic field and

sharing of common impedances. This section will explain these

effects.

Every conductor can be both a radiator of electric fields and a

receiver of electric fields. The electric field crosstalk mechanism

occurs when the electric field created by the transmitter propagates

across a stray capacitance and couples with the receiver and

induces a voltage. This voltage is an unwanted crosstalk signal

in any channel that receives it.

Currents flowing in conductors create magnetic fields that circu-

late around the currents. These magnetic fields will then gener-

ate voltages in any other conductors whose paths they link. The

undesired induced voltages in these other channels are crosstalk

signals. The channels that crosstalk can be said to have a

mutual inductance that couples signals from one channel to

another.

The power supplies, grounds and other signal return paths of a

multichannel system are generally shared by the various channels.

When a current from one channel flows in one of these paths, a

voltage that is developed across the impedance becomes an

input crosstalk signal for other channels that share the common

impedance.

All these sources of crosstalk are vector quantities, so the

magnitudes cannot be simply added together to obtain the total

crosstalk. In fact, there are conditions where driving additional

circuits in parallel in a given configuration can actually reduce

the crosstalk.

Areas of Crosstalk

For a practical AD8116 circuit, it is required that it be mounted

to some sort of circuit board in order to connect it to power

supplies and measurement equipment. Great care has been

taken to create a characterization board (also available as an

evaluation board) that adds minimum crosstalk to the intrinsic

device. This, however, raises the issue that a system’s crosstalk

is a combination of the intrinsic crosstalk of the devices and the

circuit board to which they are mounted. It is important to try

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