AD8104 datasheet(33/36 Pages) AD

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Data Sheet

AD8104/AD8105

Rev. A | Page 33 of 36

there is only one way to drive a test signal into all 31 other

inputs in parallel.

Each of these cases is legitimately different from the others and

may yield a unique value, depending on the resolution of the

measurement system, but it is hardly practical to measure all

these terms and then specify them. In addition, this describes

the crosstalk matrix for just one input channel. A similar crosstalk

matrix can be proposed for every other input. In addition, if the

possible combinations and permutations for connecting inputs

to the other outputs (not used for measurement) are taken into

consideration, the numbers rather quickly grow to astronomical

proportions. If a larger crosspoint array of multiple AD8104/

AD8105 devices is constructed, the numbers grow larger still.

Obviously, some subset of all these cases must be selected to be

used as a guide for a practical measure of crosstalk. One common

method is to measure all-hostile crosstalk; this means that the

crosstalk to the selected channel is measured while all other

system channels are driven in parallel. In general, this yields the

worst crosstalk number, but this is not always the case, due to

the vector nature of the crosstalk signal.

Other useful crosstalk measurements are those created by one

nearest neighbor or by the two nearest neighbors on either side.

These crosstalk measurements are generally higher than those

of more distant channels, so they can serve as a worst-case

measure for any other one-channel or two-channel crosstalk

measurements.

Input and Output Crosstalk

Capacitive coupling is voltage-driven (dV/dt), but is generally a

constant ratio. Capacitive crosstalk is proportional to input or

output voltage, but this ratio is not reduced by simply reducing

signal swings. Attenuation factors must be changed by changing

impedances (lowering mutual capacitance), or destructive

canceling must be utilized by summing equal and out of phase

components. For high input impedance devices such as the

AD8104/AD8105, capacitances generally dominate input-

generated crosstalk.

Inductive coupling is proportional to current (dI/dt), and often

scales as a constant ratio with signal voltage, but also shows a

dependence on impedances (load current). Inductive coupling

can also be reduced by constructive canceling of equal and out

of phase fields. In the case of driving low impedance video

loads, output inductances contribute highly to output crosstalk.

The flexible programming capability of the AD8104/AD8105

can be used to diagnose whether crosstalk is occurring more on

the input side or the output side. Some examples are illustrative.

A given input pair (IN07 in the middle for this example) can be

programmed to drive OUT07 (also in the middle). The inputs

to IN07 are just terminated to ground (via 50 Ω or 75 Ω) and no

signal is applied.

All the other inputs are driven in parallel with the same test

signal (practically provided by a distribution amplifier), with all

other outputs except OUT07 disabled. Since grounded IN07

is programmed to drive OUT07, no signal should be present.

Any signal that is present can be attributed to the other 31

hostile input signals, because no other outputs are driven

(they are all disabled). Thus, this method measures the all

hostile input contribution to crosstalk into IN07. Of course, the

method can be used for other input channels and combinations

of hostile inputs.

For output crosstalk measurement, a single input channel is

driven (IN00, for example) and all outputs other than a given

output (IN07 in the middle) are programmed to connect to

IN00. OUT07 is programmed to connect to IN15 (far away

from IN00), which is terminated to ground. Thus OUT07

should not have a signal present since it is listening to a quiet

input. Any signal measured at the OUT07 can be attributed to

the output crosstalk of the other 16 hostile outputs. Again, this

method can be modified to measure other channels and other

crosspoint matrix combinations.

Effect of Impedances on Crosstalk

The input side crosstalk can be influenced by the output

impedance of the sources that drive the inputs. The lower the

impedance of the drive source, the lower the magnitude of the

crosstalk. The dominant crosstalk mechanism on the input side

is capacitive coupling. The high impedance inputs do not have

significant current flow to create magnetically induced crosstalk.

However, significant current can flow through the input termi-

nation resistors and the loops that drive them. Thus, the PC

board on the input side can contribute to magnetically coupled

crosstalk.

From a circuit standpoint, the input crosstalk mechanism looks

like a capacitor coupling to a resistive load. For low frequencies,

the magnitude of the crosstalk is given by

(

)

(

log

where:

RS is the source resistance.

CM is the mutual capacitance between the test signal circuit and

the selected circuit.

s is the Laplace transform variable.

From the preceding equation, it can be observed that this

crosstalk mechanism has a high-pass nature; it can also be

minimized by reducing the coupling capacitance of the input

circuits and lowering the output impedance of the drivers. If the

input is driven from a 75 Ω terminated cable, the input crosstalk

can be reduced by buffering this signal with a low output

impedance buffer.

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