AD8111AST datasheet(19/28 Pages) AD

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AD8110/AD8111

–19–

REV. 0

Measuring Crosstalk

Crosstalk is measured by applying a signal to one or more chan-

nels and measuring the relative strength of that signal on a de-

sired selected channel. The measurement is usually expressed as

dB down from the magnitude of the test signal. The crosstalk is

expressed by:

|XT| = 20 log10 (Asel(s)/Atest(s))

where s = j

ω is the Laplace transform variable, Asel(s) is the

amplitude of the crosstalk-induced signal in the selected channel

and Atest(s) is the amplitude of the test signal. It can be seen

that crosstalk is a function of frequency, but not a function of

the magnitude of the test signal (to first order). In addition, the

crosstalk signal will have a phase relative to the test signal asso-

ciated with it.

A network analyzer is most commonly used to measure crosstalk

over a frequency range of interest. It can provide both magni-

tude and phase information about the crosstalk signal.

As a crosspoint system or device grows larger, the number of

theoretical crosstalk combinations and permutations can be-

come extremely large. For example, in the case of the 16

× 8

matrix of the AD8110/AD8111, we can examine the number of

crosstalk terms that can be considered for a single channel, say

IN00 input. IN00 is programmed to connect to one of the

AD8110/AD8111 outputs where the measurement can be made.

We can first measure the crosstalk terms associated with driving

a test signal into each of the other 15 inputs one at a time. We

can then measure the crosstalk terms associated with driving a

parallel test signal into all 15 other inputs taken two at a time in

all possible combinations; and then three at a time, etc., until,

finally, there is only one way to drive a test signal into all 15

other inputs.

Each of these cases is legitimately different from the others and

might yield a unique value depending on the resolution of the

measurement system, but it is hardly practical to measure all

these terms and then to 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 (not used for measurement) outputs are taken into

consideration, the numbers rather quickly grow to astronomical

proportions. If a larger crosspoint array of multiple AD8110/

AD8111s 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 com-

mon method is to measure “all hostile” crosstalk. This term

means that the crosstalk to the selected channel is measured,

while all other system channels are driven in parallel. In general,

this will yield 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 will generally be 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

The flexible programming capability of the AD8110/AD8111

can be used to diagnose whether crosstalk is occurring more on

the input side or the output side. Some examples are illustra-

tive. A given input channel (IN07 in the middle for this ex-

ample) can be programmed to drive OUT03. The input to IN07

is just terminated to ground (via 50 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 OUT03 disabled. Since grounded IN07 is

programmed to drive OUT03, there should be no signal present.

Any signal that is present can be attributed to the other 15 hos-

tile 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

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

given output (IN03 in the middle) are programmed to

connect to IN00. OUT03 is programmed to connect to IN15

(far away from IN00), which is terminated to ground. Thus

OUT03 should not have a signal present since it is listening to a

quiet input. Any signal measured at the OUT03 can be attrib-

uted to the output crosstalk of the other seven 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 imped-

ance of the sources that drive the inputs. The lower the im-

pedance 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 will be given by:

|XT| = 20 log10 [(RS CM)

× s]

where RS is the source resistance, CM is the mutual capacitance

between the test signal circuit and the selected circuit, and s is

the Laplace transform variable.

From the equation it can be observed that this crosstalk mecha-

nism has a high pass nature; it can also be minimized by reduc-

ing 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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