AD8106ASTZ datasheet(19/28 Pages) AD

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AD8106/AD8107

Rev. 0 | Page 19 of 28

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, which

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.

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

(2)

where:

RS is the source resistance.

Other useful crosstalk measurements are those created by one

of the nearest neighbors or by two of the 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.

is the mutual capacitance between the test signal circuit and

the selected circuit.

s is the Laplace transform variable.

Equation 2 shows 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.

Input and Output Crosstalk

The flexible programming capability of the AD8106/AD8107

can be used to diagnose whether crosstalk is occurring more on

the input side or the output side. For example, a given input

channel, such as IN07 in the middle, can be programmed to

drive OUT01. The input to IN07 is terminated to ground (via

50 Ω or 75 Ω) and no signal is applied.

On the output side, the crosstalk can be reduced by driving a

lighter load. Although the AD8106/AD8107 are specified with

excellent differential gain and phase when driving a standard

150 Ω video load, the crosstalk is higher than the minimum

obtainable because of the high output currents. These currents

induce crosstalk via the mutual inductance of the output pins

and bond wires of the AD8106/AD8107.

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

signal (practically provided by a distribution amplifier) with all

other outputs disabled, except OUT01. Because grounded IN07

is programmed to drive OUT01, no signal should be present. If

any signal is present, it can be attributed to the other 15 hostile

input signals because no other outputs are driven; that is, they

are all disabled. Thus, this method measures the all-hostile input

contribution to crosstalk into IN07. This method can also be used

for other input channels and combinations of hostile inputs.

From a circuit standpoint, this output crosstalk mechanism

looks like a transformer with a mutual inductance between the

windings that drive a load resistor. For low frequencies, the

magnitude of the crosstalk is given by

(

)

Mxy

log

(3)

For output crosstalk measurement, a single input channel

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

output are programmed to connect to IN00. OUT01 is

programmed to connect to IN15, which is far away from IN00,

and is terminated to ground. As a result, OUT01 should not

have a signal present because it is listening for a quiet input.

Any signal measured at the OUT01 can be attributed 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.

where:

Mxy is the mutual inductance of output x to output y.

R is the load resistance on the measured output.

This crosstalk mechanism can be minimized by keeping the

mutual inductance low and increasing RL. The mutual

inductance can be kept low by increasing the spacing of the

conductors and minimizing their parallel length.

PCB LAYOUT

Effect of Impedances on Crosstalk

Extreme care must be exercised to minimize additional

crosstalk generated by the system circuit board(s). The areas

that must be carefully detailed are grounding, shielding, signal

routing, and supply bypassing.

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

termination resistors and the loops that drive them. Thus,

the PC board on the input side can contribute to magnetically

coupled crosstalk.

The packaging of the AD8106/AD8107 is designed to help keep

the crosstalk to a minimum. Each input is separated from other

inputs by an analog ground pin. All of these AGNDs should be

connected directly to the ground plane of the circuit board.

These ground pins provide shielding, low impedance return

paths, and physical separation for the inputs. All of these help to

reduce crosstalk.

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