AD8115AST datasheet(19/26 Pages) AD

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AD8114/AD8115

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REV. 0

common 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 AD8114/AD8115

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 channel (IN07 in the middle for this example) can

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

to IN07 is 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 15 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 imped-

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

ance of the drive source, the lower the magnitude of the crosstalk.

The dominant crosstalk mechanism on the input side is capaci-

tive coupling. The high impedance inputs do not have signifi-

cant 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 be also 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.

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

lighter load. Although the AD8114/AD8115 is specified with

excellent differential gain and phase when driving a standard

150

Ω video load, the crosstalk will be higher than the minimum

obtainable due to the high output currents. These currents will

induce crosstalk via the mutual inductance of the output pins

and bond wires of the AD8114/AD8115.

From a circuit standpoint, this output crosstalk mechanism

looks like a transformer with a mutual inductance between the

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

magnitude of the crosstalk is given by:

|XT| = 20 log10 (Mxy × s/RL)

where Mxy is the mutual inductance of output X to output Y

and RL 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

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 packaging of the AD8114/AD8115 is designed to help keep

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

other input by an analog ground pin. All of these AGNDs should

be directly connected 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.

Each output is separated from its two neighboring outputs by an

analog supply pin of one polarity or the other. Each of these

analog supply pins provides power to the output stages of only

the two nearest outputs. These supply pins provide shielding,

physical separation and a low impedance supply for the outputs.

Individual bypassing of each of these supply pins with a 0.01

µF

chip capacitor directly to the ground plane minimizes high fre-

quency output crosstalk via the mechanism of sharing common

impedances.

Each output also has an on-chip compensation capacitor that

is individually tied to the nearby analog ground pins AGND00

through AGND07. This technique reduces crosstalk by prevent-

ing the currents that flow in these paths from sharing a common

impedance on the IC and in the package pins. These AGNDxx

signals should all be connected directly to the ground plane.

The input and output signals will have minimum crosstalk if

they are located between ground planes on layers above and

below, and separated by ground in between. Vias should be

located as close to the IC as possible to carry the inputs and

outputs to the inner layer. The only place the input and output

signals surface is at the input termination resistors and the out-

put series back-termination resistors. To the extent possible,

these signals should also be separated as soon as they emerge

from the IC package.

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