ADV3201ASWZ datasheet(34/36 Pages) AD

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ADV3200/ADV3201

Rev. 0 | Page 34 of 3

Effect of Impedances on Crosstalk

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 PCB 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

]

(5)

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.

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

lighter load. Although the ADV3200/ADV3201 are 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 induce crosstalk via the mutual inductance of the

output pins and bond wires of the ADV3200/ADV3201.

From a circuit standpoint, the 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

⎟

⎠

⎞

⎜

⎝

⎛

log

(6)

where:

MXY is the mutual inductance of Output X to Output Y.

RL is the load resistance on the measured output.

s is the Laplace transform variable.

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 system circuit boards. The areas that

must be carefully detailed are grounding, shielding, signal

routing, and supply bypassing.

The input and output signals have minimum crosstalk if they

are located between ground planes on layers above and below

and are separated by ground in between. Locate vias as close to

the IC as possible to carry the inputs and outputs to the inner

layer. The input and output signals surface at the input termin-

ation resistors and the output series back-termination resistors.

To the extent possible, separate these signals as soon as they

emerge from the IC package.

PCB TERMINATION LAYOUT

As frequencies of operation increase, proper routing of trans-

mission line signals becomes more important. The bandwidth

of the ADV3200/ADV3201 is large enough so that using high

impedance routing does not provide a flat in-band frequency

response for practical signal trace lengths. It is necessary for

the user to choose a characteristic impedance suitable for the

application and to properly terminate the input and output

signals of the ADV3200/ADV3201. Traditionally, video

applications use 75 Ω single-ended environments.

For flexibility, the ADV3200/ADV3201 does not contain on-

chip termination resistors. This flexibility in application comes

with some board layout challenges. The distance between the

termination of the input transmission line and the ADV3200/

ADV3201 die is a high impedance stub and causes reflections

of the input signal. With some simplification, it can be shown

that these reflections cause peaking of the input at regular

intervals in frequency, dependent on the propagation speed (vP)

of the signal in the chosen board material and the distance (d)

between the termination resistor and the ADV3200/ADV3201.

If the distance is great enough, these peaks can occur in band.

In fact, practical experience shows that these peaks are not

high-Q, and should be pushed out to three or four times the

desired bandwidth in order to not have an effect on the signal.

For a board designer using FR4 (vP = 144 × 106 m/s), this means

that the ADV3200/ADV3201 input should be placed no farther

than 2 cm after the termination resistors and, preferably, should

be placed even closer. Therefore, 2 cm PCB routing equates to

d = 2 × 10−2 m in the calculations.

(

)

PEAK

(7)

where n = {0, 1, 2, 3, …}.

In some cases, it is difficult to place the termination close to

the ADV3200/ADV3201 due to space constraints and large

resistor footprints. A better solution in this case is to maintain a

controlled transmission line past the ADV3200/ADV3201

inputs and to terminate the end of the line. This method is

known as fly-by termination. The input impedance of the

ADV3200/ADV3201 is large enough, and the stub length inside

the package is small enough, that this works well in practice.

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