AD8115AST datasheet(24/32 Pages) AD

24 / 32 page

AD8114/AD8115

Rev. B | Page 24 of 32

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 termination 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

(

)

[

log

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

mechanism has a high-pass nature; it can 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 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

)

(

log

Mxy

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.

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