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AD8159

Rev. A | Page 19 of 24

INTERFACING TO THE AD8159

TERMINATION STRUCTURES

For output ports, there are two 50 Ω resistors connected to the

termination supply. Note that the differential input resistance

for both structures is the same, 100 Ω.

To determine the best strategy for connecting to the high speed

pins of the AD8159, the user must first be familiar with the on-

chip termination structures. The AD8159 contains two types of

these structures (see

INPUT COMPLIANCE

Figure 41 and Figure 42): one type for input

and bidirectional ports and one type for output ports.

The range of allowable input voltages is determined by the

fundamental limitations of the active input circuitry. This range

of signals is normally a function of the common-mode level of

the input signal, the signal swing, and the supply voltage. For a

given input signal swing, there is a range of common-mode

voltages that keeps the high and low voltage excursions within

acceptable limits. Similarly, for a given common-mode input

voltage there is a maximum acceptable input signal swing.

There is also a minimum signal swing that the active input

circuitry can resolve reliably.

54.5Ω

1173Ω

VTTI/VTTIO/VTTOI

Figure 41. Termination Structure: Input and Bidirectional Ports

50Ω

VTTO

Figure 22 and Figure 25 summarize the input voltage ranges for

all ports. Note that the input range is different when comparing

bidirectional ports to strictly input ports. This is a consequence

of the additional circuitry required to support the bidirectional

feature on Port C.

Figure 42. Output Ports

For input and bidirectional ports, the termination structure

consists of two 54.5 Ω resistors connected to a termination

supply and an 1173 Ω resistor connected across the differential

inputs, the latter being a result of the finite differential input

impedance of the equalizer.

AC Coupling

One way to simplify the input circuit and make it compatible

with a wide variety of driving devices is to use ac coupling. This

has the effect of isolating the dc common-mode levels of the

driver and the AD8159 input circuitry. AC coupling requires a

capacitor in series with each single-ended input signal, as shown

in Figure 43. This should be done in a manner that does not

interfere with the high speed signal integrity of the PCB.

50Ω

VCC

54.5Ω

VTTI/VTTIO

DRIVER

AD8159

54.5Ω

VEE

1173Ω

Figure 43. AC-Coupling Input Signal of AD8159

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