AD8152 datasheet(18/32 Pages) AD

18 / 32 page

REV. A

–18–

AD8152

Input 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 AD8152 input circuitry. For example, the XAUI inter-

connect specification for 10 Gbps Ethernet requires ac coupling

in order to ensure that there are no interactions of dc levels

between the transmitting and receiving devices.

AC coupling requires that the signal patterns have no long-term

dc component, which may occur in any random data stream.

Codes such as 8b/10b, called for in the XAUI specification, are

used in many data communications systems to ensure that the

data pattern is benign in an ac-coupled link. This is accomplished

by run-length limiting (RLL), which sets a maximum for the

number of 1s or 0s that can occur consecutively. In addition,

residual dc components are monitored and modified by keeping

track of the running disparity, excess of 1s versus 0s or vice versa.

For the AD8152 inputs, ac coupling requires a capacitor in series

with each single-ended input signal, as shown in Figure 7. This

should be done in a manner that does not interfere with the high

speed signal integrity of the PC board. The details of this are

covered in the section on board layout guidelines. The two critical

variables are setting the proper voltage for VTTI and selecting

the correct value of coupling capacitors.

VTTI

VCC

VEE

INXXP

INXXN

CINP

CINN

Figure 7. AC-Coupling Input Signal from AD8152

On the AD8152 side of the input coupling capacitor, the average

value of the single-ended input voltage will be at the voltage set at

VTTI. The range of allowable voltages is a function of the accept-

able input voltages of the active circuitry of the AD8152 inputs

and the amplitude of the input signal. The operating input range

of the AD8152 extends from VCC + 0.2 V to 0.8 V above VEE.

The total range that will be occupied by the input signal will be

its average value (as established by the voltage applied to VTTI)

plus or minus one half the single-ended swing of the signal. For a

standard 800 mV p-p differential signal, the single-ended swing is

400 mV p-p. Thus, the signal will swing

±200 mV about the

average value equal to VTTI.

If VTTI is set equal to VCC, then the single-ended signal will

just meet the specifications where its highest excursion will be

0.2 V higher than VCC. The lowest level to set VTTI is 0.8 V

above VEE. This will cause the negative signal excursions to stay

within the operating range.

With ac-coupled inputs, there is no power consumption advan-

tage associated with varying VTTI. As a practical matter, it

might be desirable to set VTTI at the same voltage as VTTO so

that only one supply is necessary. Refer to the VTTO section for

more information.

Output Coupling

Each single-ended output of the AD8152 has a termination

resistor that ties to a common point called VTTO. When VTTO

is varied, it will change the common-mode levels of the outputs

and the power dissipation of the output stages when they are

enabled.

The individual output currents are programmable. Varying this

current will change the lower level of the output voltage (and thus

the peak-to-peak swing) and also change the power dissipation in

the output stages. To obtain a standard 800 mV p-p differential

output (single-ended = 400 mV p-p), the output current should

be programmed to 16 mA. With an effective termination resis-

tance of 25

W, this will generate the proper differential voltage.

If the AD8152 drives another device that is ac-coupled, there is no

interaction of the dc levels on each side of the coupling capacitors

(see Figure 8). The dc levels for the AD8152 can be calculated

independent of the levels of the device that is driven.

The upper allowable setting for VTTO is 0.2 V higher than VCC.

The signals will be pulled up to this level at their highest excursion.

However at this setting, the power dissipation will be a maximum.

To save power, VTTO can be lowered. The lowest level for

VTTO will be determined by the lowest output level allowable

(VOL) by the AD8152 output when it is logically low. The output

at any time should not go lower than 1.0 V below VCC. If the

single-ended swing of an output is 400 mV p-p, then the lowest

that VTTO can go is 0.6 V below VCC. For more information

on VOL, see TPC 17.

VTTO

VEE

I = 2mA

(CODE) + 2mA

OUTXXP

OUTXXN

AD8152

VEE

VCC

VTT

VCC

DRIVEN DEVICE

VEE

Figure 8. AC-Coupling Output Signal from AD8152

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