AD8156ABCZ datasheet(14/20 Pages) AD

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AD8156

Rev. 0 | Page 14 of 20

When in dual 2 × 2 mode, the A[3:0] and D[3:0] pins set the

AD8156 configuration state when CS is low. This configuration

method allows the user to have multiple AD8156s share the control

bus while each device has its own dedicated CS control signal.

AD8156

IN0P/N

IN1P/N

IN2P/N

IN3P/N

VCC

MODE

OUT0P/N

OUT1P/N

OUT2P/N

OUT3P/N

OUTPUT

DRIVERS

INPUT

RECEIVERS

INPUT

EQUALIZATION

2 × 2

SWITCH

2 × 2

SWITCH

Figure 24. AD8156 in Dual 2 × 2 Mode

INPUT EQUALIZATION

The AD8156 input equalization is an active scheme that is fully

linear over all operating ranges. The useful range of equalization

covers dc to 3.125 GHz frequencies or dc to 6.25 Gbps data

rates. Other key features include:

•

15 steps of gain, linear in dB, programmable through the

4 × 4 control interface

•

Gain has a 40 dB per decade slope

•

Peak gain of 23 dB at 3.125 GHz (~6.25 Gbps)

•

Equalizes more than 40” of typical FR4 backplane with

associated connectors and vias at all speeds

•

0.10 UI p-p residual deterministic jitter typ @ 3.125 Gbps

•

0.15 UI p-p residual deterministic jitter typ @ 6.25 Gbps

As with all equalizers, the gain setting is the key. The ideal

method of choosing the proper gain setting is to run the equalizer

with the channel, and choose the setting with minimum jitter.

If this process is not possible or is too time consuming for the

number of channels required, the loss of the channel at

3.125 GHz should be measured. The best equalizer setting is

usually 2 dB to 4 dB more than the loss at 3.125 GHz. Using the

40 dB slope of the equalizer gain, the gain at other frequencies

can be calculated based on the peak gain at 3.125 GHz. The

formula to use is

GHz

log

)

(

D[3:0]

gain

where f is the fundamental frequency of the data, or the data

rate divided by 2 (that is, 6.25 Gbps → f = 3.125 GHz).

Performance of the equalizer is heavily dependent on the channel

used. Operation at high speeds depends on features such as

dielectric used (for example, FR4, Nelco3000, or Rogers), connector

quality, via stub length, and routing geometry and topology.

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