Datasheet

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.
EQ
AD8156
IN0P/N
IN1P/N
IN2P/N
IN3P/N
VCC
MODE
OUT0P/N
OUT1P/N
OUT2P/N
OUT3P/N
EQ
EQ
EQ
OUTPUT
DRIVERS
INPUT
RECEIVERS
INPUT
EQUALIZATION
2 × 2
SWITCH
2 × 2
SWITCH
0
6305-008
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
GHz83.0
log40
15
)(
10
f
D[3:0]
fgain
×=
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.