AD8155 Datasheet, PDF (21/36 Pages) Analog Devices – 6.5 Gbps Dual Buffer Mux/Demux

AD8155
TRANSMITTERS
The AD8155 transmitter offers programmable preemphasis,
programmable output levels, output disable, and transmit
squelch. The SEL4G pin lets the user lower the transmitter
frequency of maximum boost from 3.25 GHz to 2.0 GHz,
allowing the AD8155 to offer exceptional transmit channel
compensation for legacy applications (4.5 Gbps and slower).
ON-CHIP TERMINATION
V_CC
ESD
V3
VC
RP
R_TERM
V2
V1
VP
VN
RN
R_TERM
V_TTO
OP_xx
ON_xx
Q1
Q2
IT
I_DC + I_PE
V_EE
Figure 39. Simplified Transmitter Structure
Output Level Programming and Output Structure
The output level of the transmitter of each lane is independently
programmable. In pin control mode, a default output amplitude
of 800 mV p-p diff (±400 mV diff) is delivered (see Table 17).
Register-based control allows the user to set the transmitter
output levels on a per-port or per-lane basis to four predefined
levels. Port-level programming overwrites lane-level configuration.
The ALEV, BLEV, and CLEV bits in Register 0x49, Register 0x89,
and Register 0xC9, respectively, are used to set the output levels
for all transmitters. The A[1:0]OLEV[1:0], B[1:0]OLEV[1:0],
and C[1:0]OLEV[1:0] bits in Register 0x4C, Register 0x8C, and
Register 0xCC allow per-lane settings (see Table 22).
Table 17. Predefined Output Levels
[A/B/C][1:0]OLEV[1] [A/B/C][1:0]OLEV[0]
0
0
0
1
1
0
1
1
Output Level
±200 mV diff
±300 mV diff
±400 mV diff
(default)
± 600 mV diff
Note that the choice of output level influences the output
common-mode level. A 600 mV diff output level with a full PE
range requires a supply and output termination voltage of 2.5 V
or higher (VTTO, VCC ≥ 2.5 V).
Preemphasis
Transmitter preemphasis levels can be set by pin control or
through the control registers. Pin control allows two settings of
PE, 0 dB and 6 dB. The control registers provide seven levels of
PE. Note that a larger range of boost settings is available for lower
output levels. Note that toggle pin control of PE is limited to the
400 mV diff output level settings. Table 18 lists the available
preemphasis settings for each output level.
Preemphasis can be programmed per port or per lane. Register
0x49, Register 0x89, and Register 0xC9 set all outputs in a port
at once. Registers 0x4A, 0x8A, and 0xCA allow setting PE on a
per-lane basis. The following equation sets preemphasis boost:
Gain[dB]
=
20
×
log₁₀ (1
+
V_SW
− PE ^{− V}SW
V_{SW − DC}
−
DC
)
(1)
Table 18. Setting Transmitter Preemphasis
Output Level Pin
(mV diff)
PE_[A/B/C]
Bit
PE[2:0]
PE
Boost
(%)
200
N/A
000
0
200
N/A
001
50
200
N/A
010
100
200
N/A
011
150
200
N/A
100
200
200
N/A
101
250
200
N/A
110
300
300
N/A
000
0
300
N/A
001
33
300
N/A
010
67
300
N/A
011
100
300
N/A
100
133
300
N/A
101
167
300
N/A
110
200
400
0
000
0
400
N/A
001
25
400
N/A
010
50
400
N/A
011
75
400
1
100
100
400
N/A
101
125
400
N/A
110
150
600
N/A
000
0
600
N/A
001
17
600
N/A
010
33
600
N/A
011
50
600
N/A
100
67
600
N/A
101
83
600
N/A
110
100
PE Boost
(dB)
0
3.52
6.02
7.96
9.54
10.88
12.04
0
2.5
4.44
6.02
7.36
8.52
9.54
0
1.94
3.52
4.86
6.02
7.04
7.96
0
1.34
2.5
3.52
4.44
5.26
6.02
Squelch and Disable
Each transmitter is equipped with disable and squelch controls.
Disable is a full power-down state: the transmitter current is
reduced to zero and the output pins pull up to VTTO, but there
is a delay of approximately 1 μs associated with reenabling
the transmitter. Squelch keeps the output current enabled such
that both output pins are at the output common-mode voltage.
The transmitter recovers from squelch in less than 64 ns.
Speed Select
The SEL4G pin lets the user lower the transmitter frequency of
maximum boost from 3.25 GHz to 2.0 GHz, allowing the
AD8155 to offer exceptional transmit channel compensation for
legacy applications (4.5 Gbps and slower). SEL4G = 1 lowers the
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