Datasheet

AD8158
Rev. B | Page 22 of 36
TRANSMITTERS
The AD8158 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 AD8158 to offer exceptional transmit channel
compensation for legacy applications (4.5 Gbps and slower).
V
CC
V
TTO
OP_xx
ON_xx
V
EE
Q1
Q2
RP
R
TERM
RN
R
TERM
V3
VC
V1
VN
V2
VP
IT
I
DC
+ I
PE
ON-CHIP TERMINATION ESD
06646-139
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[3:0]OLEV[1:0], B[3:0]OLEV[1:0],
and C[3:0]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][3:0]OLEV[1] [A/B/C][3:0]OLEV[0] Output Level
0 0 ±200 mV diff
0 1 ±300 mV diff
1 0
±400 mV diff
(default)
1 1 ± 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 (V
TTO
, V
CC
≥ 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 1 8 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:
)1(log20][
10
DCSW
DCSWPESW
V
VV
dBGain
−
−−
−
+×=
(1)
Table 18. Setting Transmitter Preemphasis
Output Level
(mV diff)
Pin
PE_[A/B/C]
Bit
PE[2:0]
PE Boost
(%)
PE Boost
(dB)
200 N/A 000 0 0
200 N/A 001 50 3.52
200 N/A 010 100 6.02
200 N/A 011 150 7.96
200 N/A 100 200 9.54
200 N/A 101 250 10.88
200 N/A 110 300 12.04
300 N/A 000 0 0
300 N/A 001 33 2.5
300 N/A 010 67 4.44
300 N/A 011 100 6.02
300 N/A 100 133 7.36
300 N/A 101 167 8.52
300 N/A 110 200 9.54
400 0 000 0 0
400 N/A 001 25 1.94
400 N/A 010 50 3.52
400 N/A 011 75 4.86
400 1 100 100 6.02
400 N/A 101 125 7.04
400 N/A 110 150 7.96
600 N/A 000 0 0
600 N/A 001 17 1.34
600 N/A 010 33 2.5
600 N/A 011 50 3.52
600 N/A 100 67 4.44
600 N/A 101 83 5.26
600 N/A 110 100 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 V
TTO
, 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
AD8158 to offer exceptional transmit channel compensation for
legacy applications (4.5 Gbps and slower). SEL4G = 1 lowers the