AD8155ACPZ-R7 datasheet(21/36 Pages) AD

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AD8155

Rev. 0 | Page 21 of 36

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).

VCC

VTTO

OP_xx

ON_xx

VEE

RTERM

IDC + IPE

ON-CHIP TERMINATION

ESD

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).

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

Table 17. Predefined Output Levels

[A/B/C][1:0]OLEV[1]

[A/B/C][1:0]OLEV[0]

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:

)

(

log

]

[

Gain

−

(1)

Table 18. Setting Transmitter Preemphasis

Output Level

(mV diff)

PE_[A/B/C]

Bit

PE[2:0]

Boost

(%)

PE Boost

(dB)

200

N/A

000

200

N/A

001

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

300

N/A

001

2.5

300

N/A

010

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

000

400

N/A

001

1.94

400

N/A

010

3.52

400

N/A

011

4.86

400

100

6.02

400

N/A

101

125

7.04

400

N/A

110

150

7.96

600

N/A

000

600

N/A

001

1.34

600

N/A

010

2.5

600

N/A

011

3.52

600

N/A

100

4.44

600

N/A

101

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 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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