ADN2872ACPZ-R7 datasheet(15/20 Pages) AD

15 / 20 page

ADN2872

Rev. 0 | Page 15 of 20

DATA INPUTS

Data inputs should be ac-coupled (10 nF capacitors are

recommended) and are terminated via a 100 Ω internal resistor

between the DATAP and DATAN pins. A high impedance

circuit sets the common-mode voltage and is designed to allow

maximum input voltage headroom over temperature. It is

necessary to use ac coupling to eliminate the need for matching

between common-mode voltages.

LASER DIODE INTERFACING

The schematic in Figure 32 describes the recommended circuit

for interfacing the ADN2872 to most TO-Can or coax lasers.

These lasers typically have impedances of 5 Ω to 7 Ω and have

axial leads. The circuit shown works over the full range of data

rates from 155 Mbps to 3.3 Gbps including multirate operation

(with no change to PAVCAP and ERCAP values); see Figure 18,

Figure 19, and Figure 20 in the Typical Performance Characteristics

section for multirate performance examples. Coax lasers have

special characteristics that make them difficult to interface to.

They tend to have higher inductance, and their impedance is

not well controlled. The circuit in Figure 32 operates by delibe-

rately misterminating the transmission line on the laser side, while

providing a very high quality matching network on the driver

side. The impedance of the driver side matching network is very

flat vs. frequency and enables multirate operation. A series

damping resistor should not be used.

BLM18HG601SN1D

100nF

Ω

ADN2872

IBIAS

CCBIAS

IMODP

VCC

L (0.5nH)

Ω

2.2pF

Tx LINE

Ω

Tx LINE

Ω

VCC

Figure 32. Recommended Interface for ADN2872 AC Coupling

The 30 Ω transmission line used is a compromise between drive

current required and total power consumed. Other transmission

line values can be used, with some modification of the compo-

nent values. The R and C snubber values in Figure 32, 24 Ω and

2.2 pF, respectively, represent a starting point and must be

tuned for the particular model of laser being used. RP, the pull-

up resistor, is in series with a very small (0.5 nH) inductor. In

some cases, an inductor is not required or can be accommodated

with deliberate parasitic inductance, such as a thin trace or a via

placed on the PC board.

Take care to mount the laser as close as possible to the PC

board, minimizing the exposed lead length between the laser

can and the edge of the board. The axial lead of a coax laser is

very inductive (approximately 1 nH per millimeter). Long

exposed leads result in slower edge rates and reduced eye margin.

Recommended component layouts and gerber files are available

by contacting sales at Analog Devices. Note that the circuit in

Figure 32 can supply up to 56 mA of modulation current to the

laser, sufficient for most lasers available today. Higher currents

can be accommodated by changing transmission lines and

backmatch values. This interface circuit is not recommended

for butterfly-style lasers or other lasers with 25 Ω characteristic

impedance. Instead, a 25 Ω transmission line and inductive

(instead of resistive) pull-up is recommended. Contact sales for

recommendations on transmission lines and backmatch values.

The ADN2872 also supports differential drive schemes. These

can be particularly useful when driving VCSELs or other lasers

with slow fall times. Differential drive can be implemented by

adding a few extra components. A possible implementation is

shown in Figure 33.

In Figure 32 and Figure 33, Resistor RZ is required to achieve

optimum eye quality. The recommended value is approximately

200 Ω ~ 500 Ω.

L3 = 4.7nH

L4 = BLM18HG601SN1D

VCC

L6 = BLM18HG601SN1D

SNUBBER SETTINGS: 40

Ω AND 1.5pF, NOT OPTIMIZED, OPTIMIZATION SHOULD

CONSIDER THE PARASITIC OF THE INTERFACE CIRCUITRY.

L5 = 4.7nH

VCC

R1 = 15

Ω

CCBIAS IBIAS

IMODN

IMODP

ADN2872

C1 = C2 = 100nF

Ω TRANMISSION LINES

R2 = 15

Ω

(12

Ω TO 24Ω)

SNUBBER

LIGHT

TO-CAN/VCSEL

L1 = 0.5nH

L2 = 0.5nH

VCC

Figure 33. Recommended Differential Drive Circuit

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