Datasheet

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.
08013-014
L
BLM18HG601SN1D
C1
100nF
R
P
24Ω
ADN2872
IBIAS
CCBIAS
IMODP
V
CC
L (0.5nH)
R
24Ω
C
2.2pF
Tx LINE
30Ω
Tx LINE
30Ω
V
CC
V
CC
R
Z
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. R
P
, 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 R
Z
is required to achieve
optimum eye quality. The recommended value is approximately
200 Ω ~ 500 Ω.
08013-041
L3 = 4.7nH
L4 = BLM18HG601SN1D
V
CC
L6 = BLM18HG601SN1D
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED, OPTIMIZATION SHOULD
CONSIDER THE PARASITIC OF THE INTERFACE CIRCUITRY.
L5 = 4.7nH
V
CC
R1 = 15Ω
CCBIAS IBIAS
IMODN
IMODP
ADN2872
C1 = C2 = 100nF
20Ω TRANMISSION LINES
R2 = 15Ω
(12Ω TO 24Ω)
R3
C3
SNUBBER
LIGHT
TO-CAN/VCSEL
L1 = 0.5nH
L2 = 0.5nH
V
CC
R
Z
Figure 33. Recommended Differential Drive Circuit