ADN2870 Analog Devices, Inc., ADN2870 Datasheet - Page 16

• Manufacturer Part Number: ADN2870
• Description: 3.3 V Dual-loop, 50 Mbps To 3.3 Gbps Laser Diode Driver
• Manufacturer: Analog Devices, Inc.

ADN2870
LASER DIODE INTERFACING
The schematic in Figure 32 describes the recommended circuit
for interfacing the ADN2870 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 the
Typical Operating Characteristics 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 deliberately 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 versus frequency
and enables multirate operation. A series damping resistor
should not be used.
Figure 32. Recommended Interface for ADN2870 AC Coupling
ADN2870
CCBIAS
IMODP
IBIAS
VCC
L (0.5nH)
R
24Ω
Tx LINE
BLMI8HG60ISN1D
P
30Ω
CCBIAS IBIAS
100nF
ADN2870
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED,
OPTIMIZATION SHOULD CONSIDER PARASITIC.
C
L
Tx LINE
30Ω
IMODN
IMODP
R 24Ω
C 2.2pF
V
CC
L1 = 0.5nH
Figure 33. Recommended Differential Drive Circuit
R1 = 15Ω
R1 = 15Ω
(12 TO 24Ω)
L2 = 0.5nH
VCC
20Ω TRANMISSION LINES
C1 = C2 = 100nF
Rev. 0 | Page 16 of 20
V
CC
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
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.
Care should be taken 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 are very inductive (approximately 1 nH per mm). Long
exposed leads result in slower edge rates and reduced eye margin.
Recommended component layouts and gerber files are available
by contacting the factory. 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 accom-
modated by changing transmission lines and backmatch values;
contact factory for recommendations. 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 the factory for recommendations.
The ADN2870 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.
R3
L4 = BLM18HG601SN1
L3 = 4.7nH
C3
SNUBBER
L5 = 4.7nH
L6 = BLM18HG601SN1
TOCAN/VCSEL
LIGHT
P
, the pull-up