ADN2870ACPZ Analog Devices Inc, ADN2870ACPZ Datasheet - Page 16

• Manufacturer Part Number: ADN2870ACPZ
• Description: IC,Laser Diode/LED Driver,LLCC,24PIN,PLASTIC
• Manufacturer: Analog Devices Inc
• Type: Laser Diode Driver (Fiber Optic)r

Specifications of ADN2870ACPZ

• Data Rate: 3.3Gbps
• Number Of Channels: 1
• Voltage - Supply: 3 V ~ 3.6 V
• Current - Supply: 30mA
• Current - Modulation: 90mA
• Current - Bias: 100mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 24-VFQFN, CSP Exposed Pad
• Mounting Type: Surface Mount
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 Performance 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 vs. frequency and enables multi-
rate operation. A series damping resistor should not be used.
Figure 32. Recommended Interface for ADN2870 AC Coupling
ADN2870
CCBIAS
IMODP
IBIAS
V
V
CC
CC
L (0.5nH)
R
24Ω
Tx LINE
R
BLM18HG601SN1D
P
Z
30Ω
CCBIAS IBIAS
100nF
ADN2870
C
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED,
OPTIMIZATION SHOULD CONSIDER PARASITIC.
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
V
CC
20Ω TRANMISSION LINES
C1 = C2 = 100nF
Rev. A | Page 16 of 20
V
V
CC
CC
R
Z
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
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.
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 is 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 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; contact sales at Analog Devices for recom-
mendations. 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.
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.
In the circuits shown in Figure 32 and Figure 33, Resistor R
required to achieve optimum eye quality. The recommended
value is approximately 200 Ω ~ 500 Ω.
R3
L4 = BLM18HG601SN1D
L3 = 4.7nH
C3
SNUBBER
L5 = 4.7nH
L6 = BLM18HG601SN1D
TO-CAN/VCSEL
LIGHT
P
, the pull-
Z
is