ADN2871ACPZ-RL7 Analog Devices Inc, ADN2871ACPZ-RL7 Datasheet - Page 17

• Manufacturer Part Number: ADN2871ACPZ-RL7
• Description: IC,Laser Diode/LED Driver,LLCC,24PIN,PLASTIC
• Manufacturer: Analog Devices Inc
• Type: Laser Diode Driver (Fiber Optic)r

Specifications of ADN2871ACPZ-RL7

• Data Rate: 4.25Gbps
• Number Of Channels: 1
• Voltage - Supply: 3 V ~ 3.6 V
• Current - Supply: 32mA
• 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

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
Figure 35 shows the recommended circuit for interfacing the
ADN2871 to most TO Can or coax lasers. DFB and FP 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 Multirate
Performance Using Low Cost Fabry Perot TOSA NEC NX7315UA
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 35 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 in comparison to frequency and enables multirate
operation. A series damping resistor should not be used.
Figure 35. Recommended Interface for ADN2871 AC Coupling
ADN2871
IMODP
IBIAS
V
V
CC
CC
R
24Ω
L (0.5nH)
Tx LINE
R
BLMI8HG60ISN1D
P
30Ω
Z
100nF
C
L
ADN2871
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED,
OPTIMIZATION SHOULD CONSIDER PARASITIC.
Tx LINE
30Ω
IMODN
IMODP
IBIAS
R
24Ω
C
2.2pF
V
CC
Figure 36. Recommended Differential Drive Circuit
L1 = 0.5nH
V
R1 = 15Ω
R2 = 15Ω
L2 = 0.5nH
CC
20Ω TRANMISSION LINES
C1 = C2 = 100nF
Rev. A | Page 17 of 20
V
CC
V
CC
R
Z
The 30 Ω transmission line used is a compromise between
drive current required and the total power consumed. Other
transmission line values can be used, with some modification of
the component values. In Figure 35, the R and C snubber
values, 24 Ω and 2.2 pF respectively, represent a starting point
and must be tuned for the particular model of laser being used.
R
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. Note that the circuit in Figure 35
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 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. The ADN2871 single-ended application shown
in Figure 35 is recommended for use up to 2.7 Gbps. From 2.7
Gbps to 4.25 Gbps, a differential drive is recommended when
driving VCSELs or lasers that have slow fall times. Differential
drive can be implemented by adding a few extra components. A
possible implementation is shown in Figure 36. The bias and
modulation currents that are programmed into the ADN2871
need to be larger that the bias and modulation current required
at the laser due to the laser ac coupling interface and because
some modulation current flows in pull-up Resistors R1 and R2.
Figure 35 and Figure 36, Resistor R
optimum eye quality. The recommended R
approximately 200 Ω ~ 500 Ω.
R3
P
, the pull-up resistor, is in series with a very small (0.5 nH)
L4 = BLM18HG601SN1
L3 = 4.7nH
C3
SNUBBER
L6 = BLM18HG601SN1D
TOCAN/VCSEL
LIGHT
Z
is required to achieve
Z
value is
ADN2871