ADN2873ACPZ-RL7 AD [Analog Devices], ADN2873ACPZ-RL7 Datasheet - Page 17

• Manufacturer Part Number: ADN2873ACPZ-RL7
• Description: 3.3 V, 50 Mbps to 4.25 Gbps, Single-Loop, Laser Diode Driver
• Manufacturer: AD [Analog Devices]

DATA INPUTS
Data inputs should be ac-coupled (10 nF capacitors are recom-
mended) 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 the common-mode
voltages of the data source and the ADN2873 data input pins.
LASER DIODE INTERFACING
Figure 35 shows the recommended circuit for interfacing the
ADN2873 to most TO-can or coax lasers. Uncooled 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 (without
changes to PAVCAP and ERCAP values); see Figure 23, Figure 24,
and Figure 25 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 at the laser side while provid-
ing a very high quality matching network at the driver side.
The impedance of the driver side matching network is very flat
in the designed frequency range and enables multirate operation.
A series damping resistor should not be used.
The 30 Ω transmission line used is a compromise between the
drive current required and the total power consumed. Other
Figure 35. Recommended Interface for ADN2873 AC Coupling
ADN2873
IMODP
IBIAS
V
V
CC
CC
L (0.5nH)
R
24Ω
Tx LINE
R
BLM18HG601SN1D
P
30Ω
Z
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED, OPTIMIZATION SHOULD
CONSIDER THE PARASITIC OF THE INTERFACE CIRCUITRY.
100nF
C
ADN2873
L
Tx LINE
30Ω
IMODN
IMODP
IBIAS
R
24Ω
C
2.2pF
V
CC
L1 = 0.5nH
R1 = 15Ω
R2 = 15Ω
Figure 36. Recommended Differential Drive Circuit
L2 = 0.5nH
V
CC
20Ω TRANSMISSION LINES
C1 = C2 = 100nF
Rev. 0 | Page 17 of 20
V
CC
V
CC
R
Z
R3
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
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 accommo-
dated 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 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 back-
match 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 ADN2873 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 ADN2873 need to be larger than the bias
and modulation current required at the laser due to the laser ac
coupling interface and because some modulation current flows
in the pull-up resistors, R1 and R2.
In Figure 35 and Figure 36, Resistor RZ is required to achieve
optimum eye quality. The recommended R
500 Ω ~ 800 Ω.
The interface circuit needs a special modification to support
HDTV pathological test patterns. Contact sales at Analog Devices
for HDTV support.
L4 = BLM18HG601SN1D
L3 = 4.7nH
C3
SNUBBER
L6 = BLM18HG601SN1D
TO-CAN/VCSEL
LIGHT
Z
value is approximately
ADN2873
P
, the