ADN2873(RevB) Просмотр технического описания (PDF) - Analog Devices
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ADN2873 Datasheet PDF : 20 Pages
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Data Sheet
ADN2873
DATA INPUTS
Data inputs must 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 must not be used.
VCC
IMODP
ADN2873
IBIAS
L (0.5nH)
RP
C
24Ω 100nF
Tx LINE
30Ω
VCC
Tx LINE
30Ω
RZ
L
BLM18HG601SN1D
VCC
R
24Ω
C
2.2pF
Figure 35. Recommended Interface for ADN2873 AC Coupling
The 30 Ω transmission line used is a compromise between the
drive current required and the total power consumed.
VCC
L1 = 0.5nH
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. RP, 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.
Care must 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 RZ value is approximately
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
IMODN
ADN2873
IMODP
IBIAS
R1 = 15Ω C1 = C2 = 100nF
20Ω TRANSMISSION LINES R3
R2 = 15Ω
L3 = 4.7nH TO-CAN/VCSEL
C3
SNUBBER
LIGHT
L2 = 0.5nH
VCC
VCC
RZ
L6 = BLM18HG601SN1D
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED, OPTIMIZATION SHOULD
CONSIDER THE PARASITIC OF THE INTERFACE CIRCUITRY.
Figure 36. Recommended Differential Drive Circuit
Rev. B | Page 17 of 20
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