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ADN2848 Datasheet(PDF) 5 Page - Analog Devices

Part No. ADN2848
Description  3 V Dual-Loop 50 Mbps to 1.25 Gbps Laser Diode Driver
Download  12 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADN2848 Datasheet(HTML) 5 Page - Analog Devices

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REV. 0
ADN2848
–5–
GENERAL
Laser diodes have current-in to light-out transfer functions as
shown in Figure 2. Two key characteristics of this transfer func-
tion are the threshold current, ITH, and slope in the linear region
beyond the threshold current, referred to as slope efficiency, LI.
ER =
PAV =
P1
P0
P1 + P0
2
P
I
LI =
P
I
ITH
CURRENT
P1
PAV
P0
Figure 2. Laser Transfer Function
Control
A monitor photodiode, MPD, is required to control the LD.
The MPD current is fed into the ADN2848 to control the power
and extinction ratio, continuously adjusting the bias current and
modulation current in response to the laser’s changing threshold
current and light-to-current slope efficiency.
The ADN2848 uses automatic power control, APC, to maintain
a constant average power over time and temperature.
The ADN2848 uses closed-loop extinction ratio control to
allow optimum setting of extinction ratio for every device. Thus
SONET/SDH interface standards can be met over device
variation, temperature, and laser aging. Closed-loop modulation
control eliminates the need to either overmodulate the LD or
include external components for temperature compensation.
This reduces research and development time and second
sourcing issues caused by characterizing LDs.
Average power and extinction ratio are set using the PSET and
ERSET pins, respectively. Potentiometers are connected
between these pins and ground. The potentiometer RPSET is
used to change the average power. The potentiometer RERSET is
used to adjust the extinction ratio. Both PSET and ERSET are
kept 1.2 V above GND.
For an initial setup, RPSET and RERSET potentiometers may be
calculated using the following formulas.
R
V
I
PSET
AV
=
12
.
()
R
V
I
P
ER
ER
P
ERSET
MPD
CW
CW
AV
12
1
1
.
()
_
×
+
×
where:
I
AV is the average MPD current.
P
CW is the dc optical power specified on the laser data sheet.
I
MPD_CW is the MPD current at that specified PCW.
P
AV is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
Note that I
ERSET and IPSET will change from device to device;
however, the control loops will determine the actual values.
It is not required to know the exact values for LI or MPD
optical coupling.
Loop Bandwidth Selection
For continuous operation, the user should hardwire the LBWSET
pin high and use 1
µF capacitors to set the actual loop band-
width. These capacitors are placed between the PAVCAP and
ERCAP pins and ground. It is important that these capaci-
tors are low leakage multilayer ceramics with an insulation
resistance greater than 100 G
Ω or a time constant of 1,000 sec,
whichever is less.
Operation
Recommended
Recommended
Mode
LBWSET
PAVCAP
ERCAP
Continuous
High
1
µF1 µF
50 Mbps to
1.25 Gbps
Optimized
Low
47 nF
47 nF
for 1.25 Gbps
Setting LBSET low and using 47 nF capacitors results in a
shorter loop time constant (a 10
× reduction over using 1 µF
capacitors and keeping LBWSET high).
Alarms
The ADN2848 is designed to allow interface compliance to
ITU-T-G958 (11/94) section 10.3.1.1.2 (transmitter fail) and
section 10.3.1.1.3 (transmitter degrade). The ADN2848 has two
active high alarms, DEGRADE and FAIL. A resistor between
ground and the ASET pin is used to set the current at which these
alarms are raised. The current through the ASET resistor is a ratio
of 100:1 to the FAIL alarm threshold. The DEGRADE alarm will
be raised at 90% of this level.
Example:
ImA so I
mA
I
ImA
A
R
V
IA
k
FAIL
DEGRADE
ASET
FAIL
ASET
ASET
==
==
=
==
=
50
45
100
50
100
500
12
12
500
24
..
.
*
*The smallest valid value for RASET is 1.2 k
Ω, since this corresponds to the I
BIAS
maximum of 100
A.
The laser degrade alarm, DEGRADE, is provided to give a
warning of imminent laser failure if the laser diode degrades
further or environmental conditions continue to stress the LD,
such as increasing temperature.
The laser fail alarm, FAIL, is activated when the transmitter can
no longer be guaranteed to be SONET/SDH compliant. This
occurs when one of the following conditions arise:
• The ASET threshold is reached.
• The ALS pin is set high. This shuts off the modulation and
bias currents to the LD, resulting in the MPD current drop-
ping to zero. This gives closed-loop feedback to the system
that ALS has been enabled.
DEGRADE will be raised only when the bias current exceeds
90% of ASET current.


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