ADN2841
GENERAL
LOOP BANDWIDTH SELECTION
Laser diodes have current-in to light-out transfer functions as shown
in Figure 2. Two key characteristics of this transfer function are the
threshold current, ITH, and the slope in the linear region beyond
the threshold current, referred to as slope efficiency, LI.
For anyrate 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 capacitors
be low leakage multilayer ceramics with an insulation resistance
greater than 100 GΩ or a time constant of 1000 sec, whichever
is less. The ADN2841 may be optimized for 2.7 Gbps operation
by keeping the LBWSET pin low. This results in a much shorter
loop time constant (a 10ϫ reduction). The value of PAVCAP
and ERCAP capacitors required for 2.5 Gbps operation is 22 nF.
P1
P0
ER =
P1 + P0
2
P
=
AV
P1
ꢃP
P
AV
ꢃP
ꢃI
LI =
ꢃI
ALARMS
The ADN2841 alarms are 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 ADN2841 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.
P0
I
CURRENT
TH
Figure 2. Laser Transfer Function
CONTROL
A monitor photodiode (MPD) is required to control the LD. The
MPD current is fed into the ADN2841 to control the optical
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 (LI) slope (slope efficiency).
Example:
IFAIL = 50 mA∴ IDEGRADE = 45 mA
IBIASTRIP 50 mA
IASET
=
=
= 500 µA
= 2.46 kΩ
The ADN2841 uses automatic power control (APC) to main-
tain a constant power over time and temperature.
100
100
1.23V
IASET
1.23V
500 µA
The ADN2841 uses closed-loop extinction ratio control to allow
optimum setting of extinction ratio for every device. Therefore,
SONET/SDH interface standards can be met over device varia-
tion, temperature, and time. 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.
RASET
=
=
NOTE: The smallest value for RASET is 1.2 kΩ, as this corre-
sponds to the IBIAS maximum of 100 mA.
The laser degrade alarm, DEGRADE, gives a warning of imminent
laser failure if the laser diode degrades further or environmental
conditions, e.g., increasing temperature, continue to stress the LD.
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 1.23 V above GND.
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 arises:
•
•
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
dropping to zero. This gives closed-loop feedback to the
system in which ALS has been enabled.
RPSET and RERSET can be calculated using the following formulas:
1.23V
IAV
RPSET
=
DEGRADE will only be raised when the bias current exceeds
90% of ASET current.
where IAV is the average MPD current.
1.23V
ER −1
ER +1
MONITOR CURRENTS
RERSET
=
IMPD
PCW
IBMON, IMMON, IMPDMON, and IMPDMON2 are
current controlled current sources from VCC. They mirror the
bias, modulation, and MPD current for increased monitoring
functionality. An external resistor to GND gives a voltage
proportional to the current monitored.
CW
–
×
× 0.2 × PAV
where PCW is the dc optical power specified on the laser data
sheet, IMPD_CW is the MPD current at that specified PCW, and
AV is the required average power.
P
Note that IERSET and IPSET will change from device to device.
However, the control loops will determine actual values. It is not
required to know the exact values for LI or MPD optical coupling.
DUAL MPD DWDM FUNCTION (48-LEAD LFCSP ONLY)
The ADN2841 has circuitry for an optional second monitor
photodiode, MPD2.
–6–
REV. A
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