ADN2843
GENERAL
force the PSET and ERSET pins to 1.23 V above GND. For
initial setup, RPSET and RERSET may be calculated using the
following formulas:
Laser diodes have current-in to light-out transfer functions as
shown in Figure 1. Two key characteristics of this transfer function
are the threshold current, ITH, and slope in the linear region
beyond the threshold current, referred to as the slope efficiency, LI.
The PSET resistor is given by the following formulas:
1.23 V
RPSET
=
W
(
)
IAV
where IAV is average MPD current.
ER = P1
P0
P
= P1 + P0
2
AV
P1
P
The value of the ERSET resistor is a function of the operation
mode of the ADN2843 as follows:
ꢃP
LI =
For Mode A:
ꢃP
ꢃI
AV
ER + 1
ER – 1
RERSET = RPSET
¥
ꢃI
For Mode B:
P0
RPSET ER + 1
RERSET
=
¥
I
CURRENT
TH
2
ER – 1
Figure 1. Laser Transfer Function
Note that IERSET and IPSET will change from laser diode to laser
diode, therefore RERSET and RPSET need to be adjusted for each
laser diode. When tuning the laser diode, RPSET should be
CONTROL
A monitor photodiode, MPD, is required to control the LD. The
MPD current is fed into the ADN2843 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.
adjusted first with RERSET at 25 kꢀ. Once the average power is
set, RERSET is adjusted to set the desired extinction ratio, and
RPSET is again adjusted to re-establish the desired average power.
Once the values RPSET and RERSET have been adjusted to set the
desired average power and extinction ratio, the control loops
maintain these values of average power and extinction ratio over
environmental conditions and time.
The ADN2843 uses automatic power control, APC, to maintain
a constant average power over time and temperature.
The ADN2843 uses closed-loop extinction ratio control to allow
optimum setting of the 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, thus reducing
research and development time and second sourcing issues.
PAVCAP AND ERCAP
The control loop constants are set by the PAVCAP and ERCAP
capacitors. The required value for the PAVCAP and ERCAP
capacitors is 22 nF.
The PAVCAP and ERCAP capacitors are connected between
the respective pins and GND. The capacitors should be low
leakage multilayer ceramic capacitors with an insulation resistance
The ADN2843 dual-loop control has two modes of operation.
Each mode is given by the configuration of the MODE and
D_IMOD pins as shown below.
>100 Gꢀ or an RC >1000 s, whichever is lowest.
ALARMS
The ADN2843 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 ADN2843
has two alarms, DEGRADE and FAIL. These alarms are raised
when IBIAS exceeds the respective DEGRADE and FAIL thresh-
olds. These alarms are active high. 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 1:100 to the FAIL alarm threshold. The DEGRADE
alarm will be raised at 90% of the FAIL threshold.
Operation
Mode
MODE
Pin Setting
D_IMOD
Pin Connected to
A
B
HIGH
LOW
IBIAS
IBIAS_CTRL
Configuring the ADN2843 in Mode A or Mode B (see Figures 3
and 4) enables users to achieve accurate control of the extinc-
tion ratio. Mode B is suitable for applications where an IBIAS
pin is not available to the TOSA, or where there is no space
on the TOSA for an IBIAS inductor. Experimental data and
simulation for typical lasers has shown ER to be 0.3 dB to 0.5 dB
better in Mode A, at a 5 dB extinction ratio. Care should be
taken to ensure that the extra capacitance on the IBIAS pin
due to the D_IMOD connection does not degrade the eye
quality. When physical constraints do not allow a low capaci-
tance interconnect between D_IMOD and IBIAS, the ADN2843
should be configured in Mode B (see Figure 4).
Example:
IFAIL = 50 mA so IDEGRADE = 45 mA
IFAIL 50 mA
IASET
=
=
= 500 ꢁA
100
100
1.23 V
1.23 V
IASET
RASET
=
=
= 2.46 kW
500 ꢁA
Average power and extinction ratio for both modes are set using
the PSET and ERSET pins, respectively. Potentiometers are
connected between these pins and ground. The potentiometer
The laser degrade alarm, DEGRADE, is provided to give a warn-
ing of imminent laser failure if the laser diode degrades further or
if environmental conditions continue to stress the LD, such as
increasing temperature.
RPSET is used to set the average power. The potentiometer RERSET
is used to set the extinction ratio. The internal control loops
–6–
REV. 0
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