ADN2843
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:
tor photodiode cathode should be connected directly to the PSET
node, and IMPDMON and IMPDMON2 should be tied to VCC
.
MPD currents as high as 3 mA can be used in this configuration.
∑ The ASET threshold is reached.
Another way to increase the MPD current range without sacri-
ficing the monitoring function is to use IMPD and IMPD2 in
parallel. This effectively doubles the current range but raises the
lower MPD current specification from 50 ꢀA to 100 ꢀA. If this
configuration is used, the IMPDMON and IMPDMON2 pins
should be tied together and terminated with a single resistor.
The mirror ratio of 1 is maintained in this configuration.
∑ 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 that ALS has been enabled.
DEGRADE is raised only when the bias current exceeds
90% of the alarm threshold.
DUAL MPD DWDM FUNCTION
ALARM INTERFACE
The MPD function mirrors the current in MPD to the PSET pin
and to the IMPDMON pin with a ratio of 1. A second monitor
photodiode can be connected to the IMPD2 pin. Its current is
mirrored to IMPDMON2 and also to the PSET pin, where it is
summed with the current mirrored from IMPD. The two MPD
monitor currents can be used as inputs to a DWDM wavelength
control function when used in combination with various optical
filtering techniques. If the IMPD monitor function is not required,
the monitor photodiode can be directly connected to the PSET
pin, and the IMPD pin must be tied to GND. If the IMPD2 pin
is not being used, it should be tied to GND.
The alarm voltages are open collector outputs. An internal
pull-up resistor of 30kꢁ that is used to pull the logic high
value to VCC. However, this can be overdriven with an external
resistor, allowing alarm interfacing to non-VCC levels. The
FAIL output may not be connected directly to the ALS pin to
shut down the bias and modulation currents. It can however
be latched using a flip-flop, and the output of the flip-flop can
then be used to activate ALS. Non-VCC alarm output levels
must be below the VCC used for the ADN2843.
DATA INPUTS
Figure 2 shows a simplified schematic of the ADN2845 data
inputs. The data inputs are terminated via the equivalent of a
IDTONE
The IDTONE pin is supplied for fiber identification/supervisory
channels or for control purposes. This pin modulates the optical
one level by adding a current to IMOD over a possible range of
2% of minimum IMOD to 10% of maximum IMOD. The IDTONE
current is set by an external current sink connected to the
IDTONE pin. There is a gain of 2 between the IDTONE pin
and the IMOD current. To ratio the IDTONE current to IMOD, the
input current can be derived from the IMMON output current.
100
provides 50
ꢁ
internal resistor between DATAN and DATAP. This
termination for single-ended signals. The actual
ꢁ
signal on the switching devices is attenuated by a factor of 2
internally. There is a high impedance circuit to set the common-
mode voltage, which is designed to change over temperature. It
is recommended that ac coupling be used to eliminate the need
for matching between the common-mode voltages.
If the IDTONE function is not being used, this pin must be tied
to VCC to properly disable it.
ADN2845
25ꢁ
DATAN
Note that using IDTONE during transmission may cause optical
eye degradation.
25ꢁ
25ꢁ
2kꢁ
INTERNAL
REFERENCE
AUTOMATIC LASER SHUTDOWN (ALS)
The ADN2843 ALS allows compliance to ITU-T-G958 (11/94),
Section 9.7. When ALS is asserted, both bias and modulation
currents are turned off. In ALS mode, current is sourced to the
laser from the IBIAS pin, which reverse biases the laser and ensures
that it is turned off. Correct operation of ALS can be confirmed
by the FAIL alarm being raised when ALS is asserted. Note this
is the only time that DEGRADE will be low while FAIL is high.
25ꢁ
DATAP
Figure 2. Simplified Schematic of Data Inputs
MONITOR CURRENTS
IBMON, IMMON mirror the bias, modulation current at a ratio
of 1:100 for increased monitoring functionality. IMPDMON and
IMPDMON2 mirror the current in IMPD and IMPD2, respec-
Note that for correct ALS operation, the ALS pin on the
ADN2845 and ADN2844 should be connected and termi-
nated with a 10 kW resistor. The ADN2843 ALS should be
driven with correct logic levels (see Specifications section). ALS
should never be left floating.
tively, with a ratio of 1. All monitors source current from VCC
.
If the MPD monitoring function is not required, then the IMPD
pin should be tied to ground and the monitor photodiode cathode
should be connected directly to the PSET pin. When the MPD
monitor functions are not used, IMPDMON and IMPDMON2
POWER DISSIPATION
The power dissipation of the ADN2845 can be calculated using
the following expressions:
should be tied to VCC
.
ICC = 75 mA +1.75¥ IMOD mA + 0.3¥ I
mA
(
)
(
)
BIAS
MPD CURRENT
P =VCC ¥ ICC A +V
¥ IMOD A / 2 +V
¥ IBIAS
A
( )
The maximum average MPD current is specified in the specifica-
tions section. This maximum current specified is limited by the
MPD monitoring circuitry. If the monitoring function is not
required, then IMPD and IMPD2 should be grounded, the moni-
( )
( )
IMOD
IBIAS
where VIMOD is the average voltage on the IMOD pin, and
V
IBIAS is the average voltage on the IBIAS pin.
REV. 0
–7–
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