REV. 0

ADN2843

–7–

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

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.

ALARM INTERFACE

The alarm voltages are open collector outputs. An internal

pull-up resistor of 30k

that is used to pull the logic high

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

DATA INPUTS

Figure 2 shows a simplified schematic of the ADN2845 data

inputs. The data inputs are terminated via the equivalent of a

100

internal resistor between DATAN and DATAP. This

provides 50

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.

25

25

25

25

2k

INTERNAL

REFERENCE

ADN2845

DATAN

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-

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

MPD CURRENT

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-

tor photodiode cathode should be connected directly to the PSET

MPD currents as high as 3 mA can be used in this configuration.

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.

DUAL MPD DWDM FUNCTION

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.

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

current is set by an external current sink connected to the

IDTONE pin. There is a gain of 2 between the IDTONE pin

input current can be derived from the IMMON output current.

If the IDTONE function is not being used, this pin must be tied

Note that using IDTONE during transmission may cause optical

eye degradation.

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

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.

Note that for correct ALS operation, the ALS pin on the

ADN2845 and ADN2844 should be connected and termi-

nated with a 10 k

W resistor. The ADN2843 ALS should be

driven with correct logic levels (see Specifications section). ALS

should never be left floating.

POWER DISSIPATION

The power dissipation of the ADN2845 can be calculated using

the following expressions:

ImA

I

mA

ImA

PV

I

A

V

I

A

V

I

A

CC

MOD

BIAS

CC

CC

IMOD

MOD

IBIAS

BIAS

=+

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75

1 75

0 3

2

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