ADN2848

Data Sheet

Rev. B | Page 8 of 12

Example:

mA

45

mA

50

=

=

DEGRADE

FAIL

I

so

I

μA

500

100

mA

50

100

=

=

ASET

I

I

k

Ω

4

.

2

μA

500

2

.

1

V

2

.

1

*

=

=

=

ASET

ASET

I

R

The laser degrade alarm, DEGRADE, is provided to give a

warning of imminent laser failure if the laser diode degrades

further or if environmental conditions such as increasing

temperature continue to stress the LD.

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

ASET current.

MONITOR CURRENTS

IBMON, IMMON, and IMPDMON are current controlled

and MPD current for increased monitoring functionality. An

external resistor to GND gives a voltage proportional to the

current monitored.

If the monitoring function IMPDMON is not required, the

IMPD pin must be grounded and the monitor photodiode

output must be connected directly to the PSET pin.

DATA AND CLOCK INPUTS

Data and clock inputs are ac-coupled (10 nF capacitors

recommended) and terminated via a 100 Ω internal resistor

between DATAP and DATAN and also between the CLKP and

CLKN pins. There is a high impedance circuit to set the

common-mode voltage, which is designed to allow for

maximum input voltage headroom over temperature. It is

necessary that ac coupling be used to eliminate the need for

matching between common-mode voltages.

ADN2848

TO FLIP-FLOPS

50Ω

50Ω

R

R = 2.5kΩ, DATA

R = 3kΩ, CLK

400µA TYP

DATAP

DATAN

Figure 5. AC Coupling of Data Inputs

For input signals that exceed 500 mV p-p single-ended, it is

necessary to insert an attenuation circuit as shown in Figure 6.

DATAP/CLKP

DATAN/CLKN

R3

R1

R2

ADN2848

INPUT IMPEDANCE OF THE ADN2848.

Figure 6. Attenuation Circuit

CCBIAS

When the laser is used in ac-coupled mode, the CCBIAS pin

shown in Figure 8 and Figure 9, is required. The recommended

AUTOMATIC LASER SHUTDOWN

The ADN2848 ALS allows compliance to ITU-T-G958 (11/94),

section 9.7. When ALS is logic high, both the bias and the

modulation currents are turned off. Correct operation of ALS is

confirmed by the FAIL alarm being raised when ALS is

asserted. Note that this is the only time that DEGRADE is low

while FAIL is high.

ALARM INTERFACES

The FAIL and DEGRADE outputs have an internal 30 kΩ pull-

However, the alarm output can be overdriven with an external

ADN2848.