ADN2847

Data Sheet

Rev. B | Page 10 of 16

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

warning of imminent laser failure if the laser diode degrades

further or environmental conditions continue to stress the LD,

such as increasing temperature.

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 that ALS has been enabled.

DEGRADE is raised only when the bias current exceeds 90% of

ASET current.

MONITOR CURRENTS

IBMON, IMMON, IMPDMON, and IMPDMON2 are current

modulation, and MPD current for increased monitoring

functionality. An external resistor to GND gives a voltage

proportional to the current monitored.

If the monitoring functions IMPDMON and IMPDMON2 are

not required, the IMPD pin and the IMPD2 pin must be

grounded and the monitor photodiode output must be

connected directly to the PSET pin.

DUAL MPD DWDM FUNCTION

(48-LEAD LFCSP ONLY)

The ADN2847 has circuitry for a second monitor photodiode,

MPD2. The second photodiode current is mirrored to

IMPDMON2 for wavelength control purposes and is summed

internally with the first monitor photodiode current for the

power control loop. For single MPD circuits, the MPD2 pin is

tied to GND.

This enables the system designer to use the two currents to

control the wavelength of the laser diode using various optical

filtering techniques inside the laser module.

If the monitor current functions IMPDMON and IMPDMON2

are not required, then the IMPD pin and IMPD2 pin can be

grounded and the monitor photodiode output can be connected

directly to PSET.

IDTONE (48-LEAD LFCSP ONLY)

The IDTONE pin is supplied for fiber identification/supervisory

channels or control purposes in WDM. This pin modulates the

necting an external current sink between the IDTONE pin and

Figure 12 shows how the AD9850/AD9851 or the AD9834 can

be used with the ADN2847 to allow fiber identification.

If the ID_TONE function is not used, the IDTONE pin should

can cause optical eye degradation.

DATA AND CLOCK INPUTS

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

recommended) and terminated via a 100 Ω internal resistor

between DATAP and DATAN, and also between the CLKP pin

and the CLKN pin. There is a high impedance circuit to set the

common-mode voltage that is designed to allow for maximum

input voltage headroom over temperature. It is necessary that ac

coupling is used to eliminate the need for matching between

common-mode voltages.

ADN2847

R = 2.5kΩ, DATA

R = 3kΩ, CLK

(TO FLIP-FLOPS)

400µA TYP

DATAP

DATAN

R

50Ω

50Ω

Figure 10. 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 11.

R1

R2

R3

DATAP/CLKP

DATAN/CLKN

ADN2847

NOTES

IMPEDANCE OF THE ADN2847.

Figure 11. Attenuation Circuit

CCBIAS

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

necessary, as shown in both circuits in Figure 14 and Figure 15.

200 Ω ~ 500 Ω.

AUTOMATIC LASER SHUTDOWN

The ADN2847 ALS allows compliance to ITU-T-G958 (11/94),

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

currents are turned off. Correct operation of ALS can be confirmed

if the FAIL alarm is raised when ALS is asserted. Note that this

is the only time DEGRADE is low while FAIL is high.