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ADN2848ACP-32

3 V Dual-Loop 50 Mbps to 1.25 Gbps Laser Diode Driver

Analog Devices 

ADN2848

GENERAL

Laser diodes have current-in to light-out transfer functions as

shown in Figure 2. Two key characteristics of this transfer func-

tion are the threshold current, ITH, and slope in the linear region

beyond the threshold current, referred to as slope efficiency, LI.

ER

=

P1

P0

P1

PAV =

P1

+ P0

2

⌬P

PAV

⌬I

LI =

⌬P

⌬I

P0

ITH CURRENT

Figure 2. Laser Transfer Function

Control

A monitor photodiode, MPD, is required to control the LD.

The MPD current is fed into the ADN2848 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.

The ADN2848 uses automatic power control, APC, to maintain

a constant average power over time and temperature.

The ADN2848 uses closed-loop extinction ratio control to

allow optimum setting of 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.

This reduces research and development time and second

sourcing issues caused by characterizing LDs.

Average power and extinction ratio are set using the PSET and

ERSET pins, respectively. Potentiometers are connected

between these pins and ground. The potentiometer RPSET is

used to change the average power. The potentiometer RERSET is

used to adjust the extinction ratio. Both PSET and ERSET are

kept 1.2 V above GND.

For an initial setup, RPSET and RERSET potentiometers may be

calculated using the following formulas.

RPSET

= 1.2 V

I AV

(Ω)

RERSET

I MPD _ CW

PCW

1.2V

×

ER

ER

−

+

1

1

×

PAV

(Ω)

where:

IAV is the average MPD current.

PCW is the dc optical power specified on the laser data sheet.

IMPD_CW is the MPD current at that specified PCW.

PAV is the average power required.

ER is the desired extinction ratio (ER = P1/P0).

Note that IERSET and IPSET will change from device to device;

however, the control loops will determine the actual values.

It is not required to know the exact values for LI or MPD

optical coupling.

Loop Bandwidth Selection

For continuous operation, the user should hardwire the LBWSET

pin high and use 1 µF capacitors to set the actual loop band-

width. These capacitors are placed between the PAVCAP and

ERCAP pins and ground. It is important that these capaci-

tors are low leakage multilayer ceramics with an insulation

resistance greater than 100 GΩ or a time constant of 1,000 sec,

whichever is less.

Operation

Recommended Recommended

Mode

LBWSET PAVCAP

ERCAP

Continuous High

50 Mbps to

1.25 Gbps

Optimized Low

for 1.25 Gbps

1 µF

47 nF

1 µF

47 nF

Setting LBSET low and using 47 nF capacitors results in a

shorter loop time constant (a 10× reduction over using 1 µF

capacitors and keeping LBWSET high).

Alarms

The ADN2848 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 ADN2848 has two

active high alarms, DEGRADE and FAIL. 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 100:1 to the FAIL alarm threshold. The DEGRADE alarm will

be raised at 90% of this level.

Example:

IFAIL = 50 mA so IDEGRADE = 45 mA

I ASET

=

IFAIL

100

= 50 mA

100

= 500 ␮A

1.2V 1.2

*RASET = IASET = 500 ␮A = 2.4 k⍀

*The smallest valid value for RASET is 1.2 kΩ, since this corresponds to the IBIAS

maximum of 100 ␮A.

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 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 drop-

ping to zero. This gives closed-loop feedback to the system

that ALS has been enabled.

DEGRADE will be raised only when the bias current exceeds

90% of ASET current.

REV. 0

–5–

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