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ADN2848ACP-32

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

Analog Devices 

ADN2848

Monitor Currents

IBMON, IMMON, and IMPDMON are current controlled

current sources from VCC. They mirror the bias, modulation,

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 recom-

mended) 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.

DATAP

DATAN

ADN2848

(TO FLIP-FLOPS)

50⍀ 50⍀

VREG

R

R = 2.5k⍀, DATA

R = 3k⍀, CLK

400␮A TYP

Figure 3. 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 4.

ADN2848

R1

DATAP/CLKP

R3

RIN

R2

DATAN/CLKN

NOTE THAT RIN = 100⍀ = THE DIFFERENTIAL

INPUT IMPEDANCE OF THE ADN2848

Figure 4. Attenuation Circuit

CCBIAS

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

the IBIAS pins should be tied together (see Figure 7). In dc-

coupled mode, CCBIAS should be tied to VCC.

Automatic Laser Shutdown

The ADN2848 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 con-

firmed by the FAIL alarm being raised when ALS is asserted.

Note that this is the only time that DEGRADE will be low

while FAIL is high.

Alarm Interfaces

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

up resistor that is used to pull the digital high value to VCC.

However, the alarm output may be overdriven with an external

resistor allowing alarm interfacing to non-VCC levels. Non-VCC

alarm output levels must be below the VCC used for the

ADN2848.

Power Consumption

The ADN2848 die temperature must be kept below 125oC. The

LFCSP package has an exposed paddle. The exposed paddle

should be connected in such a manner that it is at the same

potential as the ADN2848 ground pins. The θJA for the package

is shown under the Absolute Maximum Ratings. Power con-

sumption can be calculated using

ICC = ICCMIN + 0.3 IMOD

P = VCC ϫ ICC + (IBIAS ϫ VBIAS_PIN) + IMOD (VMODP_PIN +

VMODN_PIN)/2

TDIE = TAMBIENT + θJA ϫ P

Thus, the maximum combination of IBIAS + IMOD must be calcu-

lated. Where:

ICCMIN = 50 mA, the typical value of ICC provided on Page 2

with IBIAS = IMOD = 0

TDIE = die temperature

TAMBIENT = ambient temperature

VBIAS_PIN = voltage at IBIAS pin

VMODP_PIN = average voltage at IMODP pin

VMODN_PIN = average voltage at IMODN pin

Laser Disode Interfacing

Many laser diodes designed for 1.25 Gbps operation are pack-

aged with an internal resistor to bring the effective impedance

up to 25 Ω in order to minimize transmission line effects. In

high current applications, the voltage drop across this resistor,

combined with the laser diode forward voltage, makes direct

connection between the laser and the driver impractical in a 3 V

system. AC coupling the driver to the laser diode removes this

headroom constraint.

–6–

REV. 0

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