ADN2847ACP-32 データシートの表示（PDF） - Analog Devices

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ADN2847ACP-32

3 V Dual-Loop 50 Mbps to 3.3 Gbps Laser Diode Driver

Analog Devices 

ADN2847

headroom over temperature. It is necessary that ac coupling is

used to eliminate the need for matching between common-

mode voltages.

DATAP

DATAN

ADN2847

(TO FLIP-FLOPS)

50⍀ 50⍀

VREG

R

R = 2.5k⍀, DATA

R = 3k⍀, CLK

400␮A TYP

Figure 7. 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 8.

ADN2847

R1

DATAP/CLKP

R3

RIN

R2

DATAN/CLKN

NOTE THAT RIN = 100⍀ = THE DIFFERENTIAL

INPUT IMPEDANCE OF THE ADN2847

Figure 8. Attenuation Circuit

CCBIAS

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

IBIAS pins should be tied together (Figure 12). In dc-coupled

mode, CCBIAS should be tied to VCC.

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 by the FAIL alarm being raised when ALS is as-

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

while FAIL is high.

Alarm Interfaces

The FAIL and DEGRADE outputs have an internal pull-up

resistor of 30 kΩ used to pull the digital high value to VCC.

However, the alarm can 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 ADN2847.

Power Consumption

The ADN2847 die temperature must be kept below 125oC. Both

LFCSP packages have an exposed paddle, which should be

connected in such a manner that is is at the same potential as

the ADN2847 ground pins. The θJA for both packages is shown

in the Absolute Maximum Ratings. Power consumption can be

calculated using

ICC = ICCMIN + 0.3 I MOD

( ) ( ) P = VCC × ICC + IBIAS × VBIAS _ PIN + I MOD VMODP _ PIN +VMODN _ PIN / 2

TDIE = TAMBIENT + θJA × P

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

lated. Where:

ICCMIN = 50 mA, the typical value of ICC provided on page 3

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 Diode Interfacing

Many laser diodes designed for 2.5 Gbs operation are packaged

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.

REF CLOCK

20MHz–180MHz

CLKIN

9

1.25mA–20mA

21

AD9850/AD9851

AD9834

IOUT

50⍀

DDS

20

12

IOUT

RSET

50⍀

LP FILTER

(DC-COUPLED)

1/2

AD8602

10kHz–1MHz

35 IDTONE

0.125mA–2mA

BC550

ADN2847

500⍀

CONTROLLER

37.5␮A–600␮A

1/2

AD8602

BC550

50␮A–800␮A

1000⍀

32

IMMON

1300⍀

Figure 9. Application Curcuit to Allow Fiber Identification Using the AD9850/AD9851

–8–

REV. 0

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