Datasheet ADN2871 Datasheet (Analog Devices)

3.3 V, 50 Mbps to 3.3 Gbps

V

Preliminary Technical Data

FEATURES

SFP/SFF and SFF-8472 MSA-compliant
SFP reference design available
50 Mbps to 4.25 Gbps operation
Multirate 155 Mbps to 4.25 Gbps operation
Automatic average power control
Typical rise/fall time 60 ps
Bias current range 2 mA to 100 mA
Modulation current range 5 mA to 90 mA
Laser fail alarm and automatic laser shutdown (ALS)
Bias and modulation current monitoring

3.3 V operation
4 mm × 4 mm LFCSP package
Voltage setpoint control
Resistor setpoint control
Pin-compatible with ADN2870

APPLICATIONS

Multirate OC3 to OC48-FEC SFP/SFF modules
1×/2×/4× Fibre channel SFP/SFF modules
LX-4 modules
DWDM/CWDM SFP modules
1GE SFP/SFF transceiver modules

Single-Loop Laser Diode Driver

ADN2871

GENERAL DESCRIPTION

The ADN2871 laser diode driver is designed for advanced SFP
and SFF modules, using SFF-8472 digital diagnostics. The
ADN2871 supports single-rate or multi-rate operation from 50
Mbps to 4.25 Gbps.

Average power and extinction ratio can be set with a voltage
provided by a microcontroller DAC or by a trimmable resistor
or digipot. Average power control-loop is implemented using
feedback from a monitor photodiode. The part provides bias
and modulation current monitoring as well as fail alarms and
automatic laser shutdown. The device interfaces easily with the
ADI ADuC70xx family of microconverters and with the
ADN289x family of limiting amplifiers to make a complete
SFP/SFF transceiver solution. An SFP reference design is
available. The product is pin compatible with the ADN2870
Dual Loop LDD allowing one PC board layout to work with
either device. For dual loop applications, refer to the ADN2870
datasheet.

The product is available in a space-saving 4 mm ×4 mm LFCSP
package specified over the −40°C to +85°C temperature range.

CC

Tx_DISABLE

Tx_FAULT

ADI

MICROCONTROLLER

DAC

ADC

DAC

1kΩ

1kΩ

VCC

GND

GND

MPD

PAVSET

PAVREF

RPAV

ERREF

ERSET

VCC

Figure 1. Application Diagram Showing Microcontroller Interface

Protected by US patent: US6414974

Rev. PrA

Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.

GND

GND GND

VCC

IMODP

IBIAS

NC

L

R

VCC

LASER

DATAP

DATAN

CONTROL

ALSFAIL

X 100

VCC

IMODN

100Ω

IMOD

ADN2871

IBMON IMMON

470Ω1kΩ

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] © 2004 Analog Devices, Inc. All rights reserved.

GND

PAVCAP

ERCAP

GND

04510-001

ADN2871 Preliminary Technical Data

TABLE OF CONTENTS

Specifications.......................................................................................

SFP Timing Specifications.................................................................

Absolute Maximum Ratings..............................................................

ESD Caution....................................................................................

Pin Configuration and Function Descriptions...............................

Typical Operating Characteristics....................................................

Optical Waveforms Showing Multirate Performance Using

Low Cost Fabry Perot Tosa NEC NX7315UA ............................

Optical Waveforms Showing Dual-Loop Performance Over

Temperature Using DFB Tosa SUMITOMO SLT2486..............

Performance Characteristics.........................................................

Theory of Operation ..........................................................................

Control.............................................................................................

...........................................................................................................

REVISION HISTORY

Voltage Setpoint Calibration..........................................................

Resistor Setpoint Calibration.........................................................

IMPD Monitoring...........................................................................

Loop Bandwidth Selection .............................................................

Power Consumption .......................................................................

Automatic Laser Shutdown (TX_Disable)...................................

Bias and Modulation Monitor Currents.......................................

Data Inputs.......................................................................................

Laser Diode Interfacing..................................................................

Alarms...............................................................................................

Outline Dimensions............................................................................

Ordering Guide ...............................................................................

Revision 0: Initial Version

Rev. PrA | Page 2 of 19

Preliminary Technical Data ADN2871

SPECIFICATIONS

VCC = 3.0 V to 3.6 V. All specifications T

Table 1.

Parameter Min Typ Max Unit Conditions/Comments

LASER BIAS CURRENT (IBIAS)

Output Current IBIAS 2 100 mA
Compliance Voltage 1.2 VCC V
IBIAS when ALS is High 0.2 mA
CCBIAS Compliance Voltage 1.2 V

MODULATION CURRENT (IMODP, IMODN)

Output Current IMOD 5 90 mA
Compliance Voltage 1.5 VCC V
IMOD when ALS is High 0.05 mA
Rise Time
Fall Time
Random Jitter
Deterministic Jitter
Pulse-Width Distortion

2, 3

2, 3

2, 3

2, 3

2, 3

AVERAGE POWER SET (PAVSET)

Pin Capacitance 80 pF
Voltage 1.1 1.2 1.35 V

Photodiode Monitor Current (Average Current) 50 1200 µA Resistor setpoint mode

EXTINCTION RATIO SET INPUT (ERSET)

Resistance Range 1.49 25 kΩ Resistor setpoint mode
Resistance Range 0.99 1.0 1.01 kΩ Voltage setpoint mode

AVERAGE POWER REFERENCE VOLTAGE INPUT (PAVREF)

Voltage Range 0.12 1 V

Photodiode Monitor Current (Average Current) 120 1000 µA

EXTINCTION RATIO REFERENCE VOLTAGE INPUT (ERREF)

Voltage Range 0.05 0.9 V

DATA INPUTS (DATAP, DATAN)

4

V p-p (Differential) 0.4 2.4 V AC-coupled
Input Impedance (Single-Ended) 50 Ω

LOGIC INPUTS (ALS)

VIH 2 V
VIL 0.8 V

ALARM OUTPUT (FAIL)

V

> 1.8 V

OFF

V

ON

5

MIN

2

to T

,1 unless otherwise noted. Typical values as specified at 25°C.

MAX

60 104 ps
60 96 ps

0.8 1.1 ps rms
35 ps 20 mA < IMOD < 90 mA
30 ps 20 mA < IMOD < 90 mA

Voltage setpoint mode
(RPAV fixed at 1 kΩ)

Voltage setpoint mode
(RPAV fixed at 1 kΩ)

Voltage setpoint mode
(RERSET fixed at 1 kΩ)

Voltage required at FAIL for Ibias and
Imod to turn off when FAIL asserted

< 1.3 V

Voltage required at FAIL for Ibias and
Imod to stay on when FAIL asserted

Rev. PrA | Page 3 of 19

ADN2871 Preliminary Technical Data

Parameter Min Typ Max Unit Conditions/Comments

IBMON, IMMON DIVISION RATIO

IBIAS/IBMON

IBIAS/IBMON

IBIAS/IBMON STABILITY

IMOD/IMMON 50 A/A

IBMON Compliance Voltage 0 1.3 V
SUPPLY

7

I

CC

VCC (w.r.t. GND)

1

Temperature range: –40°C to +85°C.

2

Measured into a 15 Ω load (22 Ω resistor in parallel with digital scope 50 Ω input) using a 11110000 pattern at 2.5 Gbps, shown in Figure 2.

3

Guaranteed by design and characterization. Not production tested.

4

When the voltage on DATAP is greater than the voltage on DATAN, the modulation current flows in the IMODP pin.

5

Guaranteed by design. Not production tested.

6

IBIAS/IBMON ratio stability is defined in SFF-8472 revision 9 over temperature and supply variation.

7

ICC min for power calculation in the Power Consumption section.

8

All VCC pins should be shorted together.

3

3

3, 6

85 100 115 A/A 11 mA < IBIAS < 50 mA
92 100 108 A/A 50 mA < IBIAS < 100 mA
±5 % 10 mA < IBIAS < 100 mA

30 mA When IBIAS = IMOD = 0

8

3.0 3.3 3.6 V

V

CCVCC

ADN2871

IMODP

22Ω

R

L

C

BIAS TEE

80kHz  27GHz

Figure 2. High Speed Electrical Test Output Circuit

TO HIGH SPEED
DIGITAL
OSCILLOSCOPE
50Ω INPUT

04510-034

Rev. PrA | Page 4 of 19

Preliminary Technical Data ADN2871

SFP TIMING SPECIFICATIONS

Table 2.

Parameter Symbol Min Typ Max Unit Conditions/Comments

ALS Assert Time t_off 1 5 µs

ALS Negate Time

Time to Initialize, Including

Reset of FAIL

1

1

t_on 0.83 0.95 ms

t_init 25 275 ms From power-on or negation of FAIL using ALS.

FAIL Assert Time t_fault 100 µs Time to fault to FAIL on.
ALS to Reset time t_reset 5 µs Time TX_DISABLE must be held high to reset TX_FAULT.

1

Guaranteed by design and characterization. Not production tested.

V

SE

DATAP
DATAN

Time for the rising edge of ALS (TX_DISABLE) to when the bias
current falls below 10% of nominal.

Time for the falling edge of ALS to when the modulation current
rises above 90% of nominal.

SFP MODULE

VCC_Tx

1µH

3.3V

0.1µF 0.1µF 10µF

DATAP–DATAN

0V

Figure 3. Signal Level Definition

V p-p

DIFF

= 2× V

SFP HOST BOARD

SE

04510-002

Figure 4. Recommended SFP Supply

04510-003

Rev. PrA | Page 5 of 19

ADN2871 Preliminary Technical Data

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 3.

Parameter Rating

VCC to GND 4. 2 V
IMODN, IMODP –0.3 V to +4.8 V
PAVCAP –0.3 V to +3.9 V
ERCAP –0.3 V to +3.9 V
PAVSET –0.3 V to +3.9 V
PAVREF –0.3 V to +3.9 V
ERREF –0.3 V to +3.9 V
IBIAS –0.3 V to +3.9 V
IBMON –0.3 V to +3.9 V
IMMON –0.3 V to +3.9 V
ALS –0.3 V to +3.9 V
CCBIAS –0.3 V to +3.9 V
RPAV –0.3 V to +3.9 V
ERSET –0.3 V to +3.9 V
FAIL –0.3 V to +3.9 V
DATAP, DATAN

(single-ended differential)
Junction Temperature 150°C

1.5 V

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those listed in the operational sections
of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

Rev. PrA | Page 6 of 19

Preliminary Technical Data ADN2871

TEMPERATURE SPECIFICATIONS

TEMPERATURE SPECIFICATIONS
Operating Temperature Range

Industrial

Storage Temperature Range –65°C to +150°C
Junction Temperature (TJ max) 125°C

LFCSP Package

Power Dissipation1

ESD CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.

−40°C to +85°C

(TJ max – TA)/θJA W

θJA Thermal Impedance2 30°C/W
θJCThermal Impedance 29.5°C/W

Lead Temperature (Soldering 10 s) 300°C

___________________

1

Power consumption equations are provided in the Power Consumption

section.

2

θJA is defined when part is soldered on a 4-layer board.

Rev. PrA | Page 7 of 19

ADN2871 Preliminary Technical Data

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

GND

VCC
IMODP
IMODN

GND

IBIAS

18

FAIL

IBMON

ERREF

19

VCC

ADN2871

24

1

NC

VCC

GND

PAVSET

13

IMMON

ERSET

12

ALS
DATAN
DATAP
GND
PAVCAP
ERCAP

7

6

RPAV

PAVREF

04510-004

Figure 5. Pin Configuration- Top View

Table 4. Pin Fuction Descriptions

Pin No. Mnemonic Description

1 NC No Connect
2 PAVSET Average Optical Power Set Pin
3 GND Supply Ground
4 VCC Supply Voltage
5 PAVREF Reference Voltage Input for Average Optical Power Control
6 RPAV Average Power Resistor when Using PAVREF
7 ERCAP TBD
8 PAVCAP Average Power Loop Capacitor
9 GND Supply Ground
10 DATAP Data, Positive Differential Input
11 DATAN Data, Negative Differential Input
12 ALS Automatic Laser Shutdown
13 ERSET Extinction Ratio Set Pin
14 IMMON Modulation Current Monitor Current Source
15 ERREF Reference Voltage Input for Extinction Ratio Control
16 VCC Supply Voltage
17 IBMON Bias Current Monitor Current Source
18 FAIL FAIL Alarm Output
19 GND Supply Ground
20 VCC Supply Voltage
21 IMODP Modulation Current Positive Output, Connect to Laser Diode
22 IMODN Modulation Current Negative Output
23 GND Supply Ground
24 IBIAS Laser Diode Bias (Current Sink to Ground)

Note: The LFCSP package has an exposed paddle that must be connected to ground.

Rev. PrA | Page 8 of 19

Preliminary Technical Data ADN2871

TYPICAL OPERATING CHARACTERISTICS

VCC = 3.3 V and TA = 25°C, unless otherwise noted.

OPTICAL WAVEFORMS SHOWING MULTIRATE
PERFORMANCE USING LOW COST FABRY PEROT TOSA
NEC NX7315UA

Note: No change to PAVCAP and ERCAP values

(ACQ LIMIT TEST) WAVEFORMS 1000

04510-016

31-1

Figure 6. Optical Eye 2.488 Gbps,65 ps/div, PRBS 2

PAV = −4.5 dBm, ER = 9 dB, Mask Margin 25%

(ACQ LIMIT TEST) WAVEFORMS 1000

(ACQ LIMIT TEST) WAVEFORMS 1000

Figure 8. Optical Eye 155 Mbps,1.078 ns/div, PRBS 2

PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50%

EXTINCTION RATIO PERFORMANCE OVER
TEMPERATURE USING DFB TOSA ?????

Figure 9. Drift of Imod versus Temperature

31-1

04510-020

04510-017

31-1

Figure 7. Optical Eye 622 Mbps, 264 ps/div, PRBS 2

PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50%

Figure 10. Honeywekk VCSEL eye

Rev. PrA | Page 9 of 19

ADN2871 Preliminary Technical Data

PERFORMANCE CHARACTERISTICS

90

1.2

1.0

60

RISE TIME (ps)

30

0

0408020 60 100

MODULATION CURRENT (mA)

Figure 11. Rise Time vs. Modulation Current, Ibias = 20 mA

80

60

40

FALL TIME (ps)

20

04510-022

0.8

0.6

JITTER (rms)

0.4

0.2

0

0 20406080100

MODULATION CURRENT (mA)

Figure 14. Random Jitter vs. Modulation Current, Ibias = 20 mA

250

220

= 80mA

I

190

160

130

100

TOTAL SUPPLY CURRENT (mA)

70

BIAS

I

BIAS

= 40mA

I

BIAS

= 20mA

04510-037

0

0408020 60 100

MODULATION CURRENT (mA)

Figure 12. Fall Time vs. Modulation Current, Ibias = 20 mA

45

40

35

30

25

20

15

10

DETERMINISTIC JITTER (ps)

5

0

20 40 8060 100

MODULATION CURRENT (mA)

Figure 13. Deterministic Jitter vs. Modulation Current, Ibias = 20 mA

04510-025

04510-042

40

0 20406080100

MODULATION CURRENT (mA)

Figure 15. Total Supply Current vs. Modulation Current

Total Supply Current = I

+ Ibias + Imod

CC

60

55

50

45

40

35

SUPPLY CURRENT (mA)

30

25

20

–50–30–101030507090110

Figure 16. Supply Current (I

TEMPERATURE (°C)

) vs. Temperature with ALS Asserted,

CC

Ibias = 20 mA

04510-038

04510-027

Rev. PrA | Page 10 of 19

Preliminary Technical Data ADN2871

120

115

110

105

100

95

IBIAS/IBMON RATIO

90

85

80

–50–30–101030507090110

TEMPERATURE (°C)

Figure 17. IBIAS/IBMON Gain vs. Temperature, Ibias = 20 mA

OC48 PRBS31
DATA TRANSMISSION

t

LESS THAN 1µs

_OFF

04510-028

60

58

56

54

52

50

48

IMOD/IMMON RATIO

46

44

42

40

–50–30–101030507090110

TEMPERATURE (°C)

Figure 20. IMOD/IMMON Gain vs. Temperature, Imod = 30 mA

FAIL ASSERTED

04510-031

FAULT FORCED ON PAVSET

ALS

Figure 18. ALS Assert Time, 5 µs/div

OC48 PRBS31

DATA TRANSMISSION

t

_ON

ALS

Figure 19. ALS Negate Time, 200 µs/div

04510-029

04510-032

Figure 21. FAIL Assert Time,1 µs/div

TRANSMISSION ON

POWER SUPPLY TURN ON

Figure 22. Time to Initialize, Including Reset, 40 ms/div

04510-045

04510-046

Rev. PrA | Page 11 of 19

ADN2871 Preliminary Technical Data

××=

×

THEORY OF OPERATION

Laser diodes have a current-in to light-out transfer function as
shown in Figure 23. Two key characteristics of this transfer
function are the threshold current, Ith, and slope in the linear
region beyond the threshold current, referred to as slope
efficiency, LI.

P1

ER =

P

O

P1 + P

P1

P

AV

P

AV

OPTICAL POWER

P

O

Figure 23. Laser Transfer Function

O

=

2

∆

P

∆

I

Ith CURRENT

LI =

∆

P

∆

I

04510-005

LASER CONTROL

Typically laser threshold current and slope efficiency are both
functions of temperature. For FP and DFB type lasers the
threshold current increases and the slope efficiency decreases
with increasing temperature. In addition, these parameters vary
as the laser ages. To maintain a constant optical average power
and a constant optical extinction ratio over temperature and
laser lifetime, it is necessary to vary the applied electrical bias
current and modulation current to compensate for the lasers
changing LI characteristics.

Average Power Control Loop (APCL)

The APCL compensates for changes in Ith and LI by varying
Ibias. Average power control is performed by measuring MPD
current, Impd. This current is bandwidth-limited by the MPD.
This is not a problem because the APCL is required to respond
to the average current from the MPD.

Extinction Ratio (ER) Control

ER control is implemented by adjusting the Modulation current.
Temperature-calibration is required in order to adjust the
Modulation current to compensate for variation of the laser
characteristics with temperature.

CONTROL METHODS

The ADN2871 has two methods for setting the average power
(PAV) and extinction ratio (ER). The average power and
extinction ratio can be voltage-set using a microcontroller’s
voltage DACs outputs to provide controlled reference voltages
PAVREF and ERREF. Alternatively, the average power and
extinction ratio can be resistor-set using potentiometers at the
PAVSET and ERSET pins, respectively.

VOLTAGE SETPOINT CALIBRATION

The ADN2871 allows interface to a microcontroller for both
control and monitoring (see Figure 24). The average power and
extinction ratio can be set using the microcontroller’s DACs to
provide controlled reference voltages PAVREF and ERREF.

RPAVRPPAVREF

RIMOD

ERSET

(Volts)

(Volts)

ERSET

must be 1 kΩ

SPAV

ERREF

=

100

where:

R

= Impd/(Laser output power)

SP

is the average power required.

P

AV

RPAV = R

= 1kOhm

ERSET

IMOD = Modulation current

In voltage setpoint mode, RPAV and R
resistors with a 1% tolerance and a temperature coefficient of 50
ppm/°C.

Note that during power up, there is an internal sequence that
allows 25 ms before enabling the alarms; therefore the customer
must ensure that the voltage for PAVREF and ERREF are active
within 20 ms after ramp-up of the power supply.

Rev. PrA | Page 12 of 19

Preliminary Technical Data ADN2871

V

V

Tx_FAULT

Tx_FAIL

ADI

MICROCONTROLLER

DAC

ADC

DAC

1kΩ

1kΩ

VCC

GND

GND

MPD

PAVSET

PAVREF

RPAV

ERREF

ERSET

VCC

CC

GND

GND GND

ALSFAIL

CONTROL

X 100

IMOD

IBMON IMMON

470Ω1kΩ

VCC

IMODN

ADN2871

PAVCAP

GND

100Ω

GND

ERCAP

VCC

IMODP

IBIAS

NC

LASER

DATAP

DATAN

VCC

04510-001

L

R

Figure 24. ADN2871 Using Microconverter Calibration and Monitoring

CC

IMODP

IBIAS

NC

L

R

VCC

LASER

DATAP

DATAN

VCC

GND

MPD

VCC

VCC

PAVREF

RPAV

PAVSET

ERREF

ERSET

Vref

VCC

CONTROL

X 100

VCC

ALSFAIL

IMOD

IMODN

100Ω

ADN2870

GND

VCC

GND

GND GND

IBMON IMMON

470Ω1kΩ

GND

PAVCAP

GND

ERCAP

04510-010

Figure 25. ADN2871 Using Resistor Setpoint Calibration of Average Power and Extinction Ratio

Rev. PrA | Page 13 of 19

ADN2871 Preliminary Technical Data

P

AV

V

V

VCCV

µ

RESISTOR SETPOINT CALIBRATION

In resistor setpoint calibration. PAVREF, ERREF, and RPAV
must all be tied to VCC. Average power and extinction ratio can
be set using the PAVSET and ERSET pins, respectively. A
resistor is placed between the pin and GND to set the current
flowing in each pin as shown in Figure 25. The ADN2871
ensures that both PAVSET and ERSET are kept 1.23 V above
GND. The PAVSET and ERSET resistors are given by the
following:

V23.1

R

PAVSET

R

ERSET

=

=

RP

×

S

IMOD

(Ω)

100V23.1 ×

(Ω)

differential measurement across a sense resistor directly in
series with the IMPD. As shown in Figure 27, a small resistor,
Rx, is placed in series with the IMPD. If the laser used in the
design has a pinout where the monitor photodiode cathode and
the lasers anode are not connected, a sense resistor can be placed
in series with the photodiode cathode and VCC as shown in
Figure 28. When choosing the value of the resistor, the user
must take into account the expected IMPD value in normal
operation. The resistor must be large enough to make a signifi­cant signal for the buffered A/Ds to read, but small enough so
as not to cause a significant voltage reduction across the IMPD.
The voltage across the sense resistor should not exceed 250 mV
when the laser is in normal operation. It is recommended that a
10 pF capacitor be placed in parallel with the sense resistor.

CC

where:

R

= Impd/(Laser output power)

SP

IMOD is the modulation current required.

is the average power required

P

AV

IMPD MONITORING

IMPD monitoring can be implemented for voltage setpoint and
resistor setpoint as follows.

Voltage Setpoint

In voltage setpoint calibration, the following methods may be
used for IMPD monitoring.

Method 1: Measuring Voltage at RPAV

The IMPD current is equal to the voltage at RPAV divided by
the value of RPAV (see Figure 26) as long as the laser is on and
is being controlled by the control loop. This method does not
provide a valid IMPD reading when the laser is in shut-down or
fail mode. A microconverter buffered A/D input may be con­nected to RPAV to make this measurement. No decoupling or
filter capacitors should be placed on the RPAV node because
this can disturb the control loop.

CC

PHOTODIODE

PAVSET

ADN2871

R
1kΩ

RPAV

04510-043

µ

C ADC

INPUT

Figure 26. Single Measurement of IMPD RPAV in Voltage Setpoint Mode

LDPHOTODIODE

µ

C ADC

DIFFERENTIAL

INPUT

Figure 27. Differential Measurement of IMPD Across a Sense Resistor

C ADC

INPUT

PHOTODIODE

Figure 28. Single Measurement of IMPD Across a Sense Resistor

ADN2871

200Ω

RESISTOR

ADN2871

200Ω
RESISTOR

PAVSET

PAVSET

CC

10pF

04510-011

LD

04510-012

Resistor Setpoint

In resistor setpoint calibration, the current through the resistor
from PAVSET to ground is the IMPD current. The recommended
method for measuring the IMPD current is to place a small
resistor in series with PAVSET resistor (or potentiometer) and
measure the voltage across this resistor as shown in Figure 29.
The IMPD current is then equal to this voltage divided by the
value of resistor used. In resistor setpoint, PAVSET is held to

1.2 V nominal; it is recommended that the sense resistor should
be selected so that the voltage across the sense resistor does not
exceed 250 mV.

Method 2: Measuring IMPD Across a Sense Resistor

The second method has the advantage of providing a valid IMPD
reading at all times, but has the disadvantage of requiring a

Rev. PrA | Page 14 of 19

Preliminary Technical Data ADN2871

V

V

V

d

PHOTODIODE

µ

C ADC

INPUT

Figure 29. Single Measurement of IMPD Across a

Sense Resistor in Resistor Setpoint IMPD Monitoring

CC

R

PAVSET

ADN2871

4510-040

LOOP BANDWIDTH SELECTION

To ensure that the ADN2871 control loop has sufficient
bandwidth, the average power loop capacitor (PAVCAP) is
calculated using the lasers slope efficiency (watts/amps) and the
average power required.

For resistor setpoint control:

EPAVCAP ×−=

LI

PA

For voltage setpoint control:

EPAVCAP ×−=

LI

PA

where PAV is the average power required and LI (mW/mA) is
the typical slope efficiency at 25°C of a batch of lasers that are
used in a design. The capacitor value equation is used to get a
centered value for the particular type of laser that is used in a
design and average power setting. The laser LI can vary by a
factor of 7 between different physical lasers of the same type
and across temperature without the need to recalculate the
PAVCAP and ERCAP values. In ac coupling configuration the
LI can be calculated as follows:

P0P1LI−

=

Imo

(mW/mA)

where P1 is the optical power (mW) at the one level, and P0 is
the optical power (mW) at the zero level.

This capacitor is placed between the PAVCAP pin and ground.
It is important that the capacitor is a low leakage multilayer
ceramic with an insulation resistance greater than 100 GΩ or a
time constant of 1000 sec, whichever is less. Pick a standard off­the-shelf capacitor value such that the actual capacitance is within
+/-30% of the calculated value after the capacitors own tolerance
is taken into account.

)(62.3 Farad

)(628.1 Farad

= ICC min + 0.3 I

I

CC

P = V

× ICC + (I

CC

MODN_PIN

= T

DIE

)/2

AMBIENT

+ θJA × P

V

T

Thus, the maximum combination of I

BIAS

MOD

× V

BIAS_PIN

) + I

BIAS

MOD

+ I

(V

MOD

MODP_PIN

must be

+

calculated.

where:

I

min = 30 mA, the typical value of ICC provided in the

CC

= I

Specifications with I

is the die temperature.

T

DIE

T

is the ambient temperature.

AMBIENT

is the voltage at the IBIAS pin.

V

BIAS_PIN

V
V

is the voltage at the IMODP pin.

MODP_PIN

is the voltage at the IMODN pin.

MODN_PIN

BIAS

MOD

= 0.

AUTOMATIC LASER SHUTDOWN (TX_DISABLE)

ALS (TX disable) is an input that is used to shut down the
transmitter optical output. The ALS pin is pulled up internally
with a 6 kΩ resistor, and conforms to SFP MSA specification.
When ALS is logic high or when open, both the bias and
modulation currents are turned off.

BIAS AND MODULATION MONITOR CURRENTS

IBMON and IMMON are current-controlled current sources
that mirror a ratio of the bias and modulation current. The
monitor bias current, IBMON, and the monitor modulation
current, IMMON, should both be connected to ground through
a resistor to provide a voltage proportional to the bias current
and modulation current, respectively. When using a micro­controller, the voltage developed across these resistors can be
connected to two of the ADC channels, making available a
digital representation of the bias and modulation current.

DATA INPUTS

Data inputs should be ac-coupled (10 nF capacitors are
recommended) and are terminated via a 100 Ω internal resistor
between the DATAP and DATAN pins. A high impedance
circuit sets the common-mode voltage and is designed to allow
maximum input voltage headroom over temperature. It is
necessary to use ac coupling to eliminate the need for matching
between common-mode voltages.

POWER CONSUMPTION

The ADN2871 die temperature must be kept below 125°C. The
LFCSP package has an exposed paddle, which should be con­nected such that is at the same potential as the ADN2871 ground
pins. Power consumption can be calculated as follows:

Rev. PrA | Page 15 of 19

ADN2871 Preliminary Technical Data

V

LASER DIODE INTERFACING

The schematic in Figure 30 describes the recommended circuit
for interfacing the ADN2871 to most TO-Can or Coax lasers.
These lasers typically have impedances of 5 Ω to 7 Ω, and have
axial leads. The circuit shown works over the full range of data
rates from 155 Mbps to 3.3 Gbps including multirate operation
(with no change to PAVCAP and ERCAP values); see the
Typical Operating Characteristics for multirate performance
examples. Coax lasers have special characteristics that make
them difficult to interface to. They tend to have higher
inductance, and their impedance is not well controlled. The
circuit in Figure 30 operates by deliberately misterminating the
transmission line on the laser side, while providing a very high
quality matching network on the driver side. The impedance of
the driver side matching network is very flat versus frequency
and enables multirate operation. A series damping resistor
should not be used.

CC

L (0.5nH)

C

R

P

100nF

IMODP

ADN2871

IBIAS

24Ω

30Ω

Tx LINE

L

30Ω

Tx LINE

BLMI8HG60ISN1D

R 24Ω

Figure 30. Recommended Interface for ADN2871 AC Coupling

C 2.2pF

VCC

04510-014

The 30 Ω transmission line used is a compromise between drive
current required and total power consumed. Other
transmission line values can be used, with some modification of
the component values. The R and C snubber values in Figure
30, 24 Ω and 2.2 pF, respectively, represent a starting point and
must be tuned for the particular model of laser being used. R

,

P

the pull-up resistor is in series with a very small (0.5 nH)
inductor. In some cases, an inductor is not required or can be
accommodated with deliberate parasitic inductance, such as a
thin trace or a via, placed on the PC board.

Care should be taken to mount the laser as close as possible to
the PC board, minimizing the exposed lead length between the
laser can and the edge of the board. The axial lead of a coax
laser are very inductive (approximately 1 nH per mm). Long
exposed leads result in slower edge rates and reduced eye margin.

Recommended component layouts and gerber files are available
by contacting the factory. Note that the circuit in Figure 30 can
supply up to 56 mA of modulation current to the laser, sufficient
for most lasers available today. Higher currents can be accom­modated by changing transmission lines and backmatch values;
contact factory for recommendations. This interface circuit is
not recommended for butterfly-style lasers or other lasers with
25 Ω characteristic impedance. Instead, a 25 Ω transmission
line and inductive (instead of resistive) pull-up is
recommended; contact the factory for recommendations.

The ADN2871 also supports differential drive schemes. These
can be particularly useful when driving VCSELs or other lasers
with slow fall times. Differential drive can be implemented by
adding a few extra components. A possible implementation is
shown in Figure 31.

V

CC

L1 = 0.5nH

R1 = 15Ω

IMODN

ADN2871

IMODP
IBIAS

SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED,
OPTIMIZATION SHOULD CONSIDER PARASITIC.

R1 = 15Ω
(12 TO 24Ω)

V

CC

Figure 31. Recommended Differential Drive Circuit

C1 = C2 = 100nF

20Ω TRANMISSION LINES

L2 = 0.5nH

Rev. PrA | Page 16 of 19

L4 = BLM18HG601SN1

L3 = 4.7nH

C3

R3

SNUBBER

TOCAN/VCSEL

L6 = BLM18HG601SN1

LIGHT

04510-041

Preliminary Technical Data ADN2871

ALARMS

The ADN2871 has a latched active high monitoring alarm
(FAIL). The FAIL alarm output is an open drain in conformance
to SFP MSA specification requirements.

The ADN2871 has a 3-fold alarm system that covers

• Use of a bias current higher than expected, probably as a

result of laser aging.

• Undervoltage in IBIAS node (laser diode cathode) that

would increase the laser power.

The bias current alarm trip point is set by selecting the value of
resistor on the IBMON pin to GND. The alarm is triggered
when the voltage on the IBMON pin goes above 1.2 V.

FAIL is activated when the single-point faults in Table 5 occur.

• Out-of-bounds average voltage at the monitor photodiode

(MPD) input, indicating an indicating an excessive amount
of laser power or a broken loop.

Table 5. ADN2871 Single-Point Alarms

Alarm Type Pin Name Over Voltage or Short to VCC Condition Under Voltage or Short to GND Condition

1. Bias Current IBMON Alarm if > 1.2 V Ignore

2. MPD Current PAVSET Alarm if > 1.7 V Alarm if < 0.9 V
ERREF Alarm if shorted to VCC Alarm if shorted to GND 3. Crucial Nodes
IBIAS Ignore Alarm if < 400 mV

Table 6. ADN2871 Response to Various Single-Point Faults in AC-Coupled Configuration as Shown in Figure 30

Pin Short to VCC Short to GND Open

CCBIAS Fault state occurs Fault state occurs Does not increase laser average power
PAVSET Fault state occurs Fault state occurs Fault state occurs
PAVREF

RPAV

ERCAP Does not increase laser average power Does not increase laser average power Does not increase laser average power
PAVCAP Fault state occurs Fault state occurs Fault state occurs
DATAP Does not increase laser average power Does not increase laser average power Does not increase laser average power
DATAN Does not increase laser average power Does not increase laser average power Does not increase laser average power
ALS Output currents shut off Normal currents Output currents shut off
ERSET Does not increase laser average power Does not increase laser average power Does not increase laser average power
IMMON Does not affect laser power Does not increase laser average power Does not increase laser average power
ERREF

IBMON Fault state occurs Does not increase laser average power Does not increase laser average power
FAIL Fault state occurs Does not increase laser average power Does not increase laser average power
IMODP Does not increase laser average power Does not increase laser average power Does not increase laser average power
IMODN Does not increase laser average power Does not increase laser average power Does not increase laser power
IBIAS Fault state occurs Fault state occurs Fault state occurs

Voltage mode: Fault state occurs
Resistor mode: Tied to VCC

Voltage mode: Fault state occurs
Resistor mode: Tied to VCC

Voltage mode: Fault state occurs
Resistor mode: Tied to VCC

Fault state occurs Fault state occurs

Fault state occurs

Voltage mode: Does not increase
average power

Resistor mode: Fault state occurs

Voltage mode: Fault state occurs
Resistor mode: Does not increase
average power

Does not increase laser average power

Rev. PrA | Page 17 of 19

ADN2871 Preliminary Technical Data

OUTLINE DIMENSIONS

0.08

0.60 MAX

19

18

BOTTOM

13

12

VIEW

24

7

1

6

2.50 REF

PIN 1
INDICATOR

2.25

2.10 SQ

1.95

0.25MIN

4.00

PIN 1

INDICATOR

1.00

0.85

0.80

SEATING
PLANE

12° MAX

BSC SQ

TOP

VIEW

0.80 MAX

0.65TYP

COMPLIANT TOJEDECSTANDARDS MO-220-VGGD-2

0.30

0.23

0.18

3.75

BSC SQ

0.20 REF

0.60 MAX

0.05 MAX

0.02 NOM

0.50

BSC

0.50

0.40

0.30

COPLANARITY

Figure 32. 24-Lead Lead Frame Chip Scale Package [LFCSP]

(CP-24)

Dimensions shown in millimeters

Note: The LFCSP package has an exposed paddle that must be connected to ground.

ORDERING GUIDE

Model Temperature Range Package Description Package Option

ADN2871ACPZ1

ADN2871ACPZ-RL1

ADN2871ACPZ-RL71

−40°C to +85°C

−40°C to +85°C

−40°C to +85°C

24-Lead Lead Frame Chip Scale Package CP-24

24-Lead Lead Frame Chip Scale Package CP-24

24-Lead Lead Frame Chip Scale Package CP-24

1

Z = Pb-free part.

Rev. PrA | Page 18 of 19

Preliminary Technical Data ADN2871

PR05228-0-10/04(PrA)

NOTES

Rev. PrA | Page 19 of 19