3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps
Laser Diode Driver
ADN2872
Rev. 0
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2009 Analog Devices, Inc. All rights reserved.
FEATURES
SFP/SFF and SFF-8472 MSA compliant
SFP reference design available
Any rate from 50 Mbps to 3.3 Gbps operation
Dual-loop control of average power and extinction ratio
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
Voltage setpoint control
Resistor setpoint control
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
GENERAL DESCRIPTION
Like the ADN2870, the ADN28721 laser diode driver is designed
for advanced SFP and SFF modules, using SFF-8472 digital
diagnostics. The device features dual-loop control of the average
power and extinction ratio, which automatically compensates
for variations in laser characteristics over temperature and aging.
The laser needs only be calibrated at 25°C, eliminating the need
for expensive and time consuming temperature calibration. The
ADN2872 supports single-rate operation from 50 Mbps to
3.3 Gbps, or multirate operation from 155 Mbps to 3.3 Gbps.
With a new alarm scheme, this device avoids the shutdown issue
caused by the system transient generated from various lasers.
The average power and extinction ratio can be set with a
voltage provided by a microcontroller DAC or by a trimmable
resistor. The part provides both bias and modulation current
monitoring, as well as fail alarms and automatic laser shutdown.
The ADN2872, a SFF-/SFP-compliant laser diode driver, can
work with the Analog Devices, Inc., ADuC7019/ADuC702x
MicroConverter® family and the ADN289x limiting amplifier
family, to form a complete SFP/SFF transceiver solution. The
ADN2872 is available in a space-saving 4 mm × 4 mm LFCSP
specified over the −40°C to +85°C temperature range.
Figure 1 shows an application diagram with a microcontroller
interface.
1 Protected by U.S. Patent 6,414,974.
APPLICATIONS DIAGRAM
ANALOG DEV I CES
MICROCONTROLLER
Tx_FAULT
Tx_FAIL
CONTROL
PAVREF
PAVSET
ADN2872
RPAV
GND
DAC
DAC
V
CC
MPD
ERREF
ERSET
GND
I
MOD
DATAP
IMODP
DATAN
I
BIAS
IMODN
IBMON IMMON
ALSFAIL
IBIAS
100Ω
CCBIAS
V
CC
LASER
L
R
V
CC
V
CC
VCC
GND GND
PAVCAP
470Ω1kΩ
GND
GND
ERCAP
GND
1kΩ
1kΩ
ADC
V
CC
V
CC
R
Z
08013-001
Figure 1.
ADN2872
Rev. 0 | Page 2 of 20
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications ....................................................................................... 1
General Description ......................................................................... 1
Applications Diagram ...................................................................... 1
Revision History ............................................................................... 2
Specifications ..................................................................................... 3
SFP Timing Specifications ........................................................... 4
Absolute Maximum Ratings ............................................................ 5
ESD Caution .................................................................................. 5
Pin Configuration and Function Descriptions ............................. 6
Typical Performance Characteristics ............................................. 7
Optical Waveforms ........................................................................... 9
Theory of Operation ...................................................................... 10
Dual-Loop Control..................................................................... 10
Control ......................................................................................... 11
Voltage Setpoint Calibration ..................................................... 11
Resistor Setpoint Calibration .................................................... 13
IMPD Monitoring .......................................................................... 13
Loop Bandwidth Selection ........................................................ 14
Power Consumption .................................................................. 14
Automatic Laser Shutdown (Tx_Disable) ............................... 14
Bias and Modulation Monitor Currents .................................. 14
IBIAS Pin ..................................................................................... 14
Data Inputs .................................................................................. 15
Laser Diode Interfacing ............................................................. 15
Alarms .......................................................................................... 16
Outline Dimensions ....................................................................... 17
Ordering Guide .......................................................................... 17
REVISION HISTORY
3/09—Revision 0: Initial Version
ADN2872
Rev. 0 | Page 3 of 20
SPECIFICATIONS
VCC = 3.0 V to 3.6 V. All specifications TMIN to TMAX, unless otherwise noted.1 Typical values as specified at 25°C.
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 High 0.2 mA
CCBIAS Compliance Voltage 1.2 V
MODULATION CURRENT (IMODP, IMODN)2
Output Current, IMOD 5 90 mA
Compliance Voltage 1.5 VCC V
IMOD when ALS High 0.05 mA
Rise Time2, 3 60 104 ps
Fall Time2, 3 60 96 ps
Random Jitter2, 3 0.8 1.1 ps rms
Deterministic Jitter2, 3 35 ps 20 mA < IMOD < 90 mA
Pulse Width Distortion2, 3 30 ps 20 mA < IMOD < 90 mA
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 1.2 25 kΩ Resistor setpoint mode
Voltage 1.1 1.2 1.35 V Resistor setpoint mode
AVERAGE POWER REFERENCE VOLTAGE INPUT (PAVREF)
Voltage Range 0.12 1 V Voltage setpoint mode (RPAV fixed at 1 kΩ)
Photodiode Monitor Current (Average Current) 120 1000 μA Voltage setpoint mode (RPAV fixed at 1 kΩ)
EXTINCTION RATIO REFERENCE VOLTAGE INPUT (ERREF)
Voltage Range 0.1 1 V Voltage setpoint mode (RERSET fixed at 1 kΩ)
DATA INPUTS (DATAP, DATAN)4
Input Voltage Swing (Differential) 0.4 2.4 V p-p AC-coupled
Input Impedance (Single-Ended) 50 Ω
LOGIC INPUTS (ALS)
VIH 2 V
VIL 0.8 V
ALARM OUTPUT (FAIL)5
VOFF >1.8 V
Voltage required at FAIL for IBIAS and IMOD to
turn off when FAIL asserted
VON <1.3 V Voltage required at FAIL for IBIAS and IMOD to
stay on when FAIL asserted
IBMON, IMMON DIVISION RATIO
IBIAS/IBMON3 85 100 115 A/A 11 mA < IBIAS < 50 mA
IBIAS/IBMON3 92 100 108 A/A 50 mA < IBIAS < 100 mA
IBIAS/IBMON Stability3, 6 ±5 % 10 mA < IBIAS < 100 mA
IMOD/IMMON 50 A/A
IBMON Compliance Voltage 0 1.3 V
ADN2872
Rev. 0 | Page 4 of 20
Parameter Min Typ Max Unit Conditions/Comments
SUPPLY
ICC7 30 mA When IBIAS = IMOD = 0
VCC (with respect to GND)8 3.0 3.3 3.6 V
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 into 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 See the Power Consumption section for ICC minimum for power calculation.
8 All VCC pins should be shorted together.
08013-034
ADN2872
IMODP
BIAS TEE
80kHz → 27GHz
VCC VCC
TO HIGH SPEED
DIGITAL
OSCILLOSCOPE
50Ω INPUT
R
22ΩC
L
Figure 2. High Speed Electrical Test Output Circuit
SFP TIMING SPECIFICATIONS
Table 2.
Parameter Symbol Min Typ Max Unit Conditions/Comments
ALS Assert Time t_off 1 5 μs Time for the rising edge of ALS (Tx_DISABLE) to
when the bias current falls below 10% of nominal
ALS Negate Time1 t_on 0.83 0.95 ms
Time for the falling edge of ALS to when the
modulation current rises above 90% of nominal
Time to Initialize, Including Reset of FAIL1 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.
DATAP
DATAN
DATAP – DATAN Vp-p, DI FF = 2
×
VSE
VSE
0V
08013-002
Figure 3. Signal Level Definition
08013-003
0.1µF 0.1µF 10µF
1µH 3.3V
SFP HO S T BO ARD
SFP MODULE
VCC_Tx
Figure 4. Recommended SFP Supply
ADN2872
Rev. 0 | Page 5 of 20
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, ERCAP, PAVSET, PAVREF, ERREF,
IBIAS, IBMON, IMMON, ALS, CCBIAS, RPAV,
ERSET, FAIL
−0.3 V to +3.9 V
DATAP, DATAN (Single-Ended Differential) 1.5 V
Junction Temperature (TJ max) 125°C
Operating Temperature Range, Industrial −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Power Dissipation (W)1 (TJ max − TA)/θJA
θJA Thermal Impedance2 48.6°C/W
θJC Thermal Impedance2 5.0°C/W
θJB Thermal Impedance2 28.4°C/W
1 Power consumption equations are provided in the Power Consumption
section.
2 θJA, θJB, and θJC are estimated when the part’s exposed pad is soldered on a
4-layer JEDEC board at zero airflow.
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 indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
ADN2872
Rev. 0 | Page 6 of 20
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
GND
1924
181
12
7136
FAIL
IBMON
ERREF
IMMON
ERSET
VCC
CCBIAS
PAVSET
VCC
PAVREF
RPAV
GND
VCC
IMODP
IMODN
GND
IBIAS
ALS
DATAN
DATAP
GND
ADN2872
TOP V IEW
(No t to Scal e)
PAVCAP
ERCAP
NOTES
1. T HE LF CSP PACKAGE HAS AN EXP OSE D PADDLE T HAT
MUST BE CONNECT ED TO GROUND.
08013-004
Figure 5. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1 CCBIAS Control Output Current.
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 Extinction Ratio Loop Capacitor.
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 (Current Sink), Connect to Laser Diode.
22 IMODN Modulation Current Negative Output (Current Sink).
23 GND Supply Ground.
24 IBIAS Laser Diode Bias (Current Sink to Ground).
25 (EPAD) Exposed Paddle (EPAD) The LFCSP package has an exposed paddle that must be connected to ground.
ADN2872
Rev. 0 | Page 7 of 20
TYPICAL PERFORMANCE CHARACTERISTICS
90
0
30
60
0408020 60 100
08013-022
MO DUL ATIO N CURRENT (mA)
RIS E T I M E ( p s)
Figure 6. Rise Time vs. Modulation Current, IBIAS = 20 mA
0
20
40
60
80
0408020 60 100
08013-025
MO DUL ATIO N CURRENT (mA)
FALL TIME (ps)
Figure 7. Fall Time vs. Modulation Current, IBIAS = 20 mA
0
45
40
35
30
25
20
15
10
5
20 40 8060 100
08013-042
MODUL AT I ON CURRENT (mA)
DETERMINISTIC JITTER (ps)
Figure 8. Deterministic Jitter vs. Modulation Current, IBIAS = 20 mA
0
1.2
1.0
0.8
0.6
0.4
0.2
0 20406080100
08013-037
MO DUL ATIO N CURRENT (mA)
JIT TER (rms)
Figure 9. Random Jitter vs. Modulation Current, IBIAS = 20 mA
40
250
190
220
160
130
100
70
0 20406080100
08013-038
MO DUL ATIO N CURRENT (mA)
TOTAL SUPP LY CURRENT (mA)
I
BIAS
= 20mA
I
BIAS
= 40mA
I
BIAS
= 80mA
Figure 10. Total Supply Current vs. Modulation Current,
Total Supply Current = ICC + IBIAS + IMOD
20
60
55
50
45
40
35
30
25
–50 –30 –10 10 30 50 70 90 110
08013-027
TE MPE RAT URE ( ° C)
SUPPLY CURRENT ( mA)
Figure 11. Supply Current (ICC) vs. Temperature with ALS Asserted, IBIAS = 20 mA
ADN2872
Rev. 0 | Page 8 of 20
80
120
115
110
105
100
95
90
85
–50 –30 –10 10 30 50 70 90 110
08013-028
TE MPE RAT URE ( ° C)
I
BIAS
/I
BMON
GAIN
Figure 12. IBIAS/IBMON Gain vs. Temperature, IBIAS = 20 mA
08013-029
OC48 P R BS 31
DATA T RANSM ISSION
ALS
t_OFF
LESS THAN 1µs
Figure 13. ALS Assert Time, 5 μs/DIV
08013-032
ALS
t
_ON
OC48 P R BS 31
DATA T RANSM ISSION
Figure 14. ALS Negate Time, 200 μs/DIV
40
44
48
52
56
60
42
46
50
54
58
–50 –30 –10 10 30 50 70 90 110
08013-031
TEMPERATURE (°C)
IMOD/IMMON GAIN
Figure 15. IMOD/IMMON Gain vs. Temperature, IMOD = 30 mA
08013-045
FAULT FO RCE D ON PAVSET
FAIL ASSERTED
Figure 16. FAIL Assert Time,1 μs/DIV
08013-046
POWER SUP PLY TURN O N
TRANSMISSIO N ON
Figure 17. Time to Initialize, Including Reset, 40 ms/DIV
ADN2872
Rev. 0 | Page 9 of 20
OPTICAL WAVEFORMS
VCC = 3.3 V and TA = 25°C, unless otherwise noted. Note that there was no change to PAVCAP and ERCAP values when different data
rates were tested. Figure 18, Figure 19, and Figure 20 show multirate performance using the low cost Fabry Perot TOSA NEC NX7315UA;
Figure 21 and Figure 22 show dual-loop performance over temperature using the DFB TOSA Sumitomo SLT2486.
(ACQ LI MI T TEST) WA VEFO R M S 1 00
0
08013-016
Figure 18. Optical Eye 2.488 Gbps, 65 ps/DIV, PRBS 231 − 1,
PAV = −4.5 dBm, ER = 9 dB, Mask Margin 25%
(ACQ LIMIT TEST) WAVEFORM S 100
0
08013-017
Figure 19. Optical Eye 622 Mbps, 264 ps/DIV, PRBS 231 − 1,
PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50%
(ACQ LI MI T TEST) WA VEFO R M S 1 00
0
08013-020
Figure 20. Optical Eye 155 Mbps,1.078 ns/DIV, PRBS 231 − 1,
PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50%
(ACQ LIMIT TEST) WAVEFORM S 1001
08013-047
Figure 21. Optical Eye 2.488 Gbps, 65 ps/DIV, PRBS 231 − 1,
PAV = 0 dBm, ER = 9 dB, Mask Margin 22%, TA = 25°C
(ACQ LIMIT TEST) WAVEFORM S 1001
08013-048
Figure 22. Optical Eye 2.488 Gbps, 65 ps/DIV, PRBS 231 − 1,
PAV = −0.2 dBm, ER = 8.96 dB, Mask Margin 21%, TA = 85°C
ADN2872
Rev. 0 | Page 10 of 20
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.
OPTICAL PO WER
P1
P
AV
P0
I
TH
CURRENT
P
AV
=
ΔP
ΔI
ER = P1
P0
2
P1 + P0
LI = ΔP
ΔI
08013-005
Figure 23. Laser Transfer Function
DUAL-LOOP 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 laser
changing LI characteristics.
Single-loop compensation schemes use the average monitor
photodiode (MPD) current to measure and maintain the
average optical output power over temperature and laser aging.
The ADN2872 is a dual-loop device, implementing both this
primary average power control loop and a secondary control
loop, which maintains a constant optical extinction ratio. The
dual-loop control of the average power and extinction ratio
implemented in the ADN2872 can be used successfully with
both lasers that maintain good linearity of LI transfer characteris-
tics over temperature, and with those that exhibit increasing
nonlinearity of the LI characteristics over temperature.
Dual Loop
The ADN2872 uses a proprietary patented method to control both
average power and extinction ratio. The ADN2872 is constantly
sending a test signal on the modulation current signal and
reading the resulting change in the MPD current as a means of
detecting the slope of the laser in real time. This information is
used in a servo to control the ER of the laser, which is done in a
time-multiplexed manner at a low frequency, typically 80 Hz.
Figure 24 shows the dual-loop control implementation on the
ADN2872.
08013-039
ERSET
MPD
INPUT
I
PA
PAVSET
I
EX
Φ
1
Φ
2
Φ
2
Φ
2
OPTI CAL COUP LING
BIAS
SHA
MOD
SHA
MOD
CURRENT
Gm
1.2V
V
BGAP
100 2
BIAS
CURRENT
V
CC
HIGH
SPEED
SWITCH
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
A dual loop is made up of an average power control loop
(APCL) and the extinction ratio control loop (ERCL), which are
separated into two time states. During Time Φ1, the APC loop
is operating, and during Time Φ2, the ER loop is operating.
Average Power Control Loop
The APCL compensates for changes in the laser diode (LD), ITH
and LI, by varying IBIAS. APC control is performed by measuring
the MPD current, IMPD. This current is bandwidth limited by the
MPD. This is not a problem because the APCL must be low
frequency and the APCL must respond to the average current
from the MPD. The APCL compares IMPD × RPAVSET to the BGAP
voltage, VBGAP. If IMPD falls, the bias current is increased until
IMPD × RPAVSET equals VBGAP. Conversely, if the IMPD increases, IBIAS
is decreased.
Modulation Control Loop
The ERCL measures the slope efficiency, LI, of the laser diode
by monitoring the IMPD changes. During the ERCL, IMPD is
temporarily increased by ΔIMOD. The ratio between IMPD and
ΔIMOD is a fixed ratio of 50:1, but during startup, this ratio is
increased to decrease settling time.
During ERCL, switching in ΔIMOD causes a temporary increase
in average optical power, ΔPAV. However, the APC loop is disabled
during ERCL, and the increase is kept small enough so as not to
disturb the optical eye. When ΔIMOD is switched into the laser
circuit, an equal current, IEX, is switched into the PAVSET resis-
tor. The user sets the value of IEX; this is the ERSET setpoint. If
ΔIMPD is too small, the control loop knows that LI has decreased,
and increases IMPD and, therefore, ΔIMOD accordingly until ΔIMPD
is equal to IEX. The previous control cycle status of the IBIAS and IMOD
settings are stored on the hold capacitors, PAVCAP and ERCAP.
The ERCL is constantly measuring the actual LI curve; it compen-
sates for the effects of temperature and for changes in the LI
curve due to laser aging. Therefore, the laser can be calibrated
once at 25°C so that it can then automatically control the laser
over temperature. This eliminates the expensive and time
consuming temperature calibration of a laser.
ADN2872
Rev. 0 | Page 11 of 20
Operation with Lasers with Temperature-Dependent
Nonlinearity of Laser LI Curve
The ADN2872 ERCL extracts information from the monitor
photodiode signal relating to the slope of the LI characteristics
at the Optical 1 level (P1). For lasers with good linearity over
temperature, the slope measured by the ADN2872 at the Optical 1
level is representative of the slope anywhere on the LI curve.
This slope information is used to set the required modulation
current to achieve the required optical extinction ratio.
0
0.5
3.0
2.5
2.0
1.5
1.0
4.0
3.5
20 40 60
CURRENT ( mA)
OPT I CAL P O W E R (mW )
10080
08013-008
RELATIV ELY LINE AR LI CURV E AT 2 5° C
NONL INEAR L I CURVE AT 8 0° C
0
Figure 25. Measurement of a Laser LI Curve Showing
Laser Nonlinearity at High Temperatures
Some types of lasers have LI curves that become progressively
more nonlinear with increasing temperature (see Figure 25). At
temperatures where the LI curve shows significant nonlinearity,
the LI curve slope measured by the ADN2872 at the Optical 1
level is no longer representative of the overall LI curve. It is
evident that applying a modulation current based on this slope
information cannot maintain a constant extinction ratio over
temperature.
However, the ADN2872 can be configured to maintain near
constant optical bias and an extinction ratio with a laser
exhibiting a monotonic temperature-dependent nonlinearity.
To implement this correction, it is necessary to characterize a
small sample of lasers for their typical nonlinearity by measur-
ing them at two temperature points, typically 25°C and 85°C.
The measured nonlinearity is used to determine the amount of
feedback to apply.
Typically, the user must characterize five to 10 lasers of a particular
model to obtain a good number. The product can then be cali-
brated at 25°C only, avoiding the expense of temperature
calibration. Typically, the microcontroller is used to measure
the laser and apply the feedback. This scheme is particularly
suitable for circuits that already use a microcontroller for control
and digital diagnostic monitoring.
The ER correction scheme, while using the average nonlinearity
for the laser population, supplies a corrective measurement
based on the actual performance of each laser as measured during
operation. The ER correction scheme corrects for errors due to
laser nonlinearity while the dual loop continues to adjust for
changes in the Laser LI.
For more details on maintaining average optical power and
extinction ratio over temperature when working with lasers
displaying a temperature-dependent nonlinearity of LI curve,
contact sales at Analog Devices.
CONTROL
The ADN2872 has two methods for setting the average power
(PAV) and extinction ratio (ER). The average power and extinc-
tion ratio can be voltage set using the voltage DAC outputs of a
microcontroller to provide controlled reference voltages to
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 ADN2872 allows an interface to a microcontroller for both
control and monitoring (see Figure 26). The average power at
the PAVSET pin and extinction ratio at the ERSET pin can be
set using the DAC of the microcontroller to provide controlled
reference voltages to PAVREF and ERREF. Note that during
power-up, there is an internal sequence that allows 25 ms before
enabling the alarms; therefore, the user must ensure that the
voltage for PAVREF and ERREF are active within 20 ms.
PAVREF = PAV × RSP × RPAV (V)
AV
CW
CWMPD
ERSET P
ER
ER
P
I
RERREF
1
1
_ (V)
where:
PAV (mW) is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
RSP (A/W) is the monitor photodiode responsivity.
IMPD_CW (mA) is the MPD current at that specified PCW.
PCW (mW) is the dc optical power specified on the laser data sheet.
In voltage setpoint, RPAV and RERSET must be 1 kΩ resistors with
a 1% tolerance and a temperature coefficient of 50 ppm/°C.
ADN2872
Rev. 0 | Page 12 of 20
08013-009
ANALO G DEVICES
MICROCONTROLLER
Tx_FAULT
Tx_FAIL
CONTROL
PAVREF
PAVSET
ADN2872
RPAV
GND
DAC
DAC
V
CC
MPD
ERREF
ERSET
GND
I
MOD
DATAP
IMODP
DATAN
I
BIAS
IMODN
IBMON IMMON
ALSFAIL
IBIAS
100Ω
CCBIAS
V
CC
LASER
L
R
V
CC
V
CC
VCC
GND GND
PAVCAP
470Ω1kΩ
GND
GND
ERCAP
GND
1kΩ
1kΩ
ADC
V
CC
V
CC
R
Z
Figure 26. ADN2872 Using MicroConverter Calibration and Monitoring
L
V
CC
08013-010
CONTROL
ADN2872
ERSET
V
CC
ERREF
V
CC
RPAV
PAVREF
V
CC
MPD
I
MOD
DATAP
IMODP
DATAN
I
BIAS
IMODN
IBMON IMMON
ALSFAIL
IBIAS
V
CC
LASER
V
CC
VCC
GND GND
PAVCAP
GND
GND
ERCAP
GND
GND
GND
PAVSET
100Ω
CCBIAS
R
470Ω1kΩ
V
CC
V
CC
R
Z
Figure 27. ADN2872 Using Resistor Setpoint Calibration of Average Power and Energy Ratio
ADN2872
Rev. 0 | Page 13 of 20
RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration, the PAVREF, ERREF, and RPAV
pins 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 27. The ADN2872
ensures that both PAVSET and ERSET are kept 1.2 V above
GND. The PAVSET and ERSET resistors are given by:
SP
A
V
PAVSET RP
R
V23.1 (Ω)
AV
C
W
CWMPD
ERSET
P
ER
ER
P
I
R
1
1
V23.1
_
(Ω)
where:
PAV (mW) is the average power required.
RSP (A/W) is the monitor photodiode responsivity.
PCW (mW) is the dc optical power specified on the laser data
sheet.
IMPD_CW (mA) is the MPD current at that specified PCW.
ER is the desired extinction ratio (ER = P1/P0).
IMPD MONITORING
IMPD monitoring can be implemented for voltage setpoint and
resistor setpoint as follows.
Voltage Setpoint
In voltage setpoint calibration, the following methods can 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 28) 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 shutdown or
fail mode. A microconverter-buffered ADC input can be con-
nected 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.
08013-043
V
CC
PHOTODIODE
ADN2872
R
1kΩ
PAVSET
RPAV
MICROCONVERTER
ADC
INPUT
Figure 28. Single Measurement of IMPD at RPAV in Voltage Setpoint Mode
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
differential measurement across a sense resistor directly in series
with the IMPD. As shown in Figure 29, 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 30.
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 ADC to read, but small enough 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.
08013-011
V
CC
LDPHOTODIODE
MICROCONVERTER
ADC
DIFFERENTIAL
INPUT
Rx
200Ω10pF
PAVSET
ADN2872
Figure 29. Differential Measurement of IMPD Across a Sense Resistor
08013-012
V
CC
V
CC
LD
PHOTODIODE
MICROCONVERTER
ADC
INPUT
PAVSET
ADN2872
Rx
200Ω
Figure 30. Single Measurement of IMPD Across a Sense Resistor
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 the PAVSET resistor (or potentiometer)
and measure the voltage across this resistor, as shown in Figure 31.
The IMPD current is then equal to this voltage divided by the
value of the 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.
ADN2872
Rev. 0 | Page 14 of 20
0
8013-040
V
CC
PHOTODIODE
ADN2872
PAVSET
R
MICROCONVERTER
ADC
INPUT
Figure 31. Single Measurement of IMPD Across a
Sense Resistor in Resistor Setpoint IMPD Monitoring
LOOP BANDWIDTH SELECTION
To ensure that the ADN2872 control loops have sufficient
bandwidth, the average power loop capacitor (PAVCAP) and
the extinction ratio loop capacitor (ERCAP) are calculated
using the laser slope efficiency and the average power required.
For resistor setpoint control,
AV
P
LI
PAVCAP 6
102.3 (Farad)
2
PAVCAP
ERCAP (Farad)
For voltage setpoint control,
AV
P
LI
PAVCAP 6
1028.1 (Farad)
2
PAVCAP
ERCAP (Farad)
where:
PAV (mW) is the average power required.
LI (mW/mA) is the typical slope efficiency at 25°C of a batch of
lasers that are used in a design.
The preceding capacitor estimation formulas are used to obtain
a centered value for the particular type of laser that is used in a
design and average power setting. 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 the ac coupling configuration, LI can be
calculated as
MOD
I
P0P1
LI
(mW/mA)
where P1 is the optical power (mW) at the one level, and P0 is
the optical power (mW) at the zero level.
These capacitors are placed between the PAVCAP and ERCAP
pins and ground. It is important that these capacitors are low
leakage multilayer ceramics with an insulation resistance
greater than 100 GΩ or a time constant of 1000 sec, whichever
is less. The capacitor tolerance can be ±30% from the calculated
value to the available off-the-shelf value, including the capacitor’s
own tolerance.
POWER CONSUMPTION
The ADN2872 die temperature must be kept below 125°C. The
LFCSP package has an exposed paddle that should be connected
such that it is at the same potential as the ADN2872 ground pins.
Power consumption can be calculated as:
ICC = ICC min + 0.3 IMOD
P = VCC × ICC + (IBIAS × VBIAS_PIN) + IMOD (VMODP_PIN +
VMODN_PIN)/2
TDIE = TAMBIENT + θJA × P
where:
ICC min is 30 mA, the typical value of ICC provided in Table 1
with IBIAS = IMOD = 0.
TDIE is the die temperature.
TAMBIENT is the ambient temperature.
VBIAS_PIN is the voltage at the IBIAS pin.
VMODP_PIN is the voltage at the IMODP pin.
VMODN_PIN is the voltage at the IMODN pin.
Thus, the maximum combination of IBIAS + IMOD must be
calculated.
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 specifications.
When ALS is logic high or 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 micro-
controller, 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.
IBIAS PIN
ADN2872 has one on-chip, 800 Ωpull-up resistor. The current
sink from this resistor is VIBIAS dependent.
8.0
IBIASCC
UP
VV
I
(mA)
where VIBIAS is the voltage measured at the IBIAS pin after setup
of one laser bias current, IBIAS. Usually, when set up, a maximum
laser bias current of 100 mA results in a VIBIAS of about 1.2 V. In
a worst-case scenario, VCC = 3.6 V, VIBIAS = 1.2 V, and IUP ≤ 3 mA.
This on-chip resistor helps to damp out the low frequency
oscillation observed from some inexpensive lasers. If the on-
chip resistance does not provide enough damping, one external
RZ may be necessary (see Figure 32 and Figure 33).
ADN2872
Rev. 0 | Page 15 of 20
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.
LASER DIODE INTERFACING
The schematic in Figure 32 describes the recommended circuit
for interfacing the ADN2872 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 Figure 18,
Figure 19, and Figure 20 in the Typical Performance Characteristics
section 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 32 operates by delibe-
rately 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 vs. frequency and enables multirate operation. A series
damping resistor should not be used.
08013-014
L
BLM18HG601SN1D
C1
100nF
R
P
24Ω
ADN2872
IBIAS
CCBIAS
IMODP
V
CC
L (0.5nH)
R
24Ω
C
2.2pF
Tx LINE
30Ω
Tx LINE
30Ω
V
CC
V
CC
R
Z
Figure 32. Recommended Interface for ADN2872 AC Coupling
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 compo-
nent values. The R and C snubber values in Figure 32, 24 Ω and
2.2 pF, respectively, represent a starting point and must be
tuned for the particular model of laser being used. RP, 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.
Take care 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 is
very inductive (approximately 1 nH per millimeter). Long
exposed leads result in slower edge rates and reduced eye margin.
Recommended component layouts and gerber files are available
by contacting sales at Analog Devices. Note that the circuit in
Figure 32 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. 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 sales for
recommendations on transmission lines and backmatch values.
The ADN2872 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 33.
In Figure 32 and Figure 33, Resistor RZ is required to achieve
optimum eye quality. The recommended value is approximately
200 Ω ~ 500 Ω.
08013-041
L3 = 4.7nH
L4 = BLM18HG601SN1D
V
CC
L6 = BLM18HG601SN1D
SNUBBER SETT INGS : 40Ω AND 1. 5p F, NOT OPT IMIZE D, OPTIMIZATION SHOULD
CONSIDER T HE P ARASIT IC OF THE INT ERFACE CIRCUIT RY .
L5 = 4.7n H
V
CC
R1 = 15 Ω
CCBIAS IBIAS
IMODN
IMODP
ADN2872
C1 = C2 = 100nF
20Ω TRANMISSION LINES
R2 = 15 Ω
(12Ω TO 24Ω)
R3 C3
SNUBBER LIGHT
TO-CAN/VCSEL
L1 = 0.5nH
L2 = 0.5nH
V
CC
R
Z
Figure 33. Recommended Differential Drive Circuit
ADN2872
Rev. 0 | Page 16 of 20
ALARMS
The ADN2872 has a latched active high monitoring alarm
(FAIL). The FAIL alarm output is an open drain in conformance
with SFP MSA specification requirements.
The ADN2872 has a three-fold alarm system that covers:
Use of a bias current higher than expected, likely as a result
of laser aging.
Out-of-bounds average voltage at the monitor photodiode
(MPD) input, indicating an excessive amount of laser
power or a broken loop.
Undervoltage in the IBIAS pin (laser diode cathode) that
increases 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.
Table 5. ADN2872 Single-Point Alarms
Alarm Type Mnemonic Overvoltage or Short to VCC Condition
Undervoltage or Short to
GND Condition
Bias Current IBMON Alarm if > 1.2 V Ignore
MPD Current PAVSET Alarm if > 2.0 V Alarm if < 0.4 V
Crucial Nodes ERREF (the ERRREF is designed tied to
VCC in resistor setting mode)
Alarm if shorted to VCC (the alarm is valid for
voltage setting mode only)
Alarm if shorted to GND
IBIAS Ignore Alarm if < 0.6 V
Table 6. ADN2872 Response to Various Single-Point Faults in AC-Coupled Configuration, as Shown in Figure 32
Mnemonic 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 Voltage mode: fault state occurs Fault state occurs Fault state occurs
Resistor mode: tied to VCC
RPAV Voltage mode: fault state occurs Fault state occurs Voltage mode: fault state occurs
Resistor mode: tied to VCC Resistor mode: does not increase
average power
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 Voltage mode: fault state occurs Voltage mode: does not increase
average power
Does not increase laser average power
Resistor mode: tied to VCC Resistor mode: fault state occurs
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
ADN2872
Rev. 0 | Page 17 of 20
OUTLINE DIMENSIONS
*COMP L I ANT T O JEDE C ST ANDARDS M O -220- VG GD-2
EXCEPT FOR EXPOSED PAD DIMENSION
1
24
6
7
13
19
18
12
*2.45
2.30 S Q
2.15
0.60 M A X
0.50
0.40
0.30
0.30
0.23
0.18
2.50 RE F
0.50
BSC
12° MA X 0.80 M A X
0.65 TYP 0.05 M A X
0.02 NOM
1.00
0.85
0.80
SEATING
PLANE
PIN 1
INDICATOR TOP
VIEW 3.75
BSC SQ
4.00
BSC SQ PIN 1
INDICATOR
0.60 M A X
COPLANARITY
0.08
0.20 REF
0.23 M IN
EXPOSED
PA D
(BOTTOMVIEW)
FO R PRO P ER CONNE CT I O N O F
THE EXPOSED PAD, REFER TO
THE P IN CONFIGURATIO N AND
FUNCT ION DE S CRIPT IONS
SECT ION OF THIS DATA SHEE T .
080808-A
Figure 34. 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
4 mm × 4 mm Body, Very Thin Quad
(CP-24-2)
Dimensions shown in millimeters
ORDERING GUIDE
Model Temperature Range Package Description Package Option Ordering Quantity
ADN2872ACPZ1 −40°C to +85°C 24-Lead LFCSP_VQ CP-24-2 490
ADN2872ACPZ-RL1 −40°C to +85°C 24-Lead LFCSP_VQ CP-24-2 5,000
ADN2872ACPZ-R71 −40°C to +85°C 24-Lead LFCSP_VQ CP-24-2 1,500
1 Z = RoHS Compliant Part.
ADN2872
Rev. 0 | Page 18 of 20
NOTES
ADN2872
Rev. 0 | Page 19 of 20
NOTES
ADN2872
Rev. 0 | Page 20 of 20
NOTES
©2009 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D08013-0-3/09(0)
