3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps
Laser Diode Driver
Data Sheet
ADN2870
Rev. C Document Feedback
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FEATURES
SFP/SFF MSA and SFF-8472 compliant
SFP reference design available
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
RoHS compliant
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
The ADN28701 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
to be calibrated at 25°C, eliminating the expensive and time
consuming temperature calibration. The ADN2870 supports
single-rate operation from 50 Mbps to 3.3 Gbps or multirate
operation from 155 Mbps to 3.3 Gbps.
Average power and extinction ratios can be set with reference
voltages provided by a microcontroller DAC or by adjustable
resistors. The ADN2870 provides bias and modulation current
monitoring as well as fail alarms and automatic laser shutdown.
The device interfaces easily with the ADuC702x 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 available in a
space-saving 4 mm × 4 mm LFCSP specified over the −40°C to
+85°C temperature range.
1 Protected by U.S. Patent 6,414,974.
APPLICATIONS DIAGRAM
04510-001
ANALOG DEVICES
MICROCONTROLLER
Tx_FAULT
Tx_FAIL
CONTROL
PAVREF
PAVSET
ADN2870
RPAV
GND
DAC
DAC
V
CC
MPD
ERREF
ERSET
GND
IMOD
DATAP
IMODP
DATAN
IBIAS
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 1. Application Diagram Showing Microcontroller Interface
ADN2870 Data Sheet
Rev. C | 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 ............................................................... 5
Absolute Maximum Ratings ............................................................ 6
ESD Caution .................................................................................. 6
Pin Configuration and Function Descriptions ............................. 7
Typical Performance Characteristics ............................................. 8
Optical Waveforms ......................................................................... 10
Multirate Performance Using Low Cost Fabry Perot Tosa
NEC NX7315UA ........................................................................ 10
Dual-Loop Performance Over Temperature Using DFB Tosa
SUMITOMO SLT2486 ............................................................... 10
Theory of Operation ...................................................................... 11
Dual-Loop Control .................................................................... 11
Control ......................................................................................... 12
Voltage Setpoint Calibration ..................................................... 12
Resistor Setpoint Calibration .................................................... 14
IMPD Monitoring .......................................................................... 14
Loop Bandwidth Selection ........................................................ 15
Power Consumption .................................................................. 15
Automatic Laser Shutdown (Tx_Disable) ............................... 15
Bias and Modulation Monitor Currents .................................. 15
Data Inputs .................................................................................. 15
Laser Diode Interfacing ............................................................. 16
Alarms .......................................................................................... 17
Outline Dimensions ....................................................................... 18
Ordering Guide .......................................................................... 18
REVISION HISTORY
11/2017—Rev B to Rev. C
Changed CP-24-14 to CP-24-2 .................................... Throughout
Updated Outline Dimensions ....................................................... 18
Changes to Ordering Guide .......................................................... 18
8/2016—Rev A to Rev. B
Changed CP-24-2 to CP-24-14 .................................... Throughout
Changes to Table 4 ............................................................................ 7
Updated Outline Dimensions ....................................................... 18
Changes to Ordering Guide .......................................................... 18
3/2008—Rev 0 to Rev. A
Changes to Features, General Description, and Figure 1 ............ 1
Changes to Table 3 ............................................................................ 6
Changes to Figure 5 .......................................................................... 7
Reorganized Layout .......................................................................... 8
Changes to Modulation Control Loop Section .......................... 11
Changes to Operation with Lasers with Temperature-
Dependent Nonlinearity of Laser LI Curve Section .................. 12
Changes to Voltage Setpoint Calibration Section ...................... 12
Changes to Figure 26 and Figure 27 ............................................. 13
Changes to Resistor Setpoint Calibration Section and IMPD
Monitoring Section ........................................................................ 14
Changes to Loop Bandwidth Selection Section ......................... 15
Changes to Laser Diode Interfacing Section, Figure 32, and
Figure 33 .......................................................................................... 16
Changes to Table 5 .......................................................................... 17
Updated Outline Dimensions ....................................................... 18
Changes to Ordering Guide .......................................................... 18
8/2004—Revision 0: Initial Version
Data Sheet ADN2870
Rev. C | 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 (PAVSE T )
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
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)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
ADN2870 Data Sheet
Rev. C | 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 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 minimum for power calculation (see the Power Consumption section).
8 All VCC pins should be shorted together.
04510-034
ADN2870
IMODP
BIAS TEE
80kHz 27GHz
V
CC
V
CC
C
L
Figure 2. High Speed Electrical Test Output Circuit
Data Sheet ADN2870
Rev. C | Page 5 of 20
SFP TIMING SPECIFICATIONS
Table 2.
Parameter Symbol Min Typ Max Unit Conditions/Comments
ALS Assert Time t_OFF 1 5 μs
Time from 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 from 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 Time from power-on or negation of FAIL using ALS.
FAIL Assert Time t_FAULT 100 μs Time from fault to FAIL on.
ALS to Reset Time t_RESET 5 μs Tx_DISABLE must be held high to reset Tx_FAULT.
1 Guaranteed by design and characterization. Not production tested.
04510-002
DATAP
DATAN
DATAP–DATAN
V p-p
DIFF
= 2
×
V
SE
V
SE
0V
Figure 3. Signal Level Definition
04510-003
0.1F 0.1F 10F
1H
3.3V
SFP HOST BOARD
SFP MODULE
VCC_Tx
Figure 4. Recommended SFP Supply
ADN2870 Data Sheet
Rev. C | Page 6 of 20
ABSOLUTE MAXIMUM RATINGS
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
D ATA P, DATAN (Single-Ended Differential) 1.5 V
Junction Temperature
150°C
Operating Temperature Range
Industrial −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Junction Temperature (TJ max) 125°C
LFCSP Package
Power Dissipation (W)1 (TJ max − TA)/θJA
θJA Thermal Impedance2 30°C/W
θJC Thermal Impedance 29.5°C/W
1 Power consumption equations are provided in the Power Consumption
section.
2 θJA is defined when the part is soldered on a 4-layer board.
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
ESD CAUTION
Data Sheet ADN2870
Rev. C | Page 7 of 20
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
04510-004
GND
FAIL
IBMON
ERREF
IMMON
ERSET
VCC
CCBIAS
PAVSET
VCC
PAVREF
RPAV
GND
VCC
IMODP
IMODN
GND
IBIAS
ALS
DATAN
DATAP
GND
PAVCAP
ERCAP
1
2
3
4
5
6
15
16
17
18
14
13
7
8
9
11
12
10 21
22
23
24
20
19
NOTES
1. THE LFCSP PACKAGE HAS AN EXPOSED PAD
THAT MUST BE CONNECTED TO GROUND.
ADN2870
TOP VIEW
(Not to Scale)
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)
EPAD Exposed Pad. The LFCSP package has an exposed paddle that must be connected to ground.
ADN2870 Data Sheet
Rev. C | Page 8 of 20
TYPICAL PERFORMANCE CHARACTERISTICS
90
0
30
60
040 80
20 60 100
04510-022
MODULATION CURRENT (mA)
RISE TIME (ps)
Figure 6. Rise Time vs. Modulation Current, IBIAS = 20 mA
0
20
40
60
80
0 40 8020 60 100
04510-025
MODULATION 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
04510-042
MODULATION 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 20 40 60 80 100
04510-037
MODULATION CURRENT (mA)
JITTER (rms)
Figure 9. Random Jitter vs. Modulation Current, IBIAS = 20 mA
40
250
190
220
160
130
100
70
0 20 40 60 80 100
04510-038
MODULATION CURRENT (mA)
TOTAL SUPPLY 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
04510-027
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
Figure 11. Supply Current (ICC) vs. Temperature with ALS Asserted, IBIAS = 20 mA
Data Sheet ADN2870
Rev. C | Page 9 of 20
80
120
115
110
105
100
95
90
85
–50 –30 –10 10 30 50 70 90 110
04510-028
TEMPERATURE (°C)
IBIAS/IBMON RATIO
Figure 12. IBIAS/IBMON Gain vs. Temperature, IBIAS = 20 mA
04510-029
OC48 PRBS31
DATA TRANSMISSION
t_OFF LESS THAN 1µs
ALS
Figure 13. ALS Assert Time, 5 µs/DIV
04510-032
ALS
t_ON
OC48 PRBS31
DATA TRANSMISSION
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
04510-031
TEMPERATURE (°C)
IMOD/IMMON RATIO
Figure 15. IMOD/IMMON Gain vs. Temperature, IMOD = 30 mA
04510-045
FAULT FORCED ON PAVSET
FAIL ASSERTED
Figure 16. FAIL Assert Time,1 µs/DIV
04510-046
POWER SUPPLY TURN ON
TRANSMISSION ON
Figure 17. Time to Initialize, Including Reset, 40 ms/DIV
ADN2870 Data Sheet
Rev. C | Page 10 of 20
OPTICAL WAVEFORMS
VCC = 3.3 V and TA = 25°C, unless otherwise noted. Note that in Figure 18 through Figure 22, there is no change to the PAVCAP and
ERCAP values using either of the lasers or at any of the data rates tested.
MULTIRATE PERFORMANCE USING LOW COST
FABRY PEROT TOSA NEC NX7315UA
(ACQ LIMIT TEST) WAVEFORMS 1000
04510-016
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) WAVEFORMS 1000
04510-017
Figure 19. Optical Eye 622 Mbps, 264 ps/DIV, PRBS 231-1,
PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50%
(ACQ LIMIT TEST) WAVEFORMS 1000
04510-020
Figure 20. Optical Eye 155 Mbps,1.078 ns/DIV, PRBS 231-1,
PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50%
DUAL-LOOP PERFORMANCE OVER TEMPERATURE
USING DFB TOSA SUMITOMO SLT2486
(ACQ LIMIT TEST) WAVEFORMS 1001
04510-047
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) WAVEFORMS 1001
04510-048
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
Data Sheet ADN2870
Rev. C | Page 11 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.
04510-005
OPTICAL POWER
P1
P
AV
P
0
Ith CURRENT
P
AV
=
∆P
∆I
ER = P1
P
0
2
P1 + P
0
LI = ∆P
∆I
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 current to measure and maintain the average
optical output power over temperature and laser aging. The
ADN2870 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 ADN2870 can be used successfully both
with lasers that maintain good linearity of LI transfer character-
istics over temperature and with those that exhibit increasing
nonlinearity of the LI characteristics over temperature.
Dual Loop
The ADN2870 uses a proprietary patented method to control
both average power and extinction ratio. The ADN2870 is
constantly sending a test signal on the modulation current
signal and reading the resulting change in the monitor photo-
diode (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 ADN2870.
04510-039
ERSET
MPD
INPUT
I
PA
PAVSET
I
EX
1
2
2
2
OPTICAL COUPLING
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 Ith and LI by varying
IBIAS. APC control is performed by measuring MPD current,
IMPD. This current is bandwidth limited by the MPD. This is not
a problem because the APCL must be low frequency; 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 × RPAVSE T equals VBGAP.
Conversely, if the IMPD increases, IBIAS is decreased.
Modulation Control Loop
The ERCL measures the slope efficiency, LI, of the LD and
changes IMPD as LI changes. During the ERCL, IMPD is tempor-
arily 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
resistor. 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 time state values of the bias and mod
settings are stored on the hold capacitors, PAVCAP and ERCAP.
The ERCL is constantly measuring the actual LI curve; there-
fore, it compensates 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 and can then automatically
control the laser over temperature. This eliminates expensive
and time consuming temperature calibration of the laser.
ADN2870 Data Sheet
Rev. C | Page 12 of 20
Operation with Lasers with Temperature-Dependent
Nonlinearity of Laser LI Curve
The ADN2870 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 ADN2870 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)
OPTICAL POWER (mW)
10080
04510-008
RELATIVELY LINEAR LI CURVE AT 25°C
NONLINEAR LI CURVE AT 80°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 ADN2870 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 ADN2870 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
measuring 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, one must characterize 5 to 10 lasers of a particular
model to get a good number. The product can then be cali-
brated at 25°C only, avoiding the expense of temperature
calibration. Typically, the microcontroller supervisor is used
to measure the laser and apply the feedback. This scheme is
particularly suitable for circuits that already use a microcon-
troller for control and digital diagnostic monitoring.
The ER correction scheme, while using the average nonlinearity
for the laser population, in fact, supplies a corrective measure-
ment based on each laser’s actual performance 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,
refer to the Application Note AN-743 available through Analog
Devices sales.
CONTROL
The ADN2870 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 DAC outputs 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 ADN2870 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 microcontroller’s DAC to provide controlled
reference voltages PAVREF and ERREF. After power-on, the
ADN2870 starts an initial process that takes 25 ms before
enabling the alarms; therefore, the customer must ensure that
stable reference voltages to PAVREF and ERREF are available
within 20 ms.
PAVREF = PAV × RSP × RPAV (Volts)
AV
CW
CWMPD
ERSET
P
ER
ER
P
I
RERREF ×
+
−
××= 1
1
_
(Volts)
where:
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 the specified PCW.
PAV (mW) is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
In voltage setpoint, RPAV and RERSET must be 1 kΩ resistors with
a 1% tolerance and a temperature coefficient of 50 ppm/°C.
Data Sheet ADN2870
Rev. C | Page 13 of 20
04510-009
ANALOG DEVICES
MICROCONTROLLER
Tx_FAULT
Tx_FAIL
CONTROL
PAVREF
PAVSET
ADN2870
RPAV
GND
DAC
DAC
V
CC
MPD
ERREF
ERSET
GND
IMOD
DATAP
IMODP
DATAN
IBIAS
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. Using MicroConverter Calibration and Monitoring
L
V
CC
04510-010
CONTROL
ADN2870
ERSET
V
CC
ERREF
V
CC
RPAV
PAVREF
V
CC
MPD
IMOD
DATAP
IMODP
DATAN
IBIAS
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. Using Resistor Setpoint Calibration of Average Power and Energy Ratio
ADN2870 Data Sheet
Rev. C | Page 14 of 20
RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration, 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 ADN2870
ensures that both PAVSET and ERSET are kept 1.2 V above
GND. The resistors connected to PAVSET and ERSET are
given by
SPAV
PAVSET
RP
R×
=V23.1
(Ω)
AV
CW
CWMPD
ERSET
P
ER
ER
P
I
R
×
+
−
×
=
1
1
V23.1
_
(Ω)
where:
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 MPD current at that specified PCW.
PAV (mW) is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
IMPD MONITORING
IMPD monitoring can be implemented for voltage setpoint and
resistor setpoint as described in the sections that follow.
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 A/D 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.
04510-043
V
CC
PHOTODIODE
ADN2870
R
1kΩ
µC ADC
INPUT
PAVSET
RPAV
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 signifi-
cant 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.
04510-011
VCC
LD
PHOTODIODE
µC ADC
DIFFERENTIAL
INPUT
200Ω
RESISTOR 10pF
PAVSET
ADN2870
Figure 29. Differential Measurement of IMPD Across a Sense Resistor
04510-012
V
CC
V
CC
LD
PHOTODIODE
µC ADC
INPUT
200Ω
RESISTOR
PAVSET
ADN2870
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 resistor used. In resistor setpoint, PAVSET is held to
1.2 V nominal; it is recommended that the sense resistor be
selected so that the voltage across the sense resistor does not
exceed 250 mV.
Data Sheet ADN2870
Rev. C | Page 15 of 20
04510-040
VCC
PHOTODIODE
ADN2870
PAVSET
R
µC ADC
INPUT
Figure 31. Single Measurement of IMPD Across a
Sense Resistor in Resistor Setpoint IMPD Monitoring
LOOP BANDWIDTH SELECTION
To ensure that the ADN2870 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 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 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 may be ±30% from the calculated
value to the available off-the-shelf value, including the tolerance
of the capacitors.
POWER CONSUMPTION
The ADN2870 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 ADN2870 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
Thus, the maximum combination of IBIAS + IMOD must be
calculated.
where:
ICC min is 30 mA, the typical value of ICC provided in the
Specifications 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.
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 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.
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.
ADN2870 Data Sheet
Rev. C | Page 16 of 20
LASER DIODE INTERFACING
The schematic in Figure 32 describes the recommended circuit
for interfacing the ADN2870 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 Performance 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 32
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 vs. frequency and enables multi-
rate operation. A series damping resistor should not be used.
04510-014
L
BLM18HG601SN1D
C
100nF
R
P
24Ω
ADN2870
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 ADN2870 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.
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 is 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 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; contact sales at Analog Devices for recom-
mendations. 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.
The ADN2870 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 the circuits shown in Figure 32 and Figure 33, Resistor RZ is
required to achieve optimum eye quality. The recommended
value is approximately 200 Ω ~ 500 Ω.
04510-041
L3 = 4.7nH
L4 = BLM18HG601SN1D
V
CC
L6 = BLM18HG601SN1D
SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED,
OPTIMIZATION SHOULD CONSIDER PARASITIC.
L5 = 4.7nH
V
CC
R1 = 15Ω
CCBIAS IBIAS
IMODN
IMODP
ADN2870
C1 = C2 = 100nF
20Ω TRANMISSION LINES
R1 = 15Ω
(12 TO 24Ω)
R3 C3
SNUBBER LIGH
T
TO-CAN/VCSEL
L1 = 0.5nH
L2 = 0.5nH
V
CC
R
Z
Figure 33. Recommended Differential Drive Circuit
Data Sheet ADN2870
Rev. C | Page 17 of 20
ALARMS
The ADN2870 has a latched active high monitoring alarm
(FAIL). The FAIL alarm output is an open drain in conformance
to SFP MSA specification requirements.
The ADN2870 has a three-fold alarm system that recognizes:
• Use of a bias current higher than expected, most likely as
a result of laser aging.
• Out-of-bounds average voltage at the MPD input, indicat-
ing an excessive amount of laser power or a broken loop.
• Undervoltage in IBIAS pin (laser diode cathode) that
increases 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. ADN2870 Single-Point Alarms
Alarm Type Mnemonic
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
3. Crucial Nodes ERREF (the ERRREF 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 <600 mV
Table 6. ADN2870 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
ADN2870 Data Sheet
Rev. C | Page 18 of 20
OUTLINE DIMENSIONS
COMPLIANT
TO
JEDEC STANDARDS MO-220-VGGD-2
04-09-2012-A
1
0.50
BSC
PIN 1
INDICATOR
2.50 REF
0.50
0.40
0.30
TOP VIEW
12° MAX 0.80 MAX
0.65 TYP
SEATING
PLANE
COPLANARITY
0.08
1.00
0.85
0.80
0.30
0.23
0.18
0.05 MAX
0.02 NOM
0.20 REF
0.25 MIN
2.45
2.30 SQ
2.15
24
7
19
12
13
18
6
0.60 MAX
0.60 MAX
PIN 1
INDICATOR
4.10
4.00 SQ
3.90
3.75 BSC
SQ
EXPOSED
PAD
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
BOTTOM VIEW
Figure 34. 24-Lead Lead Frame Chip Scale Package [LFCSP]
4 mm × 4 mm Body and 0.85 mm Package Height
(CP-24-2)
Dimensions shown in millimeters
ORDERING GUIDE
Model1
Temperature
Range Package Description
Package
Option
Ordering
Quantity
ADN2870ACPZ −40°C to +85°C 24-Lead Lead Frame Chip Scale Package [LFCSP] CP-24-2 490
ADN2870ACPZ-RL7 −40°C to +85°C 24-Lead Lead Frame Chip Scale Package [LFCSP], 7” Tape and Reel CP-24-2 1,500
1 Z = RoHS Compliant Part.
Data Sheet ADN2870
Rev. 0 | Page 19 of 20
NOTES
