3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps
Laser Diode Driver
ADN2870
Rev. 0
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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
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registered trademarks are the property of their respective owners.
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Tel: 781.329.4700
www.analog.com
Fax: 781.326.8703
© 2004 Analog Devices, Inc. All rights reserved.
FEATURES
SFP/SFF and SFF-8472 MSA-compliant
SFP reference design available
50 Mbps to 3.3 Gbps operation
Multirate 155 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 package
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
The ADN2870 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 need only
be calibrated at 25°C, eliminating the need for expensive and
time consuming temperature calibration. The ADN2870 supports
single-rate operation from 50 Mbps to 3.3 Gbps or multirate
from 155 Mbps to 3.3 Gbps.
Average power and extinction ratio can be set with a voltage
provided by a microcontroller DAC or by a trimmable resistor.
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 micro-
converters 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 package specified over the -40°C to
+85°C temperature range.
04510-001
ADI
MICROCONTROLLER
Tx_FAULT
Tx_FAIL
CONTROL
PAVREF
PAVSET
ADN2870
RPAV
GND
DAC
DAC
VCC
MPD
ERREF
ERSET
GND
IMOD
DATAP
IMODP
DATAN
IBIAS
IMODN
IBMON
IMMON
ALS
FAIL
IBIAS
100
CCBIAS
VCC
LASER
L
R
VCC
VCC
VCC
GND
GND
PAVCAP
470
1k
GND
GND
ERCAP
GND
1k
1k
ADC
VCC
Figure 1. Application Diagram Showing Microcontroller Interface
Protected by US patent: US6414974
ADN2870
Rev. 0 | Page 2 of 20
TABLE OF CONTENTS
Specifications..................................................................................... 3
SFP Timing Specifications............................................................... 5
Absolute Maximum Ratings............................................................ 6
ESD Caution.................................................................................. 6
Pin Configuration and Function Descriptions............................. 7
Typical Operating Characteristics.................................................. 8
Optical Waveforms Showing Multirate Performance Using
Low Cost Fabry Perot Tosa NEC NX7315UA .......................... 8
Optical Waveforms Showing Dual-Loop Performance Over
Temperature Using DFB Tosa SUMITOMO SLT2486............ 8
Performance Characteristics....................................................... 9
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
8/04--Revision 0: Initial Version
ADN2870
Rev. 0 | Page 3 of 20
SPECIFICATIONS
V
CC
= 3.0 V to 3.6 V. All specifications T
MIN
to T
MAX
,
1
unless otherwise noted. 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
V
CC
V
IBIAS when ALS is 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
V
CC
V
IMOD when ALS is High
0.05
mA
Rise Time
2, 3
60
104
ps
Fall Time
2, 3
60
96
ps
Random Jitter
2, 3
0.8
1.1
ps
rms
Deterministic Jitter
2, 3
35
ps
20 mA < IMOD < 90 mA
Pulse-Width Distortion
2, 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
k
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 k)
Photodiode Monitor Current (Average Current)
120
1000
µA
Voltage setpoint mode
(RPAV fixed at 1 k)
EXTINCTION RATIO REFERENCE VOLTAGE INPUT (ERREF)
Voltage Range
0.1
1
V
Voltage setpoint mode
(RERSET fixed at 1 k)
DATA INPUTS (DATAP, DATAN)
4
V p-p (Differential)
0.4
2.4
V
AC-coupled
Input Impedance (Single-Ended)
50
LOGIC INPUTS (ALS)
V
IH
2 V
V
IL
0.8
V
ALARM OUTPUT (FAIL)
5
V
OFF
>
1.8
V
Voltage required at FAIL for Ibias and
Imod to turn off when FAIL asserted
V
ON
<
1.3
V
Voltage required at FAIL for Ibias and
Imod to stay on when FAIL asserted
ADN2870
Rev. 0 | Page 4 of 20
Parameter
Min Typ Max Unit Conditions/Comments
IBMON, IMMON DIVISION RATIO
IBIAS/IBMON
3
85
100
115
A/A
11 mA < IBIAS < 50 mA
IBIAS/IBMON
3
92
100
108
A/A
50 mA < IBIAS < 100 mA
IBIAS/IBMON STABILITY
3, 6
±5
%
10 mA < IBIAS < 100 mA
IMOD/IMMON
50
A/A
IBMON Compliance Voltage
0
1.3
V
SUPPLY
I
CC
7
30
mA
When IBIAS = IMOD = 0
V
CC
(w.r.t. GND)
8
3.0 3.3 3.6 V
1
Temperature range: 40°C to +85°C.
2
Measured into a 15 load (22 resistor in parallel with digital scope 50 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
I
CC
min for power calculation in the Power Consumption section.
8
All VCC pins should be shorted together.
04510-034
ADN2870
IMODP
BIAS TEE
80kHz
27GHz
V
CC
V
CC
TO HIGH SPEED
DIGITAL
OSCILLOSCOPE
50
INPUT
R
22
C
L
Figure 2. High Speed Electrical Test Output Circuit
ADN2870
Rev. 0 | 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 for the rising edge of ALS (TX_DISABLE) to when the bias
current falls below 10% of nominal.
ALS Negate Time
1
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 FAIL
1
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.
04510-002
DATAP
DATAN
DATAPDATAN
V p-p
DIFF
= 2
×
V
SE
V
SE
0V
Figure 3. Signal Level Definition
04510-003
0.1
µ
F
0.1
µ
F
10
µ
F
1
µ
H
3.3V
SFP HOST BOARD
SFP MODULE
VCC_Tx
Figure 4. Recommended SFP Supply
ADN2870
Rev. 0 | Page 6 of 20
ABSOLUTE MAXIMUM RATINGS
T
A
= 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)
1.5 V
TEMPERATURE SPECIFICATIONS
Operating Temperature Range
Industrial
-40°C to +85°C
Storage Temperature Range
65°C to +150°C
Junction Temperature (T
J
max)
150°C
LFCSP Package
Power Dissipation
1
(T
J
max T
A
)/
JA
W
JA
Thermal Impedance
2
30°C/W
JC
Thermal Impedance
29.5°C/W
Lead Temperature (Soldering 10 s)
300°C
___________________
1
Power consumption equations are provided in the Power Consumption
section.
2
JA
is defined when part is soldered on a 4-layer board.
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.
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.
ADN2870
Rev. 0 | Page 7 of 20
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
04510-004
GND
19
24
18
1
12
7
13
6
FAIL
IB
MON
E
RRE
F
IMMON
ER
SET
VC
C
CCBI
AS
PA
VSET
VC
C
PA
VR
EF
RP
AV
GND
VCC
IMODP
IMODN
GND
IBIAS
ALS
DATAN
DATAP
GND
ADN2870
PAVCAP
ERCAP
Figure 5. Pin Configuration
Table 4. Pin Fuction 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, 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.
ADN2870
Rev. 0 | Page 8 of 20
TYPICAL OPERATING CHARACTERISTICS
V
CC
= 3.3 V and T
A
= 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
Figure 6. Optical Eye 2.488 Gbps,65 ps/div, PRBS 2
31-1
PAV =
-4.5 dBm, ER = 9 dB, Mask Margin 25%
(ACQ LIMIT TEST) WAVEFORMS 1000
04510-017
Figure 7. Optical Eye 622 Mbps, 264 ps/div, PRBS 2
31-1
PAV =
-4.5 dBm, ER = 9 dB, Mask Margin 50%
(ACQ LIMIT TEST) WAVEFORMS 1000
04510-020
Figure 8. Optical Eye 155 Mbps,1.078 ns/div, PRBS 2
31-1
PAV =
-4.5 dBm, ER = 9 dB, Mask Margin 50%
OPTICAL WAVEFORMS SHOWING DUAL-LOOP
PERFORMANCE OVER TEMPERATURE USING DFB TOSA
SUMITOMO SLT2486
(ACQ LIMIT TEST) WAVEFORMS 1001
04510-047
Figure 9. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 2
31-1
PAV = 0 dBm, ER = 9 dB, Mask Margin 22%, T
A
= 25°C
(ACQ LIMIT TEST) WAVEFORMS 1001
04510-048
Figure 10. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 2
31-1
PAV = -0.2 dBm, ER = 8.96 dB, Mask Margin 21%, T
A
= 85°C
ADN2870
Rev. 0 | Page 9 of 20
PERFORMANCE CHARACTERISTICS
90
0
30
60
0
40
80
20
60
100
04510-022
MODULATION CURRENT (mA)
R
I
SE TIM
E
(
p
s)
Figure 11. Rise Time vs. Modulation Current, Ibias = 20 mA
0
20
40
60
80
0
40
80
20
60
100
04510-025
MODULATION CURRENT (mA)
FALL TIME (ps)
Figure 12. Fall Time vs. Modulation Current, Ibias = 20 mA
0
45
40
35
30
25
20
15
10
5
20
40
80
60
100
04510-042
MODULATION CURRENT (mA)
DE
TE
RMINIS
TIC J
I
TTE
R (ps
)
Figure 13. 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
1
04510-037
MODULATION CURRENT (mA)
JITTER (rms)
00
Figure 14. Random Jitter vs. Modulation Current, Ibias = 20 mA
40
250
190
220
160
130
100
70
0
20
40
60
80
1
04510-038
MODULATION CURRENT (mA)
TOTAL S
U
P
P
L
Y
CURRE
NT (mA)
00
I
BIAS
= 20mA
I
BIAS
= 40mA
I
BIAS
= 80mA
Figure 15. Total Supply Current vs. Modulation Current
Total Supply Current = I
CC
+ Ibias + Imod
20
60
55
50
45
40
35
30
25
50
30
10
10
30
50
70
90
110
04510-027
TEMPERATURE (
°
C)
S
U
P
P
L
Y
CURRE
NT (mA)
Figure 16. Supply Current (I
CC
) vs. Temperature with ALS Asserted,
Ibias = 20 mA
ADN2870
Rev. 0 | Page 10 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/IBM
O
N RATIO
Figure 17. IBIAS/IBMON Gain vs. Temperature, Ibias = 20 mA
04510-029
OC48 PRBS31
DATA TRANSMISSION
t
_OFF
LESS THAN 1
µ
s
ALS
Figure 18. ALS Assert Time, 5 µs/div
04510-032
ALS
t
_ON
OC48 PRBS31
DATA TRANSMISSION
Figure 19. 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)
IM
OD/IM
M
O
N RATIO
Figure 20. IMOD/IMMON Gain vs. Temperature, Imod = 30 mA
04510-045
FAULT FORCED ON PAVSET
FAIL ASSERTED
Figure 21. FAIL Assert Time,1 µs/div
04510-046
POWER SUPPLY TURN ON
TRANSMISSION ON
Figure 22. Time to Initialize, Including Reset, 40 ms/div
ADN2870
Rev. 0 | 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
OP
TICAL P
O
W
E
R
P1
P
AV
P
O
Ith
CURRENT
P
AV
=
P
I
ER = P1
P
O
2
P1 + P
O
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 lasers
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, additionally, a
secondary control loop, which maintains constant optical
extinction ratio. The dual-loop control of average power and
extinction ratio implemented in the ADN2870 can be used
successfully both with lasers that maintain good linearity of LI
transfer characteristics 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 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
VCC
HIGH
SPEED
SWITCH
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
A dual loop is made up of an APCL (average power control
loop) and the ERCL (extinction ratio control loop), which are
separated into two time states. During time 1, the APC loop is
operating, and during time 2, 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 since
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 LD, and
changes Imod as LI changes. During the ERCL, Imod is
temporarily increased by Imod. The ratio between Imod and
Imod is a fixed ratio of 50:1, but during startup, this ratio is
increased in order to decrease settling time.
During ERCL, switching in Imod causes a temporary increase
in average optical power, Pav. However the APC loop is dis-
abled during ERCL, and the increase is kept small enough so as
not to disturb the optical eye. When Imod 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 Impd is too small, the control loop knows that LI has
decreased and increases Imod and, therefore, Imod accordingly
until Impd 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, therefore
it compensates for the effects of temperature and for changes in
the LI curve due to laser aging. Thus the laser may 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
Rev. 0 | 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)
OP
TICAL P
O
WE
R (mW)
100
80
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 laser 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 extinction ratio with a
laser exhibiting a monotonic temperature-dependant nonlinear-
ity. 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. Then
the product can be calibrated 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 microcontroller 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 dependant nonlinearity of LI curve,
see Application Note AN-743.
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 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 ADN2870 allows 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 DACs to provide controlled
reference voltages PAVREF and ERREF. 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.
RPAV
R
P
PAVREF
SP
AV
×
×
=
(Volts)
AV
CW
CW
MPD
ERSET
P
ER
ER
P
I
R
ERREF
×
+
-
×
×
=
1
1
_
(Volts)
where:
R
SP
is the monitor photodiode responsivity.
P
CW
is the dc optical power specified on the laser data sheet.
I
MPD_CW
is MPD current at that specified P
CW
.
P
AV
is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
In voltage setpoint, RPAV and R
ERSET
must be 1 k resistors with
a 1% tolerance and a temperature coefficient of 50 ppm/°C.
ADN2870
Rev. 0 | Page 13 of 20
04510-009
ADI
MICROCONTROLLER
CONTROL
PAVREF
PAVSET
ADN2870
RPAV
GND
DAC
DAC
VCC
MPD
ERREF
1k
1k
ERSET
GND
IMOD
DATAP
IMODP
DATAN
IBIAS
IMODN
IBMON
IMMON
ALS
FAIL
IBIAS
VCC
LASER
VCC
VCC
GND
GND
PAVCAP
GND
GND
ERCAP
GND
100
CCBIAS
R
470
1k
ADC
Tx_FAULT
Tx_FAIL
VCC
L
VCC
Figure 26. ADN2870 Using Microconverter Calibration and Monitoring
L
VCC
04510-010
CONTROL
ADN2870
ERSET
VCC
ERREF
VCC
RPAV
PAVREF
VCC
MPD
IMOD
DATAP
IMODP
DATAN
IBIAS
IMODN
IBMON
IMMON
ALS
FAIL
IBIAS
VCC
LASER
VCC
VCC
GND
GND
PAVCAP
GND
GND
ERCAP
GND
GND
GND
PAVSET
100
CCBIAS
R
470
1k
VCC
Figure 27. ADN2870 Using Resistor Setpoint Calibration of Average Power and Energy Ratio
ADN2870
Rev. 0 | Page 14 of 20
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 27. The ADN2870
ensures that both PAVSET and ERSET are kept 1.2 V above
GND. The PAVSET and ERSET resistors are given by the
following:
SP
AV
PAVSET
R
P
R
×
=
V
23
.
1
()
AV
CW
CW
MPD
ERSET
P
ER
ER
P
I
R
+
+
-
×
=
1
1
V
23
.
1
_
()
where:
R
SP
is the monitor photodiode responsivity.
P
CW
is the dc optical power specified on the laser data sheet.
I
MPD_CW
is MPD current at that specified P
CW
.
P
AV
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 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 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 shut-down or
fail mode. A microconverter buffered A/D input may 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
VCC
PHOTODIODE
ADN2870
R
1k
µ
C ADC
INPUT
PAVSET
RPAV
Figure 28. Single Measurement of IMPD 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
µ
C ADC
DIFFERENTIAL
INPUT
200
RESISTOR
10pF
PAVSET
ADN2870
Figure 29. Differential Measurement of IMPD Across a Sense Resistor
04510-011
VCC
VCC
LD
PHOTODIODE
µ
C ADC
INPUT
200
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 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 should
be selected so that the voltage across the sense resistor does not
exceed 250 mV.
ADN2870
Rev. 0 | Page 15 of 20
04510-040
VCC
PHOTODIODE
ADN2870
PAVSET
R
µ
C 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 lasers slope efficiency (watts/amps) and the average
power required.
For resistor point control:
)
(
6
2
.
3
Farad
PAV
LI
E
PAVCAP
×
-
=
)
(
2
Farad
PAVCAP
ERCAP =
For voltage setpoint control:
)
(
6
28
.
1
Farad
PAV
LI
E
PAVCAP
×
-
=
)
(
2
Farad
PAVCAP
ERCAP =
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:
Imod
P0
P1
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 G 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 capacitors
own tolerance.
POWER CONSUMPTION
The ADN2870 die temperature must be kept below 125°C. The
LFCSP package has an exposed paddle, which should be con-
nected such that is at the same potential as the ADN2870 ground
pins. Power consumption can be calculated as follows:
I
CC
= I
CC
min + 0.3 I
MOD
P = V
CC
× I
CC
+ (I
BIAS
× V
BIAS_PIN
) + I
MOD
(V
MODP_PIN
+
V
MODN_PIN
)/2
T
DIE
= T
AMBIENT
+
JA
× P
Thus, the maximum combination of I
BIAS
+ I
MOD
must be
calculated.
where:
I
CC
min = 30 mA, the typical value of I
CC
provided in the
Specifications with I
BIAS
= I
MOD
= 0.
T
DIE
is the die temperature.
T
AMBIENT
is the ambient temperature.
V
BIAS_PIN
is the voltage at the IBIAS pin.
V
MODP_PIN
is the voltage at the IMODP pin.
V
MODN_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 k 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 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 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
Rev. 0 | 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 to 7 , 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 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 versus frequency
and enables multirate operation. A series damping resistor
should not be used.
04510-014
L
BLMI8HG60ISN1D
C
100nF
R
P
24
ADN2870
IBIAS
CCBIAS
IMODP
VCC
L (0.5nH)
R 24
C 2.2pF
Tx LINE
30
Tx LINE
30
VCC
Figure 32. Recommended Interface for ADN2870 AC Coupling
The 30 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 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 32 can
supply up to 56 mA of modulation current to the laser, sufficient
for most lasers available today. Higher currents can be accom-
modated 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 characteristic impedance. Instead, a 25 transmission line
and inductive (instead of resistive) pull-up is recommended;
contact the factory 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.
04510-041
L3 = 4.7nH
L4 = BLM18HG601SN1
V
CC
L6 = BLM18HG601SN1
SNUBBER SETTINGS: 40
AND 1.5pF, NOT OPTIMIZED,
OPTIMIZATION SHOULD CONSIDER PARASITIC.
L5 = 4.7nH
V
CC
R1 = 15
CCBIAS IBIAS
IMODN
IMODP
ADN2870
C1 = C2 = 100nF
20
TRANMISSION LINES
R1 = 15
(12 TO 24
)
R3
C3
SNUBBER
LIGHT
TOCAN/VCSEL
L1 = 0.5nH
L2 = 0.5nH
Figure 33. Recommended Differential Drive Circuit
ADN2870
Rev. 0 | 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 3-fold alarm system that covers
·
Use of a bias current higher than expected, probably as a
result of laser aging.
·
Out-of-bounds average voltage at the monitor photodiode
(MPD) input, indicating an indicating an excessive amount
of laser power or a broken loop.
·
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.
Table 5. ADN2870 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 < 600 mV
Table 6. ADN2870 Response to Various Single-Point Faults in AC-Coupled Configuration as Shown in Figure 32
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
Voltage mode: Fault state occurs
Resistor mode: Tied to VCC
Fault state occurs
Fault state occurs
RPAV
Voltage mode: Fault state occurs
Resistor mode: Tied to VCC
Fault state occurs
Voltage mode: Fault state occurs
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
Resistor mode: Tied to VCC
Voltage mode: Does not increase
average power
Resistor mode: Fault state occurs
Does not increase laser average power
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
Rev. 0 | Page 18 of 20
OUTLINE DIMENSIONS
1
24
6
7
13
19
18
BOTTOM
VIEW
12
2.25
2.10 SQ
1.95
0.60 MAX
0.50
0.40
0.30
0.30
0.23
0.18
2.50 REF
0.50
BSC
12° MAX
0.80 MAX
0.65 TYP
0.05 MAX
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 MAX
COPLANARITY
0.08
0.20 REF
0.25 MIN
COMPLIANT TO JEDECSTANDARDS MO-220-VGGD-2
Figure 34. 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
ADN2870ACPZ
1
-40°C to +85°C
24-Lead Lead Frame Chip Scale Package
CP-24
ADN2870ACPZ-RL
1
-40°C to +85°C
24-Lead Lead Frame Chip Scale Package
CP-24
ADN2870ACPZ-RL7
1
-40°C to +85°C
24-Lead Lead Frame Chip Scale Package
CP-24
1
Z = Pb-free part.
Preliminary Technical Data
ADN2870
Rev. 0 | Page 19 of 20
NOTES
ADN2870
Rev. 0 | Page 20 of 20
NOTES
© 2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D0451008/04(0)
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