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3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps Laser Diode Driver ADN2870
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 x 4 mm LFCSP package Voltage setpoint control Resistor setpoint control
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 25C, 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 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 spacesaving 4 mm x4 mm LFCSP package specified over the -40C to +85C temperature range.
VCC VCC VCC VCC L VCC MPD FAIL ALS IMODN R IMODP DATAP ADI MICROCONTROLLER DAC ADC 1k DAC GND ERREF PAVSET IMOD PAVREF CONTROL RPAV IBIAS CCBIAS IBIAS 100 DATAN LASER
APPLICATIONS
Multirate OC3 to OC48-FEC SFP/SFF modules 1x/2x/4x Fibre channel SFP/SFF modules LX-4 modules DWDM/CWDM SFP modules 1GE SFP/SFF transceiver modules
Tx_FAULT Tx_FAIL
ADN2870
1k ERSET GND IBMON VCC GND GND 1k GND 470 GND GND
04510-001
IMMON
PAVCAP
ERCAP
Figure 1. Application Diagram Showing Microcontroller Interface
Protected by US patent: US6414974
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: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved.
ADN2870 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
Rev. 0 | Page 2 of 20
ADN2870 SPECIFICATIONS
VCC = 3.0 V to 3.6 V. All specifications TMIN to TMAX,1 unless otherwise noted. Typical values as specified at 25C. Table 1.
Parameter LASER BIAS CURRENT (IBIAS) Output Current IBIAS Compliance Voltage IBIAS when ALS is High CCBIAS Compliance Voltage MODULATION CURRENT (IMODP, IMODN)2 Output Current IMOD Compliance Voltage IMOD when ALS is High Rise Time2, 3 Fall Time2, 3 Random Jitter2, 3 Deterministic Jitter2, 3 Pulse-Width Distortion2, 3 AVERAGE POWER SET (PAVSET) Pin Capacitance Voltage Photodiode Monitor Current (Average Current) EXTINCTION RATIO SET INPUT (ERSET) Resistance Range Voltage AVERAGE POWER REFERENCE VOLTAGE INPUT (PAVREF) Voltage Range Photodiode Monitor Current (Average Current) EXTINCTION RATIO REFERENCE VOLTAGE INPUT (ERREF) Voltage Range DATA INPUTS (DATAP, DATAN)4 V p-p (Differential) Input Impedance (Single-Ended) LOGIC INPUTS (ALS) VIH VIL ALARM OUTPUT (FAIL)5 VOFF VON Min 2 1.2 1.2 5 1.5 60 60 0.8 90 VCC 0.05 104 96 1.1 35 30 80 1.35 1200 25 1.35 1 1000 Typ Max 100 VCC 0.2 Unit mA V mA V mA V mA ps ps ps ps ps pF V A k V V A Conditions/Comments
rms 20 mA < IMOD < 90 mA 20 mA < IMOD < 90 mA
1.1 50 1.2 1.1 0.12 120
1.2
Resistor setpoint mode Resistor setpoint mode Resistor setpoint mode Voltage setpoint mode (RPAV fixed at 1 k) Voltage setpoint mode (RPAV fixed at 1 k) Voltage setpoint mode (RERSET fixed at 1 k) AC-coupled
1.2
0.1
1
V
0.4 50 2
2.4
V V V V V
0.8 > 1.8 < 1.3
Voltage required at FAIL for Ibias and Imod to turn off when FAIL asserted Voltage required at FAIL for Ibias and Imod to stay on when FAIL asserted
Rev. 0 | Page 3 of 20
ADN2870
Parameter IBMON, IMMON DIVISION RATIO IBIAS/IBMON3 IBIAS/IBMON3 IBIAS/IBMON STABILITY3, 6 IMOD/IMMON IBMON Compliance Voltage SUPPLY ICC7 VCC (w.r.t. GND)8 Min 85 92 Typ 100 100 50 0 30 3.3 1.3 Max 115 108 5 Unit A/A A/A % A/A V mA V Conditions/Comments 11 mA < IBIAS < 50 mA 50 mA < IBIAS < 100 mA 10 mA < IBIAS < 100 mA
When IBIAS = IMOD = 0
3.0
3.6
1 2
Temperature range: -40C to +85C. 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 ICC min for power calculation in the Power Consumption section. 8 All VCC pins should be shorted together.
VCC
VCC L C BIAS TEE 80kHz 27GHz
ADN2870
IMODP
R 22
Figure 2. High Speed Electrical Test Output Circuit
Rev. 0 | Page 4 of 20
04510-034
TO HIGH SPEED DIGITAL OSCILLOSCOPE 50 INPUT
ADN2870 SFP TIMING SPECIFICATIONS
Table 2.
Parameter ALS Assert Time ALS Negate Time1 Time to Initialize, Including Reset of FAIL1 FAIL Assert Time ALS to Reset time Symbol t_off t_on t_init t_fault t_reset Min Typ 1 0.83 25 Max 5 0.95 275 100 5 Unit s ms ms s s Conditions/Comments Time for the rising edge of ALS (TX_DISABLE) to when the bias current falls below 10% of nominal. Time for the falling edge of ALS to when the modulation current rises above 90% of nominal. From power-on or negation of FAIL using ALS. Time to fault to FAIL on. Time TX_DISABLE must be held high to reset TX_FAULT.
1
Guaranteed by design and characterization. Not production tested.
VSE DATAP DATAN
SFP MODULE
1H VCC_Tx 0.1F 0.1F 10F
04510-003
3.3V
SFP HOST BOARD
DATAP-DATAN 0V
04510-002
V p-p DIFF = 2 x VSE
Figure 4. Recommended SFP Supply
Figure 3. Signal Level Definition
Rev. 0 | Page 5 of 20
ADN2870 ABSOLUTE MAXIMUM RATINGS
TA = 25C, unless otherwise noted. Table 3.
Parameter VCC to GND IMODN, IMODP PAVCAP ERCAP PAVSET PAVREF ERREF IBIAS IBMON IMMON ALS CCBIAS RPAV ERSET FAIL DATAP, DATAN (single-ended differential) TEMPERATURE SPECIFICATIONS Operating Temperature Range Industrial Storage Temperature Range Junction Temperature (TJ max) LFCSP Package Power Dissipation1 JA Thermal Impedance2 JCThermal Impedance Lead Temperature (Soldering 10 s) Rating 4. 2 V -0.3 V to +4.8 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V -0.3 V to +3.9 V 1.5 V
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
-40C to +85C -65C to +150C 150C (TJ max - TA)/JA W 30C/W 29.5C/W 300C
___________________ 1 Power consumption equations are provided in the Power Consumption section. 2 JA is defined when part is soldered on a 4-layer board.
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.
Rev. 0 | Page 6 of 20
ADN2870 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
IMMON ERREF IBMON
18 19
ERSET RPAV
FAIL
VCC
13 12
GND VCC IMODP IMODN GND IBIAS
24
ALS DATAN DATAP
ADN2870
GND PAVCAP ERCAP
7
PAVSET
CCBIAS
Figure 5. Pin Configuration
Table 4. Pin Fuction Descriptions
Pin No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Mnemonic CCBIAS PAVSET GND VCC PAVREF RPAV ERCAP PAVCAP GND DATAP DATAN ALS ERSET IMMON ERREF VCC IBMON FAIL GND VCC IMODP IMODN GND IBIAS Description Control Output Current Average Optical Power Set Pin Supply Ground Supply Voltage Reference Voltage Input for Average Optical Power Control Average Power Resistor when Using PAVREF Extinction Ratio Loop Capacitor Average Power Loop Capacitor Supply Ground Data, Positive Differential Input Data, Negative Differential Input Automatic Laser Shutdown Extinction Ratio Set Pin Modulation Current Monitor Current Source Reference Voltage Input for Extinction Ratio Control Supply Voltage Bias Current Monitor Current Source FAIL Alarm Output Supply Ground Supply Voltage Modulation Current Positive Output, Connect to Laser Diode Modulation Current Negative Output Supply Ground Laser Diode Bias (Current Sink to Ground)
Note: The LFCSP package has an exposed paddle that must be connected to ground.
Rev. 0 | Page 7 of 20
04510-004
1
PAVREF
GND
VCC
6
ADN2870 TYPICAL OPERATING CHARACTERISTICS
VCC = 3.3 V and TA = 25C, 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
OPTICAL WAVEFORMS SHOWING DUAL-LOOP PERFORMANCE OVER TEMPERATURE USING DFB TOSA SUMITOMO SLT2486
(ACQ LIMIT TEST) WAVEFORMS 1001
04510-016
Figure 6. 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
Figure 9. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 231-1 PAV = 0 dBm, ER = 9 dB, Mask Margin 22%, TA = 25C
(ACQ LIMIT TEST) WAVEFORMS 1001
04510-017
Figure 7. Optical Eye 622 Mbps, 264 ps/div, PRBS 231-1 PAV = -4.5 dBm, ER = 9 dB, Mask Margin 50%
Figure 10. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 231-1 PAV = -0.2 dBm, ER = 8.96 dB, Mask Margin 21%, TA = 85C
(ACQ LIMIT TEST) WAVEFORMS 1000
Figure 8. Optical Eye 155 Mbps,1.078 ns/div, PRBS 231-1 PAV = -4.5 dBm, ER = 9 dB, Mask Margin 50%
Rev. 0 | Page 8 of 20
04510-020
04510-048
04510-047
ADN2870
PERFORMANCE CHARACTERISTICS
90 1.2
1.0
60
0.8
RISE TIME (ps)
JITTER (rms)
30
04510-022
0.6
0.4
0.2
04510-037
0 0 20 40 60 MODULATION CURRENT (mA) 80
0 0 20 40 60 MODULATION CURRENT (mA) 80
100
100
Figure 11. Rise Time vs. Modulation Current, Ibias = 20 mA
80
Figure 14. Random Jitter vs. Modulation Current, Ibias = 20 mA
250
220
TOTAL SUPPLY CURRENT (mA)
60
IBIAS = 80mA 190 IBIAS = 40mA
FALL TIME (ps)
160
40
130 IBIAS = 20mA 100
20
04510-025
0 0 20 40 60 MODULATION CURRENT (mA) 80
40 0 20 40 60 MODULATION CURRENT (mA) 80
100
100
Figure 12. Fall Time vs. Modulation Current, Ibias = 20 mA
Figure 15. Total Supply Current vs. Modulation Current Total Supply Current = ICC + Ibias + Imod
60 55 50 45 40 35 30
04510-027
45 40
DETERMINISTIC JITTER (ps)
35 30 25 20 15 10 5 0 20
04510-042
SUPPLY CURRENT (mA)
25 20 -50
40 60 80 MODULATION CURRENT (mA)
100
-30
-10
10 30 50 TEMPERATURE (C)
70
90
110
Figure 13. Deterministic Jitter vs. Modulation Current, Ibias = 20 mA
Figure 16. Supply Current (ICC) vs. Temperature with ALS Asserted, Ibias = 20 mA
Rev. 0 | Page 9 of 20
04510-038
70
ADN2870
120 115 110 105 100 95 90 44
04510-028
04510-031
60 58 56
IMOD/IMMON RATIO
IBIAS/IBMON RATIO
54 52 50 48 46
85 80 -50
42 40 -50 -30 -10 10 30 50 TEMPERATURE (C) 70 90
-30
-10
10 30 50 TEMPERATURE (C)
70
90
110
110
Figure 17. IBIAS/IBMON Gain vs. Temperature, Ibias = 20 mA
Figure 20. IMOD/IMMON Gain vs. Temperature, Imod = 30 mA
OC48 PRBS31 DATA TRANSMISSION
t_OFF LESS THAN 1s
FAIL ASSERTED
FAULT FORCED ON PAVSET
ALS
04510-029 04510-045
Figure 18. ALS Assert Time, 5 s/div
Figure 21. FAIL Assert Time,1 s/div
OC48 PRBS31 DATA TRANSMISSION
TRANSMISSION ON
t_ON
ALS
POWER SUPPLY TURN ON
04510-032 04510-046
Figure 19. ALS Negate Time, 200 s/div
Figure 22. Time to Initialize, Including Reset, 40 ms/div
Rev. 0 | Page 10 of 20
ADN2870 THEORY OF OPERATION
Laser diodes have a current-in to light-out transfer function as shown in Figure 23. Two key characteristics of this transfer function are the threshold current, Ith, and slope in the linear region beyond the threshold current, referred to as slope efficiency, LI.
P1 ER = PO P1 + PO PAV = 2
ERSET PAVSET MOD SHA MPD INPUT Gm OPTICAL COUPLING BIAS SHA
1
BIAS CURRENT
VCC
2
IEX IPA
1.2V VBGAP
HIGH SPEED SWITCH
OPTICAL POWER
P1
2
04510-039
2
PAV
P I
LI =
MOD CURRENT 100
2
Figure 24. Dual-Loop Control of Average Power and Extinction Ratio
04510-005
P I
PO Ith CURRENT
Figure 23. Laser Transfer Function
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.
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.
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 x Rpavset to the BGAP voltage, Vbgap. If Impd falls, the bias current is increased until Impd x 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 disabled 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 25C and can then automatically control the laser over temperature. This eliminates expensive and time consuming temperature calibration of the laser.
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.
Rev. 0 | Page 11 of 20
ADN2870
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.
4.0 RELATIVELY LINEAR LI CURVE AT 25C 3.5 3.0 2.5 2.0 1.5 1.0 NONLINEAR LI CURVE AT 80C
04510-008
The ER correction scheme, while using the average nonlinearity for the laser population, in fact, supplies a corrective measurement 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.
OPTICAL POWER (mW)
0.5 0 0 20 40 60 CURRENT (mA) 80 100
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.
PAVREF = PAV x RSP x RPAV (Volts) ERREF = R ERSET x I MPD _ CW PCW x ER - 1 x PAV (Volts) ER + 1
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 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 25C and 85C. 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 25C 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.
where: RSP is the monitor photodiode responsivity. PCW is the dc optical power specified on the laser data sheet. IMPD_CW is MPD current at that specified PCW. PAV is the average power required. ER is the desired extinction ratio (ER = P1/P0). In voltage setpoint, RPAV and RERSET must be 1 k resistors with a 1% tolerance and a temperature coefficient of 50 ppm/C.
Rev. 0 | Page 12 of 20
ADN2870
VCC Tx_FAULT Tx_FAIL VCC MPD FAIL ALS IMODN VCC VCC L R IMODP DATAP ADI MICROCONTROLLER DAC ADC 1k PAVSET IMOD PAVREF CONTROL RPAV IBIAS CCBIAS IBIAS 100 DATAN VCC LASER
DAC
GND ERREF
ADN2870
1k ERSET GND IBMON VCC GND GND 1k GND 470 GND
04510-009
IMMON
PAVCAP
ERCAP GND
Figure 26. ADN2870 Using Microconverter Calibration and Monitoring
VCC
VCC
VCC L VCC LASER
FAIL VCC VCC PAVREF MPD RPAV
ALS
IMODN
R IMODP
DATAP DATAN
IMOD PAVSET GND CONTROL IBIAS
100 IBIAS
CCBIAS ERSET GND VCC ERREF IBMON VCC GND 1k GND IMMON 470 GND PAVCAP GND ERCAP GND
ADN2870
Figure 27. ADN2870 Using Resistor Setpoint Calibration of Average Power and Energy Ratio
Rev. 0 | Page 13 of 20
04510-010
ADN2870
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:
RPAVSET =
R ERSET =
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 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.
VCC
1.23 V PAV x RSP
()
1.23 V I MPD _ CW ER - 1 + PAV x P CW ER + 1
()
where: RSP is the monitor photodiode responsivity. PCW is the dc optical power specified on the laser data sheet. IMPD_CW is MPD current at that specified PCW. PAV is the average power required. ER is the desired extinction ratio (ER = P1/P0).
PHOTODIODE
LD
IMPD MONITORING
IMPD monitoring can be implemented for voltage setpoint and resistor setpoint as follows.
C ADC DIFFERENTIAL INPUT
200 RESISTOR
10pF
PAVSET
ADN2870
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 connected to RPAV to make this measurement. No decoupling or filter capacitors should be placed on the RPAV node because this can disturb the control loop.
VCC PHOTODIODE PAVSET
200 RESISTOR
Figure 29. Differential Measurement of IMPD Across a Sense Resistor
VCC
VCC
C ADC
INPUT PHOTODIODE
LD
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
C ADC
INPUT
04510-043
RPAV R 1k
Figure 28. Single Measurement of IMPD RPAV in Voltage Setpoint Mode
Rev. 0 | Page 14 of 20
04510-011
04510-011
ADN2870
VCC PHOTODIODE PAVSET
POWER CONSUMPTION
The ADN2870 die temperature must be kept below 125C. The LFCSP package has an exposed paddle, which should be connected such that is at the same potential as the ADN2870 ground pins. Power consumption can be calculated as follows:
04510-040
ADN2870
C ADC
INPUT R
ICC = ICC min + 0.3 IMOD P = VCC x ICC + (IBIAS x VBIAS_PIN) + IMOD (VMODP_PIN + VMODN_PIN)/2 TDIE = TAMBIENT + JA x P Thus, the maximum combination of IBIAS + IMOD must be calculated. where: ICC min = 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.
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:
PAVCAP = 3.2 E - 6 x ERCAP = LI (Farad) PAV
PAVCAP (Farad) 2
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.
For voltage setpoint control:
PAVCAP = 1.28 E - 6 x ERCAP = LI (Farad) PAV
PAVCAP (Farad) 2
BIAS AND MODULATION MONITOR CURRENTS
IBMON and IMMON are current-controlled current sources that mirror a ratio of the bias and modulation current. The monitor bias current, IBMON, and the monitor modulation current, IMMON, should both be connected to ground through a resistor to provide a voltage proportional to the bias current and modulation current, respectively. When using a microcontroller, the voltage developed across these resistors can be connected to two of the ADC channels, making available a digital representation of the bias and modulation current.
where PAV is the average power required and LI (mW/mA) is the typical slope efficiency at 25C 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:
LI = P1 - P0 (mW/mA) Imod
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.
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.
Rev. 0 | Page 15 of 20
ADN2870
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.
VCC L (0.5nH) RP 24 IMODP C 100nF Tx LINE 30 VCC
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 component 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. 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 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 accommodated by changing transmission lines and backmatch values; contact factory for recommendations. This interface circuit is not recommended for butterfly-style lasers or other lasers with 25 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.
ADN2870
IBIAS CCBIAS
Tx LINE 30
R 24 C 2.2pF
L BLMI8HG60ISN1D
04510-014
Figure 32. Recommended Interface for ADN2870 AC Coupling
VCC L1 = 0.5nH L4 = BLM18HG601SN1
R1 = 15 IMODN
C1 = C2 = 100nF
L3 = 4.7nH
TOCAN/VCSEL
ADN2870
IMODP CCBIAS IBIAS
20 TRANMISSION LINES
R3
C3 SNUBBER
LIGHT
R1 = 15 (12 TO 24) L2 = 0.5nH VCC
L5 = 4.7nH
L6 = BLM18HG601SN1
SNUBBER SETTINGS: 40 AND 1.5pF, NOT OPTIMIZED, OPTIMIZATION SHOULD CONSIDER PARASITIC.
Figure 33. Recommended Differential Drive Circuit
Rev. 0 | Page 16 of 20
04510-041
ADN2870
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 1. Bias Current 2. MPD Current 3. Crucial Nodes Pin Name IBMON PAVSET ERREF IBIAS Over Voltage or Short to VCC Condition Alarm if > 1.2 V Alarm if > 1.7 V Alarm if shorted to VCC Ignore Under Voltage or Short to GND Condition Ignore Alarm if < 0.9 V Alarm if shorted to GND Alarm if < 600 mV
Table 6. ADN2870 Response to Various Single-Point Faults in AC-Coupled Configuration as Shown in Figure 32
Pin CCBIAS PAVSET PAVREF RPAV Short to VCC Fault state occurs Fault state occurs Voltage mode: Fault state occurs Resistor mode: Tied to VCC Voltage mode: Fault state occurs Resistor mode: Tied to VCC Does not increase laser average power Fault state occurs Does not increase laser average power Does not increase laser average power Output currents shut off Does not increase laser average power Does not affect laser power Voltage mode: Fault state occurs Resistor mode: Tied to VCC Fault state occurs Fault state occurs Does not increase laser average power Does not increase laser average power Fault state occurs Short to GND Fault state occurs Fault state occurs Fault state occurs Fault state occurs Open Does not increase laser average power Fault state occurs Fault state occurs Voltage mode: Fault state occurs Resistor mode: Does not increase average power Does not increase laser average power Fault state occurs Does not increase laser average power Does not increase laser average power Output currents shut off Does not increase laser average power Does not increase laser average power Does not increase laser average power
ERCAP PAVCAP DATAP DATAN ALS ERSET IMMON ERREF
IBMON FAIL IMODP IMODN IBIAS
Does not increase laser average power Fault state occurs Does not increase laser average power Does not increase laser average power Normal currents Does not increase laser average power Does not increase laser average power Voltage mode: Does not increase average power Resistor mode: Fault state occurs Does not increase laser average power Does not increase laser average power Does not increase laser average power Does not increase laser average power Fault state occurs
Does not increase laser average power Does not increase laser average power Does not increase laser average power Does not increase laser power Fault state occurs
Rev. 0 | Page 17 of 20
ADN2870 OUTLINE DIMENSIONS
4.00 BSC SQ 0.60 MAX 0.60 MAX
19 18 24 1
PIN 1 INDICATOR 2.25 2.10 SQ 1.95
6
PIN 1 INDICATOR
TOP VIEW
3.75 BSC SQ
0.50 BSC 0.50 0.40 0.30
BOTTOM VIEW
13 12 7
0.25 MIN 2.50 REF
1.00 0.85 0.80
12 MAX
0.80 MAX 0.65 TYP 0.05 MAX 0.02 NOM 0.30 0.23 0.18 0.20 REF COPLANARITY 0.08
SEATING PLANE
COMPLIANT TO JEDEC STANDARDS 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 ADN2870ACPZ1 ADN2870ACPZ-RL1 ADN2870ACPZ-RL71 Temperature Range -40C to +85C -40C to +85C -40C to +85C Package Description 24-Lead Lead Frame Chip Scale Package 24-Lead Lead Frame Chip Scale Package 24-Lead Lead Frame Chip Scale Package Package Option CP-24 CP-24 CP-24
1
Z = Pb-free part.
Rev. 0 | Page 18 of 20
Preliminary Technical Data NOTES
ADN2870
Rev. 0 | Page 19 of 20
ADN2870 NOTES
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04510-0-8/04(0)
Rev. 0 | Page 20 of 20

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