Datasheet ADN2870 Datasheet (Analog Devices)

3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps

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

Laser Diode Driver

ADN2870

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 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 ×4 mm LFCSP package specified over the −40°C to +85°C temperature range.

VCC

Tx_FAUL

Tx_FAIL

ADI

MICROCONTROLLER

DAC

ADC

DAC

1kΩ

VCC

GND

MPD

PAVSET

PAVREF

RPAV

ERREF

ERSET

VCC

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.

GND

GND GND

VCC

IMODP

IBIAS

CCBIAS

VCC

LASER

DATAP

DATAN

ALSFAIL

CONTROL

IMOD

IBIAS

VCC

IMODN

100Ω

ADN2870

IBMON IMMON

470Ω1kΩ

PAVCAP

GND

ERCAP

GND

04510-001

www.analog.com

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

Optical Waveforms Showing Dual-Loop Performance Over

Temperature Using DFB Tosa SUMITOMO SLT2486

Performance Characteristics....................................................... 9

Theory of Operation ...................................................................... 11

Dual-Loop Control .................................................................... 11

Control......................................................................................... 12

.......................... 8

............ 8

REVISION HISTORY

Volt a ge S etp oint C al i brat ion ..................................................... 12

Resistor Setpoint Calibration.................................................... 14

IMPD Monitoring...................................................................... 14

Loop Bandwidth Selection ........................................................ 15

Power Consumption .................................................................. 15

Automatic Laser Shutdown (T X_Disable).............................. 15

Bias and Modulation Monitor Currents.................................. 15

Data Inputs .................................................................................. 15

Laser Diode Interfacing............................................................. 16

Alarms.......................................................................................... 17

Outline Dimensions....................................................................... 18

Ordering Guide .......................................................................... 18

8/04—Revision 0: Initial Version

Rev. 0 | Page 2 of 20

ADN2870

SPECIFICATIONS

VCC = 3.0 V to 3.6 V. All specifications T

Table 1.

Parameter Min Typ Max Unit Conditions/Comments

LASER BIAS CURRENT (IBIAS)

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

MODULATION CURRENT (IMODP, IMODN)

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

2, 3

AVERAGE POWER SET (PAVSET)

Pin Capacitance 80 pF Voltage 1.1 1.2 1.35 V

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

EXTINCTION RATIO SET INPUT (ERSET)

Resistance Range 1.2 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

Photodiode Monitor Current (Average Current) 120 1000 µA

EXTINCTION RATIO REFERENCE VOLTAGE INPUT (ERREF)

Voltage Range 0.1 1 V

DATA INPUTS (DATAP, DATAN)

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

LOGIC INPUTS (ALS)

ALARM OUTPUT (FAIL)

OFF

MIN

to T

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

MAX

60 104 ps 60 96 ps

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

2 V

0.8 V

> 1.8 V

< 1.3 V

Voltage setpoint mode (RPAV fixed at 1 kΩ)

Voltage setpoint mode (RERSET fixed at 1 kΩ)

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

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

Rev. 0 | Page 3 of 20

ADN2870

Parameter Min Typ Max Unit Conditions/Comments

IBMON, IMMON DIVISION RATIO

IBIAS/IBMON

IBIAS/IBMON STABILITY

IMOD/IMMON 50 A/A

IBMON Compliance Voltage 0 1.3 V SUPPLY

VCC (w.r.t. GND)

Temperature range: –40°C to +85°C.

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.

Guaranteed by design and characterization. Not production tested.

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

Guaranteed by design. Not production tested.

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

ICC min for power calculation in the Power Consumption section.

All VCC pins should be shorted together.

3, 6

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

30 mA When IBIAS = IMOD = 0

3.0 3.3 3.6 V

CCVCC

ADN2870

IMODP

22Ω

BIAS TEE

80kHz → 27GHz

Figure 2. High Speed Electrical Test Output Circuit

TO HIGH SPEED DIGITAL OSCILLOSCOPE 50Ω INPUT

04510-034

Rev. 0 | Page 4 of 20

ADN2870

SFP TIMING SPECIFICATIONS

Table 2.

Parameter Symbol Min Typ Max Unit Conditions/Comments

ALS Assert Time t_off 1 5 µs

ALS Negate Time

Time to Initialize, Including

Reset of FAIL

t_on 0.83 0.95 ms

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

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

Guaranteed by design and characterization. Not production tested.

DATAP DATAN

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

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

SFP MODULE

VCC_Tx

1µH

3.3V

0.1µF 0.1µF 10µF

DATAP–DATAN

Figure 3. Signal Level Definition

V p-p

DIFF

= 2× V

SFP HOST BOARD

04510-002

Figure 4. Recommended SFP Supply

04510-003

Rev. 0 | Page 5 of 20

ADN2870

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 3.

Parameter Rating

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

(single-ended differential)

TEMPERATURE SPECIFICATIONS Operating Temperature Range

Industrial

Storage Temperature Range –65°C to +150°C

Junction Temperature (TJ max) 150°C LFCSP Package

Power Dissipation

θJA Thermal Impedance

θJCThermal Impedance 29.5°C/W Lead Temperature (Soldering 10 s) 300°C

___________________

Power consumption equations are provided in the Power Consumption

section.

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

1.5 V

−40°C to +85°C

(TJ max – TA)/θJA W 30°C/W

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.

Rev. 0 | Page 6 of 20

ADN2870

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

GND

VCC

IMODP

IMODN

GND

IBIAS

FAIL

IBMON

ERREF

VCC

ADN2870

CCBIAS

PAVSET

GND

VCC

IMMON

PAVREF

ERSET

RPAV

ALS DATAN DATAP GND PAVCAP ERCAP

04510-004

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.

Rev. 0 | Page 7 of 20

ADN2870

TYPICAL OPERATING CHARACTERISTICS

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

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

Note: No change to PAVCAP and ERCAP values

(ACQ LIMIT TEST) WAVEFORMS 1000

04510-016

31-1

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

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

(ACQ LIMIT TEST) WAVEFORMS 1000

OPTICAL WAVEFORMS SHOWING DUAL-LOOP PERFORMANCE OVER TEMPERATURE USING DFB TOSA SUMITOMO SLT2486

(ACQ LIMIT TEST) WAVEFORMS 1001

04510-047

31-1

Figure 9. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 2 PAV = 0 dBm, ER = 9 dB, Mask Margin 22%, T

(ACQ LIMIT TEST) WAVEFORMS 1001

= 25°C

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

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

(ACQ LIMIT TEST) WAVEFORMS 1000

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

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

31-1

04510-017

04510-020

Rev. 0 | Page 8 of 20

Figure 10. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 2

PAV = −0.2 dBm, ER = 8.96 dB, Mask Margin 21%, T

31-1

= 85°C

04510-048

ADN2870

PERFORMANCE CHARACTERISTICS

1.2

1.0

RISE TIME (ps)

0408020 60 100

MODULATION CURRENT (mA)

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

FALL TIME (ps)

04510-022

0.8

0.6

JITTER (rms)

0.4

0.2

0 204060801

MODULATION CURRENT (mA)

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

250

220

= 80mA

190

160

130

100

TOTAL SUPPLY CURRENT (mA)

BIAS

= 40mA

BIAS

= 20mA

04510-037

0408020 60 100

MODULATION CURRENT (mA)

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

DETERMINISTIC JITTER (ps)

20 40 8060 100

MODULATION CURRENT (mA)

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

04510-025

04510-042

0 204060801

MODULATION CURRENT (mA)

Figure 15. Total Supply Current vs. Modulation Current

Total Supply Current = I

+ Ibias + Imod

SUPPLY CURRENT (mA)

–50 –30 –10 10 30 50 70 90 110

Figure 16. Supply Current (I

TEMPERATURE (°C)

) vs. Temperature with ALS Asserted,

Ibias = 20 mA

04510-038

04510-027

Rev. 0 | Page 9 of 20

ADN2870

120

115

110

105

100

IBIAS/IBMON RATIO

–50 –30 –10 10 30 50 70 90 110

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

TEMPERATURE (°C)

OC48 PRBS31 DATA TRANSMISSION

LESS THAN 1µs

_OFF

04510-028

IMOD/IMMON RATIO

–50 –30 –10 10 30 50 70 90 110

TEMPERATURE (°C)

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

FAIL ASSERTED

04510-031

ALS

Figure 18. ALS Assert Time, 5 µs/div

OC48 PRBS31

DATA TRANSMISSION

_ON

ALS

Figure 19. ALS Negate Time, 200 µs/div

04510-029

04510-032

FAULT FORCED ON PAVSET

04510-045

Figure 21. FAIL Assert Time,1 µs/div

TRANSMISSION ON

POWER SUPPLY TURN ON

04510-046

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

Rev. 0 | Page 10 of 20

ADN2870

THEORY OF OPERATION

OPTICAL COUPLING

BIAS

SHA

CURRENT

MOD

SHA

BIAS

SWITCH

CURRENT

HIGH

SPEED

MOD

VCC

100 2

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.

ER =

P1 + P

OPTICAL POWER

∆

Ith CURRENT

LI =

∆

04510-005

Figure 23. Laser Transfer Function

DUAL-LOOP CONTROL

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

MPD

INPUT

1.2V

BGAP

ERSET

PAVSET

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 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 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 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 a n d 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.

04510-039

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 25°C

3.5

3.0

2.5

2.0

1.5

OPTICAL POWER (mW)

1.0

0.5

20 40 60

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 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.

NONLINEAR LI CURVE AT 80°C

CURRENT (mA)

04510-008

10080

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.

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.

RPAVRPPAVREF

SPAV

RERREF ×

ERSET

where:

is the monitor photodiode responsivity.

is the dc optical power specified on the laser data sheet.

is MPD current at that specified PCW.

MPD_CW

is the average power required.

ER is the desired extinction ratio (ER = P1/P0).

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

(Volts)

−

CWMPD

ERSET

××=

(Volts)

must be 1 kΩ resistors with

Rev. 0 | Page 12 of 20

ADN2870

Tx_FAUL

Tx_FAIL

MICROCONTROLLER

ADI

DAC

ADC

DAC

1kΩ

VCC

GND

MPD

PAVSET

PAVREF

RPAV

ERREF

ERSET

VCC

GND

GND GND

ALSFAIL

IMOD

CONTROL

IBMON IMMON

IBIAS

470Ω1kΩ

VCC

IMODN

ADN2870

PAVCAP

GND

100Ω

GND

ERCAP

VCC

IMODP

IBIAS

CCBIAS

LASER

DATAP

DATAN

VCC

04510-009

Figure 26. ADN2870 Using Microconverter Calibration and Monitoring

ERCAP

VCC

IMODP

IBIAS

CCBIAS

LASER

DATAP

DATAN

VCC

04510-010

VCC

GND

MPD

VCC

PAVREF

RPAV

PAVSET

ERSET

ERREF

VCC

GND

GND GND

ALSFAIL

IMOD

CONTROL

IBMON IMMON

IBIAS

470Ω1kΩ

VCC

IMODN

ADN2870

PAVCAP

GND

100Ω

GND

Figure 27. ADN2870 Using Resistor Setpoint Calibration of Average Power and Energy Ratio

Rev. 0 | Page 13 of 20

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:

V23.1

CWMPD

C ADC INPUT

(Ω)

V23.1

−

R 1kΩ

(Ω)

PAVSET

ADN2870

RPAV

04510-043

PAVSET

ERSET

where:

is the monitor photodiode responsivity.

is the dc optical power specified on the laser data sheet.

is MPD current at that specified PCW.

MPD_CW

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 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.

PHOTODIODE

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 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

LDPHOTODIODE

C ADC

DIFFERENTIAL

INPUT

Figure 29. Differential Measurement of IMPD Across a Sense Resistor

C ADC

INPUT

PHOTODIODE

Figure 30. Single Measurement of IMPD Across a Sense Resistor

ADN2870

200Ω

RESISTOR

ADN2870

200Ω RESISTOR

PAVSET

VCC VCC

PAVSET

10pF

04510-011

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.

Rev. 0 | Page 14 of 20

ADN2870

PHOTODIODE

C ADC

INPUT

Figure 31. Single Measurement of IMPD Across a

Sense Resistor in Resistor Setpoint IMPD Monitoring

PAVSET

ADN2870

04510-040

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:

EPAVCAP ×−=

ERCAP =

PAVCAP

For voltage setpoint control:

EPAVCAP ×−=

ERCAP =

PAVCAP

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

P0P1LI−

Imo

(mW/mA)

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

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 capacitors own tolerance.

)(62.3 Farad

)(2Farad

)(628.1 Farad

)(2Farad

POWER CONSUMPTION

The ADN2870 die temperature must be kept below 125°C. The LFCSP package has an exposed paddle, which should be connected such that is at the same potential as the ADN2870 ground pins. Power consumption can be calculated as follows:

= ICC min + 0.3 I

P = V

× ICC + (I

MODN_PIN

= T

DIE

)/2

AMBIENT

+ θJA × P

Thus, the maximum combination of I

BIAS

MOD

× V

BIAS_PIN

) + I

BIAS

MOD

+ I

MOD

MODP_PIN

must be

calculated.

where:

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

= I

Specifications with I

is the die temperature.

DIE

is the ambient temperature.

AMBIENT

is the voltage at the IBIAS pin.

BIAS_PIN

V V

is the voltage at the IMODP pin.

MODP_PIN

is the voltage at the IMODN pin.

MODN_PIN

BIAS

MOD

= 0.

AUTOMATIC LASER SHUTDOWN (TX_DISABLE)

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

BIAS AND MODULATION MONITOR CURRENTS

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

DATA INPUTS

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

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 Ω to 7 Ω, and have axial leads. The circuit shown works over the full range of data rates from 155 Mbps to 3.3 Gbps including multirate operation (with no change to PAVCAP and ERCAP values); see the Typical Operating Characteristics for multirate performance examples. Coax lasers have special characteristics that make them difficult to interface to. They tend to have higher inductance, and their impedance is not well controlled. The circuit in Figure 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)

100nF

IMODP

ADN2870

IBIAS

CCBIAS

24Ω

30Ω

Tx LINE

30Ω

Tx LINE

BLMI8HG60ISN1D

Figure 32. Recommended Interface for ADN2870 AC Coupling

R 24Ω

C 2.2pF

VCC

04510-014

The 30 Ω transmission line used is a compromise between drive current required and total power consumed. Other transmission line values can be used, with some modification of the component values. The R and C snubber values in Figure 32, 24 Ω and

2.2 pF, respectively, represent a starting point and must be tuned for the particular model of laser being used. R

, 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 Ω characteristic impedance. Instead, a 25 Ω 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.

L1 = 0.5nH

R1 = 15Ω

IMODN

ADN2870

IMODP

CCBIAS IBIAS

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

R1 = 15Ω (12 TO 24Ω)

Figure 33. Recommended Differential Drive Circuit

C1 = C2 = 100nF

20Ω TRANMISSION LINES

L2 = 0.5nH

Rev. 0 | Page 16 of 20

L4 = BLM18HG601SN1

L3 = 4.7nH

SNUBBER

L5 = 4.7nH

L6 = BLM18HG601SN1

TOCAN/VCSEL

LIGHT

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.

• Undervoltage in IBIAS node (laser diode cathode) that

would increase the laser power.

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

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

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

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

Table 5. 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

RPAV

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

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

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

Fault state occurs Fault state occurs

Fault state occurs

Voltage mode: Does not increase average power

Resistor mode: Fault state occurs

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

Does not increase laser average power

Rev. 0 | Page 17 of 20

ADN2870

OUTLINE DIMENSIONS

0.08

0.60 MAX

BOTTOM

VIEW

2.50 REF

PIN 1 INDICATOR

2.25

2.10 SQ

1.95

0.25 MIN

4.00

PIN 1

INDICATOR

1.00

0.85

0.80

SEATING PLANE

12° MAX

BSC SQ

TOP

VIEW

0.80 MAX

0.65TYP

COMPLIANT TO JEDECSTANDARDSMO-220-VGGD-2

0.30

0.23

0.18

3.75

BSC SQ

0.20 REF

0.60 MAX

0.05 MAX

0.02 NOM

0.50

BSC

0.50

0.40

0.30

COPLANARITY

Figure 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

ADN2870ACPZ-RL

ADN2870ACPZ-RL7

−40°C to +85°C

24-Lead Lead Frame Chip Scale Package CP-24

Z = Pb-free part.

Rev. 0 | Page 18 of 20

Preliminary Technical Data ADN2870

NOTES

Rev. 0 | Page 19 of 20

ADN2870

NOTES

D04510–0–8/04(0)

Rev. 0 | Page 20 of 20

Datasheet ADN2870 Datasheet (Analog Devices)

Specifications and Main Features

Frequently Asked Questions

User Manual

FEATURES

APPLICATIONS

GENERAL DESCRIPTION

SPECIFICATIONS

SFP TIMING SPECIFICATIONS

ABSOLUTE MAXIMUM RATINGS

ESD CAUTION

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

TYPICAL OPERATING CHARACTERISTICS

PERFORMANCE CHARACTERISTICS

THEORY OF OPERATION

DUAL-LOOP CONTROL

Dual Loop

Average Power Control Loop

Modulation Control Loop

CONTROL

VOLTAGE SETPOINT CALIBRATION

RESISTOR SETPOINT CALIBRATION

IMPD MONITORING

Voltage Setpoint

Method 1: Measuring Voltage at RPAV

Method 2: Measuring IMPD Across a Sense Resistor

Resistor Setpoint

LOOP BANDWIDTH SELECTION

POWER CONSUMPTION

AUTOMATIC LASER SHUTDOWN (TX_DISABLE)

BIAS AND MODULATION MONITOR CURRENTS

DATA INPUTS

LASER DIODE INTERFACING

ALARMS

OUTLINE DIMENSIONS

ORDERING GUIDE