Application Note 1010: 2002 Methods Controlling Laser Drivers: PO

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Application Note 1010: 2002 Methods Controlling Laser Drivers: PO

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CONVERSION/SAMPLING CIRCUITS DIGITAL POTENTIOMETERS FIBER OPTIC CIRCUITS
Application Note 1010: 2002
Methods Controlling Laser Drivers: POTs DACs laser module designer fixed resistor, mechanical pot, digital pot, digital-to-analog converter (DAC) control laser driver's modulation bias currents. advantages programmable method (POT DAC) that manufacturing process automated digital control applied (e.g., compensate temperature). Using POTs more simple approach than DAC. There slight cost advantage using POT, this usually significant relative other pieces design. Using offer advantages, including improved linearity (translating ease software implementation ability required accuracy), increased board density, wider range resolutions, better optimization range, ease with negative voltage laser driver, unit-tounit consistency. This application note discusses advantages using these devices these applications provides overview biasing circuit requirements laser drivers.
Several families laser drivers from variety vendors designed resistors control modulation bias currents supplied laser driver. Potentiometers digital-to-analog converters (DACs) also used control these parameters. With each control method, there benefits tradeoffs that will affect overall laser driver's performance. fully understand advantages disadvantages choices laser driver control systems, some familiarity with laser drivers fiber module design essential. Originally, resistors were used control modulation bias currents, shown many laser driver data sheets. While least expensive component option, resistors don't allow real-time adjustments tweaking them during system calibration manufacturing step difficult time consuming. better solution mechanical potentiometer (variable resistor), which allows manual adjustment. volumes increase automation becomes important, mechanical potentiometers replaced digital potentiometers. Digital potentiometers also offer active digital control. Active digital control technique monitoring parameter (such output power) measuring with ADC, processing with digital engine, then using this information adjust parameter (such laser driver output current). Active digital engines compensate temperature dependence laser diodes. DACs used most places that potentiometer used, offer several advantages some designs.
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Control Section Laser Driver Many laser drivers have very simple structure that translates programming resistance (RPROGRAM) into output bias modulation depth currents (see Figure Typically, internal circuitry behind laser driver current control pins (i.e., IMOD, IBIAS) includes internal reference voltage source. output current this internal voltage source measured, amplified current amplifier, output which output laser driver. simple resistor connected between ground these control pins, provides consistent control current that circuit. Note that laser driver only cares about amount current pulled this pin, value resistor connected Hence, resistor replaced that controls this current, will described later. Typically, gain current amplifier order 100-200 (mA/mA), typical output currents 50-80mA.
Figure Laser driver internal architecture sensing value programming resistor Digital Options (and Advantages) Using digital place control resistor simplest most obvious approach (see Figure
Figure Programming laser driver with digital potentiometer
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However, resistance value available pots precisely line with desired range programming current. Additional resistors used more directly range onto range currents desired (see Figure Note that current varies with inverse resistance. Digital potentiometers usually have evenly spaced steps, which means that resulting step size programming current will large resistance range pot, small high resistance range.
Figure Programming laser driver current with optimized programming range potentiometer
Options (and Advantages) used place control laser driver currents. voltage output used with series resistor, such that current drawn from laser driver control programming current (see Figure ideal full scale/reference voltage same reference voltage internal particular laser driver. larger, usable range compromised
Figure Programming laser driver current with
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Some reasons design engineer would consider using include: Linearity: relationship between digital code resulting control current relationship. This linear relationship DAC. linear design primary advantages: linear design very regular predictable step size, easy calculate design calibrated appropriate end-target accuracy. discussed above, design have undesired step-size problem, relationship, hence design might able accuracy. control algorithm used adjust output current based measuring module output power much simpler design optimize throughput using linear control loop software algorithm. Density/Board Size: some designs, board layout actually made smaller with DAC. Since typically, there other control parameters requiring board, with extra channels (for laser driver modulation depth bias current control currents) chosen. Hence, function fulfilled another that already required another part design, rather than adding just this purpose. Resolution: Pots with resolution greater than bits commonly available. DACs found with wide range resolutions. Range optimization: full-scale voltage range correspond directly range currents desired choice resistor. laser driver available programming range needs change, with approach, resistor that necessary change; with approach, need changed different because resistance value embedded pot. Bipolar ease implementation: laser driver must from voltage rail, there fewer pots from which choose. Using bipolar DAC, similar voltage translation technique allows alternative. Unit Unit Consistency: Most digital pots have large (30% more) error resistance value. DACs resistors have errors significantly below depending voltage reference resistors used. This error trimmed calibration process. However, error needs taken into account error budget/calibration range calibration algorithm. Also, desired understand distribution required voltages understand control calibration process, data better controls more consistent. Conclusion laser module designer either fixed resistor, mechanical pot, digital pot, control laser driver's modulation bias currents. advantages programmable method (POT DAC) that manufacturing process automated digital control applied (e.g. compensate temperature). Using POTs more simple approach than DAC. There slight cost advantage using POT, this usually significant relative other pieces design.
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Using offer advantages, including improved linearity (translating ease software implementation ability required accuracy), increased board density, wider range resolutions, better optimization range, ease with negative voltage laser driver, unit-to-unit consistency.
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Application Note 1010: 2002 Methods Controlling Laser Drivers: PO

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