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MAX3934BE Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX3934BE Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 10 page Current Monitors The MAX3934 features a bias current monitor output (BIASMON) and a modulation current monitor output (MODMON). The voltage at BIASMON is equal to (IBIAS × RBIAS) + VEE, and the voltage at MODMON is equal to (IMOD × RMOD) + VEE, where IBIAS represents the laser bias current, IMOD represents the modulation current, and RBIAS and RMOD are internal 6 Ω and 3Ω (±10%) resistors, respectively. BIASMON and MODMON should be connected to the inverting input of an operational amplifier to program the bias and modulation current (see the Design Procedure section). Design Procedure When designing a laser transmitter, the optical output usually is expressed in terms of average power and extinction ratio. Table 1 gives relationships helpful in converting between the optical average power and the modulation current. These relationships are valid if the mark density and duty cycle of the optical waveform are 50%. Programming the Modulation Current For a desired laser average optical power (PAVG) and optical extinction ratio (re) the required modulation cur- rent can be calculated based on the laser slope effi- ciency ( η) using the equations in Table 1. To program the desired modulation current, connect the inverting input of an op amp (such as the MAX4281) to MODMON and connect the output to MODSET. Connect the positive op-amp voltage supply to VCC and the negative supply to VEE (for +5V opera- tion, VCC = +5V and VEE = ground; for -5.2V operation VCC = ground and VEE = -5.2V). Connect a reference voltage (VMOD) to the noninverting input of the op amp to set the modulation current. See the IMOD vs. VMOD graph in the Typical Operating Characteristics to select the value of VMOD that corresponds to the required modulation current. Programming the Bias Current To program the desired laser bias current, connect the inverting input of an op amp (such as the MAX4281) to BIASMON and connect the output to BIASSET. Connect the positive op-amp voltage supply to VCC and the negative supply to VEE (for +5V operation, VCC = +5V and VEE = ground; for -5.2V operation, VCC = ground and VEE = -5.2V). Connect a reference voltage (VBIAS) to the noninverting input of the op amp to set the laser bias current. Refer to the IBIAS vs. VBIAS graph in the Typical Operating Characteristics to select the value of VBIAS that corresponds to the required laser bias current. External Op-Amp Selection External op amps are required for regulating the bias and modulation currents. The ability to operate from a single supply with input common-mode range extending to the negative supply rail is critical in op-amp selection. Low bias current and high PSNR are also important. The op-amp gain bandwidth must be high enough to regu- late at the power-supply ripple frequency to maintain the PSNR of the laser driver. Filtering the op-amp output is recommended (see the Typical Application Circuit). To maintain stability, the filter capacitor should be smaller than the op-amp capacitive load specification. Interfacing with Laser Diodes Refer to Maxim application note HFAN-2.0: Interfacing Maxim Laser Drivers with Laser Diodes for detailed information. The MAX3934 contains an integrated damping resistor (RD) with values of 12 Ω or 15Ω, depending on part ver- sion. The modulation output is optimized for driving a 20 Ω load; therefore, the total series load of RD and RLD 10.7Gbps Compact Laser Diode Driver 8 _______________________________________________________________________________________ PARAMETER SYMBOL RELATION Average Power PAVG PAVG = (P0 + P1) / 2 Extinction Ratio re re = P1 / P0 Optical Power of a “1” P1 P1 = 2PAVG re / (re + 1) Optical Power of a “0” P0 P0 = 2PAVG / (re + 1) Optical Amplitude PP-P PP-P = P1 - P0 = 2PAVG(re - 1) / (re + 1) Laser Slope Efficiency ηη = PP-P / IMOD Modulation Current IMOD IMOD = PP-P / η Table 1. Optical Power Relations Note: Assuming a 50% average duty cycle and mark density. TIME P0 P1 OPTICAL POWER PAVG PP-P Figure 5. Optical Power Definitions |
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