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AD96606 Datasheet(PDF) 7 Page - Analog Devices |
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AD96606 Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page AD9660 REV. 0 –7– Choosing CBIAS HOLD and CWRITE HOLD Choosing values for the hold capacitors, CWRITE and CHOLD, is a tradeoff between output current droop when the control loops are open, and the time it takes to calibrate and recalibrate the laser power when the loops are closed. The amount of output current droop is determined by the value of the hold capacitor and the leakage current at that node. When either of the two control loops are open (WRITE CAL or BIAS CAL logic LOW), the pin connections for the hold capacitors (WRITE HOLD and BIAS HOLD) are high imped- ance inputs. Leakage currents will range from ±200 nA; this low current minimizes the droop in the output power level. As- suming the worst case current of ±200 nA, the output current will change as follows: ±∆I BIAS = 18 ×10 −9 t BIAS HOLD C BIAS HOLD ±∆I MODULATED = 18 ×10 −9 t WRITE HOLD C WRITE HOLD To choose a value, the user will need to determine the amount of time the loop will be in hold mode, tWRITE HOLD or tBIAS HOLD, the maximum change in laser output power the application can tolerate, and the laser efficiency (defined as the change in laser output power to the change in laser diode current). As an ex- ample, if an application requires 5 mW of laser power ±5%, and the laser diode efficiency is 0.25 mW/mA, then ∆I MAX = 5 mW × (5% ) / 0. 25 mW mA = 1.0 mA If the same application had a hold time requirement of 250 µs, then the minimum value of the hold capacitor would be: C HOLD = 18 ×10 −9 × 250 µs 1.0 mA = 4.5 nF When determining the calibration time, the T/H and the exter- nal hold capacitor can be modeled using the simple RC circuit illustrated in Figure 7. CHOLD R AD9660 TZA T/H WRITE LEVEL OR BIAS LEVEL POWER MONITOR WRITE HOLD OR BIAS HOLD EXTERNAL HOLD CAPACITOR Figure 7. Circuit Model for Determining Calibration Times Minimum specifications for IMONITOR MAX should be used when choosing RGAIN. Users are cautioned that laser diode/photo di- ode combinations that produce monitor currents that are less than IMONITOR MAX in the equation above will produce higher la- ser output power than predicted, which may damage the laser diode. Such a condition is possible if RGAIN is calculated using typical instead of minimum monitor current specifications. In that case the input range to the AD9660 BIAS LEVEL and WRITE LEVEL inputs should be limited to avoid damaging laser diodes. Although not recommended, another approach would be to use a potentiometer for RGAIN. This allows users to optimize the value of RGAIN for each laser diode/photo diode combina- tion’s monitor current. The drawback to this approach is that potentiometer’s stray inductance and capacitance may cause the transimpedance amplifier to overshoot and degrade its settling, and the value of CGAIN may not be optimized for the entire potentiometer’s range. CGAIN optimizes the response of the transimpedance amplifier and should be chosen as from the table below. Choosing CGAIN larger than the recommended value will slow the response of the amplifier. Lower values improve TZA bandwidth but may cause the amplifier to oscillate. Table I. Recommended RGAIN CGAIN ≥2.5 kΩ 2 pF 1.5 k Ω 3 pF 1 k Ω 4 pF 500 Ω 8 pF The circuit in Figure 6 allows an adjustable gain with low vari- ance in bandwidth, but requires several external components. VREF RINTERNAL 50 Ω GAIN C1 RF CF POWER MONITOR VREF R1 R2 R3 1.7k Ω 2I PIN 1:2 IMONITOR SENSE IN AD9660 EQUIVALENT CIRCUIT REQ VREF RINTERNAL 50 Ω CEQ REQ + 50Ω = (RF + 50Ω) (1+ + ) R2 R1 R1 RF Figure 6. Adjustable Gain Configuration |
Número de pieza similar - AD96606_15 |
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Descripción similar - AD96606_15 |
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