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REV. 0 –6– AD9660 RECALIBRATE THE BIAS LOOP POWER-UP OR LASER NOT IN USE BIAS CAL TIME WRITE LOOP CAL TIME RECALIBRATE WRITE LOOP LASER POWER MODULATED BETWEEN BIAS AND WRITE LEVELS DISABLE BIAS CAL WRITE CAL WRITE PULSE LASER OUTPUT POWER CALIBRATED WRITE CALIBRATED BIAS WRITE LOOP HOLD TIME Figure 4. Normal Operating Mode The disable circuit turns off IOUT and returns the hold capacitor voltages to their minimum levels (minimum output current) when DISABLE = logic HIGH. It is used during initial power- up of the AD9660 or during time periods when the laser is inac- tive. When the AD9660 is re-enabled the control loops must be recalibrated. Normal operation of the AD9660 involves (in order, see figure): 1. The AD9660 is enabled (DISABLE = logic LOW). 2. The input voltages (BIAS LEVEL and WRITE LEVEL) are driven to the appropriate levels to set the calibrated laser di- ode output power levels. 3. The bias loop is closed for calibration (BIAS CAL = logic HIGH), and then opened (BIAS CAL = logic LOW). 4. The write loop is closed for calibration (WRITE PULSE and WRITE CAL = logic HIGH) and then opened. 5. While both loops are open, the laser is pulsed between the two calibrated levels by WRITE PULSE. 6. The bias and write loops are periodically recalibrated as needed. 7. The AD9660 is disabled when the laser will not be pulsed for an indefinite period of time. Control Loop Transfer Functions The relationship between IMONITOR and VBIAS LEVEL is I MONITOR = V BIAS LEVEL − VREF 1.85 × (R GAIN + 50 Ω) once the bias loop is calibrated. When the bias loop is open (BIAS CAL = logic LOW), its output current, IBIAS, is propor- tional to the held voltage at BIAS HOLD; the external hold ca- pacitor on this pin determines the droop error in the output bias current between calibrations. The relationship between IMONITOR and VWRITE LEVEL is I MONITOR = V WRITE LEVEL − VREF 1.85 × (R GAIN + 50 Ω) once the write loop is calibrated. The current supplied by the write loop output is referred to as the modulation current, IMODULATION. When the write loop is open (WRITE CAL logic LOW), IMODULATION is proportional to the held voltage at WRITE HOLD. The external hold capacitor (WRITE HOLD) determines the droop error between calibrations. IMODULATION may be switched on and off by WRITE PULSE when the write loop is open. 4 3 0 2 1 0 120 20 40 60 80 100 CONSTANT WRITE POWER BIAS CALIBRATION POWER 0 °C CASE 25 °C CASE 50 °C CASE BIAS MOD FORWARD CURRENT – mA Figure 5. Typical Laser Diode Current-to-Optical Power Curve The sections below discuss choosing the external components in the feedback loops for a particular application. Choosing RGAIN The gain resistor, RGAIN, allows the user to match the feedback loop’s transfer function to the laser diode/photo diode combination. The user should define the maximum laser diode output power for the intended application, PLD MAX, and the corresponding photo diode monitor current, IMONITOR MAX. A typical laser di- ode transfer function is illustrated in Figure 5. RGAIN should be chosen as: R GAIN = 1.6 V 1.85 × I MONITOR MAX − 50 Ω . The laser diode’s output power will then vary from 0 to PLD MAX for an input range of VREF to VREF +1.6 V @ the BIAS LEVEL and WRITE LEVEL inputs. |
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