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ADN2847 Datasheet(PDF) 10 Page - Analog Devices |
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ADN2847 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADN2847 Data Sheet Rev. B | Page 10 of 16 The laser degrade alarm, DEGRADE, is provided to give a warning of imminent laser failure if the laser diode degrades further or environmental conditions continue to stress the LD, such as increasing temperature. The laser fail alarm, FAIL, is activated when the transmitter can no longer be guaranteed to be SONET/SDH compliant. This occurs when one of the following conditions arises: • The ASET threshold is reached. • The ALS pin is set high. This shuts off the modulation and bias currents to the LD, resulting in the MPD current dropping to zero. This gives closed-loop feedback to the system that ALS has been enabled. DEGRADE is raised only when the bias current exceeds 90% of ASET current. MONITOR CURRENTS IBMON, IMMON, IMPDMON, and IMPDMON2 are current controlled current sources from VCC. They mirror the bias, modulation, and MPD current for increased monitoring functionality. An external resistor to GND gives a voltage proportional to the current monitored. If the monitoring functions IMPDMON and IMPDMON2 are not required, the IMPD pin and the IMPD2 pin must be grounded and the monitor photodiode output must be connected directly to the PSET pin. DUAL MPD DWDM FUNCTION (48-LEAD LFCSP ONLY) The ADN2847 has circuitry for a second monitor photodiode, MPD2. The second photodiode current is mirrored to IMPDMON2 for wavelength control purposes and is summed internally with the first monitor photodiode current for the power control loop. For single MPD circuits, the MPD2 pin is tied to GND. This enables the system designer to use the two currents to control the wavelength of the laser diode using various optical filtering techniques inside the laser module. If the monitor current functions IMPDMON and IMPDMON2 are not required, then the IMPD pin and IMPD2 pin can be grounded and the monitor photodiode output can be connected directly to PSET. IDTONE (48-LEAD LFCSP ONLY) The IDTONE pin is supplied for fiber identification/supervisory channels or control purposes in WDM. This pin modulates the optical one level over a possible range of 2% of minimum IMOD to 10% of maximum IMOD. The level of modulation is set by con- necting an external current sink between the IDTONE pin and ground. There is a gain of two from this pin to the IMOD current. Figure 12 shows how the AD9850/AD9851 or the AD9834 can be used with the ADN2847 to allow fiber identification. If the ID_TONE function is not used, the IDTONE pin should be tied to VCC. Note that using IDTONE during transmission can cause optical eye degradation. DATA AND CLOCK INPUTS Data and clock inputs are ac-coupled (10 nF capacitors are recommended) and terminated via a 100 Ω internal resistor between DATAP and DATAN, and also between the CLKP pin and the CLKN pin. There is a high impedance circuit to set the common-mode voltage that is designed to allow for maximum input voltage headroom over temperature. It is necessary that ac coupling is used to eliminate the need for matching between common-mode voltages. ADN2847 R = 2.5kΩ, DATA R = 3kΩ, CLK (TO FLIP-FLOPS) 400µA TYP DATAP DATAN VREG R 50Ω 50Ω Figure 10. AC Coupling of Data Inputs For input signals that exceed 500 mV p-p single-ended, it is necessary to insert an attenuation circuit as shown in Figure 11. R1 R2 R3 DATAP/CLKP DATAN/CLKN RIN ADN2847 NOTES 1. RIN = 100Ω = THE DIFFERENTIAL INPUT IMPEDANCE OF THE ADN2847. Figure 11. Attenuation Circuit CCBIAS When the laser is used in ac-coupled mode, the CCBIAS pin and the IBIAS pin should be tied together (Figure 15). In dc- coupled mode, CCBIAS should be tied to VCC. IBIAS To achieve optimum eye quality, one pull-up resistor (RZ) is necessary, as shown in both circuits in Figure 14 and Figure 15. The recommended resistor RZ value is approximately 200 Ω ~ 500 Ω. AUTOMATIC LASER SHUTDOWN The ADN2847 ALS allows compliance to ITU-T-G958 (11/94), section 9.7. When ALS is logic high, both bias and modulation currents are turned off. Correct operation of ALS can be confirmed if the FAIL alarm is raised when ALS is asserted. Note that this is the only time DEGRADE is low while FAIL is high. |
Similar Part No. - ADN2847_15 |
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Similar Description - ADN2847_15 |
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