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ADN2843CHIPSET Datasheet(PDF) 7 Page - Analog Devices |
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ADN2843CHIPSET Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page REV. 0 ADN2843 –7– 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 arise: ∑ 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 the alarm threshold. ALARM INTERFACE The alarm voltages are open collector outputs. An internal pull-up resistor of 30k that is used to pull the logic high value to VCC. However, this can be overdriven with an external resistor, allowing alarm interfacing to non-VCC levels. The FAIL output may not be connected directly to the ALS pin to shut down the bias and modulation currents. It can however be latched using a flip-flop, and the output of the flip-flop can then be used to activate ALS. Non-VCC alarm output levels must be below the VCC used for the ADN2843. DATA INPUTS Figure 2 shows a simplified schematic of the ADN2845 data inputs. The data inputs are terminated via the equivalent of a 100 internal resistor between DATAN and DATAP. This provides 50 termination for single-ended signals. The actual signal on the switching devices is attenuated by a factor of 2 internally. There is a high impedance circuit to set the common- mode voltage, which is designed to change over temperature. It is recommended that ac coupling be used to eliminate the need for matching between the common-mode voltages. 25 25 25 25 2k INTERNAL REFERENCE ADN2845 DATAN DATAP Figure 2. Simplified Schematic of Data Inputs MONITOR CURRENTS IBMON, IMMON mirror the bias, modulation current at a ratio of 1:100 for increased monitoring functionality. IMPDMON and IMPDMON2 mirror the current in IMPD and IMPD2, respec- tively, with a ratio of 1. All monitors source current from VCC. If the MPD monitoring function is not required, then the IMPD pin should be tied to ground and the monitor photodiode cathode should be connected directly to the PSET pin. When the MPD monitor functions are not used, IMPDMON and IMPDMON2 should be tied to VCC. MPD CURRENT The maximum average MPD current is specified in the specifica- tions section. This maximum current specified is limited by the MPD monitoring circuitry. If the monitoring function is not required, then IMPD and IMPD2 should be grounded, the moni- tor photodiode cathode should be connected directly to the PSET node, and IMPDMON and IMPDMON2 should be tied to VCC. MPD currents as high as 3 mA can be used in this configuration. Another way to increase the MPD current range without sacri- ficing the monitoring function is to use IMPD and IMPD2 in parallel. This effectively doubles the current range but raises the lower MPD current specification from 50 A to 100 A. If this configuration is used, the IMPDMON and IMPDMON2 pins should be tied together and terminated with a single resistor. The mirror ratio of 1 is maintained in this configuration. DUAL MPD DWDM FUNCTION The MPD function mirrors the current in MPD to the PSET pin and to the IMPDMON pin with a ratio of 1. A second monitor photodiode can be connected to the IMPD2 pin. Its current is mirrored to IMPDMON2 and also to the PSET pin, where it is summed with the current mirrored from IMPD. The two MPD monitor currents can be used as inputs to a DWDM wavelength control function when used in combination with various optical filtering techniques. If the IMPD monitor function is not required, the monitor photodiode can be directly connected to the PSET pin, and the IMPD pin must be tied to GND. If the IMPD2 pin is not being used, it should be tied to GND. IDTONE The IDTONE pin is supplied for fiber identification/supervisory channels or for control purposes. This pin modulates the optical one level by adding a current to IMOD over a possible range of 2% of minimum IMOD to 10% of maximum IMOD. The IDTONE current is set by an external current sink connected to the IDTONE pin. There is a gain of 2 between the IDTONE pin and the IMOD current. To ratio the IDTONE current to IMOD, the input current can be derived from the IMMON output current. If the IDTONE function is not being used, this pin must be tied to VCC to properly disable it. Note that using IDTONE during transmission may cause optical eye degradation. AUTOMATIC LASER SHUTDOWN (ALS) The ADN2843 ALS allows compliance to ITU-T-G958 (11/94), Section 9.7. When ALS is asserted, both bias and modulation currents are turned off. In ALS mode, current is sourced to the laser from the IBIAS pin, which reverse biases the laser and ensures that it is turned off. Correct operation of ALS can be confirmed by the FAIL alarm being raised when ALS is asserted. Note this is the only time that DEGRADE will be low while FAIL is high. Note that for correct ALS operation, the ALS pin on the ADN2845 and ADN2844 should be connected and termi- nated with a 10 k W resistor. The ADN2843 ALS should be driven with correct logic levels (see Specifications section). ALS should never be left floating. POWER DISSIPATION The power dissipation of the ADN2845 can be calculated using the following expressions: ImA I mA ImA PV I A V I A V I A CC MOD BIAS CC CC IMOD MOD IBIAS BIAS =+ ¥ ()+¥ () =¥ ()+¥ () +¥ () 75 1 75 0 3 2 .. / where VIMOD is the average voltage on the IMOD pin, and VIBIAS is the average voltage on the IBIAS pin. |
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