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ADN2531ACPZ-WP Datasheet(PDF) 12 Page - Analog Devices |
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ADN2531ACPZ-WP Datasheet(HTML) 12 Page - Analog Devices |
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12 / 20 page  ADN2531 Rev. 0 | Page 12 of 20 The equivalent circuits of the BSET, IBIAS, and IBMON pins are shown in Figure 26 to Figure 28. VCC BSET VCC 800 Ω 200 Ω Figure 26. Equivalent Circuit of the BSET Pin 10 Ω 2k Ω 100 Ω IBIAS VCC VCC Figure 27. Equivalent Circuit of the IBIAS Pin IBMON 100 Ω 500 Ω VCC VCC VCC Figure 28. Equivalent Circuit of the IBMON Pin The recommended configuration for the BSET, IBIAS, and IBMON pins is shown in Figure 29. ADN2531 BSET VBSET GND IBMON IBIAS TO LASER CATHODE L RIBMON 750 Ω IBIAS Figure 29. Recommended Configuration for BSET, IBIAS, and IBMON Pins The circuit used to drive the BSET voltage must be able to drive the 1 kΩ input resistance of the BSET pin. For proper operation of the bias current source, the voltage at the IBIAS pin must be between the compliance voltage specifications for this pin over supply, temperature, and bias current range (see Table 1). The maximum compliance voltage is specified for only two bias current levels (10 mA and 100 mA), but it can be calculated for any bias current by VCOMPLIANCE (V) = VCC (V) − 0.75 − 4.4 × IBIAS (A) See the Headroom Calculations section for examples. The function of Inductor L is to isolate the capacitance of the IBIAS output from the high frequency signal path. For recommended components, see Table 6. AUTOMATIC LASER SHUTDOWN (ALS) The ALS pin is a digital input that enables/disables both the bias and modulation currents, depending on the logic state applied, as shown in Table 5. Table 5. ALS Logic States ALS Logic State IBIAS and IMOD High Disabled Low Enabled Floating Enabled The ALS pin is compatible with 3.3 V CMOS and LVTTL logic levels. Its equivalent circuit is shown in Figure 30. VCC VCC ALS 100 Ω 42k Ω 2k Ω Figure 30. Equivalent Circuit of the ALS Pin MODULATION CURRENT The modulation current can be controlled by applying a dc voltage to the MSET pin. This voltage is converted into a dc current via a voltage-to-current converter that uses an operational amplifier and a bipolar transistor, as shown in Figure 31. 100 Ω 200 Ω 400 Ω MSET GND IMODP IMODN ADN2531 VCC FROM CPA STAGE IMOD Figure 31. Generation of Modulation Current on the ADN2531 The dc current is switched by the data signal applied to the input stage (DATAP and DATAN pins) and gained up by the output stage to generate the differential modulation current at the IMODP and IMODN pins. The output stage also generates the active back-termination, which provides proper transmission line termination. Active back-termination uses feedback around an active circuit to synthesize a broadband termination resistance. This provides excellent transmission line termination while dissipating less power than a traditional resistor passive back- termination. No portion of the modulation current flows in the active back-termination resistance. All of the preset modulation current (IMOD), the range of which is specified in Table 1, flows into the external load. |
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