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AD8015ARZ Fiches technique(PDF) 4 Page - Analog Devices |
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AD8015ARZ Fiches technique(HTML) 4 Page - Analog Devices |
4 / 8 page REV. A –4– AD8015 SONET OC-3 SENSITIVITY ANALYSIS OC-3 Minimum Bandwidth = 0.7 × 155 MHz ≈ 110 MHz Total Current Noise = ( π/2) × 26.5 nA = 42 nA (assuming single pole response) To maintain a BER < 1 × 10–10 (1 error per 10 billion bits): Minimum current level needs to be > 13 × Total Current Noise = 541 nA (peak) Assume a typical photodiode current/power conversion ratio = 0.85 A/W Sensitivity (minimum power level) = 541/0.85 nW = 637 nW (peak) = –32.0 dBm (peak) = –35.0 dBm (average) The SONET OC-3 specification allows for a minimum power level of –31 dBm peak, or –34 dBm average. Using the AD8015 provides 1 dB margin. FDDI SENSITIVITY ANALYSIS FDDI Minimum Bandwidth = 0.7 × 125 MHz ≈ 88 MHz Total Current Noise = (π / 2) × 88 MHz 100 MHz × 26.5nA = 39 nA (assuming single pole response) To maintain a BER < 2.5 × 10–10 (1 error per 4 billion bits): Minimum current level needs to be > 12.6 × Total Current Noise = 492 nA (peak) Assume a typical photodiode current/power conversion ratio = 0.85 A/W Sensitivity (minimum power level) = 492/0.85 nW = 579 nW (peak) = –32.4 dBm (peak) = –35.4 dBm (average) The FDDI specification allows for a minimum power level of –28 dBm peak, or –31 dBm average. Using the AD8015 pro- vides 4.4 dB margin. THEORY OF OPERATION The simplified schematic is shown in Figure 5. Q1 and Q3 make up the input stage, with Q3 running at 300 µA and Q1 running at 2.7 mA. Q3 runs essentially as a grounded emitter. A large capacitor (0.01 µF) placed from V BYP to the positive supply shorts out the noise of R17, R21, and Q16. The first stage of the amplifier (Q3, R2, Q4, and C1) functions as an integrator, inte- grating current into the IIN pin. The integrator drives a differen- tial stage (Q5, Q6, R5, R3, and R4) with gains of +3 and –3. The differential stage then drives emitter followers (Q41, Q42, Q60 and Q61). The positive output of the differential stage pro- vides the feedback by driving RFB. The differential outputs are buffered using Q7 and Q8. The bandwidth of the AD8015 is set to within +20% of the nominal value, 240 MHz, by factory trimming R5 to 60 Ω. The following formula describes the AD8015 bandwidth: Bandwidth = 1/(2 π × C1 × R FB × (R5 + 2 re)/R4) where re (of Q5 and Q6) = 9 Ω each, constant over temperature, and RFB/R4 = 43.5, constant over temperature. The bandwidth equation simplifies, and the bandwidth depends only on the value of C1: Bandwidth = 1/(2 π × 3393 × C1). Q3 INPUT CLAMPS Q1 IIN Q16 R17 635 R1 300 R21 1.8k VBYP R2 3k +VS I10 0.75MA C1 0.2pF Q4 Q5 Q56 I1 1.5MA I2 3MA R5 60 R3 230 Q41 RFB Q6 R4 230 Q7 +VS Q42 Q8 330 330 –VS +OUTPUT R44 50 R43 50 I3 1MA I4 3MA I5 3MA I6 1MA I7 1MA I8 1MA I9 1MA 10k Q61 Q60 –OUTPUT Figure 5. AD8015 Simplified Schematiic |
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