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AD8015 Datasheet PDF : 8 Pages
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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
88 MHz
Total Current Noise = (π / 2) ×
× 26.5 nA
100 MHz
= 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 VBYP 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 × RFB × (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).
+VS
R17 R1
R2
635 300
3k
Q4
R21
1.8k
Q16
INPUT
CLAMPS
VBYP
Q1 Q3
IIN
C1 0.2pF
+VS
I10
0.75MA
R3
R4
230
230
Q41
Q5 Q6
R5 60
Q42
Q8
Q7
330
+OUTPUT
R44 50
Q60
330
–OUTPUT
Q61 R43 50
I1
1.5MA
Q56
I2
3MA
I3
1MA
I4
3MA
RFB
10k
I5
3MA
I6
1MA
I7
1MA
I8
1MA
I9
1MA
–VS
Figure 5. AD8015 Simplified Schematiic
–4–
REV. A
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