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
A and Q1 running
at 2.7 mA. Q3 runs essentially as a grounded emitter. A large
capacitor (0.01
F) 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
. 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
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
–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