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

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

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

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-

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-

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,

The bandwidth equation simplifies, and the bandwidth depends

only on the value of C1:

Bandwidth = 1/(2

π × 3393 × C1).

Q3

INPUT

CLAMPS

Q1

Q16

R17

635

R1

300

R21

1.8k

R2

3k

I10

0.75MA

C1 0.2pF

Q4

Q5

Q56

I1

1.5MA

I2

3MA

R5 60

R3

230

Q41

RFB

Q6

R4

230

Q7

Q42

Q8

330

330

+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