AD8015

REV. A

–3–

.

V1

CLOCK

RECOVERY

LPF:

3dB@

0.7 x F

LPF:

3dB@

0.7 x F

QUANTIZER

R > 40

Ω

C1 >100pF

CLK

DATA

R

R

C1

10k

Ω

5

6

7

8

4

3

2

1

AD8015

50

Ω

+1

G = 3

G = 30

50

Ω

+1

– +

1.7V

1.7V

Figure 3. Fiber Optic Receiver Application: Photodiode

Referred to Positive Supply

PHOTODIODE REFERRED TO NEGATIVE SUPPLY

Figure 4 shows the AD8015 used in a circuit where the photo-

diode is referred to the negative supply. This results in a larger

back bias voltage than when referring the photodiode to the

positive supply. The larger back bias voltage on the photodiode

decreases the photodiode’s capacitance thereby increasing its

bandwidth. The R2, C2 network shown in Figure 4 is added to

decouple the photodiode to the positive supply. This improves

PSRR.

1.7V

R2

C2

R > 40

Ω

C1 >100pF

R2 AND C2 OPTIONAL

FOR IMPROVED PSRR

V1

CLOCK

RECOVERY

LPF:

3dB@

0.7 x F

LPF:

3dB@

0.7 x F

QUANTIZER

CLK

DATA

R

R

C1

10k

Ω

5

6

7

8

4

3

2

1

AD8015

50

Ω

+1

G = 3

G = 30

50

Ω

+1

– +

1.7V

Figure 4. Fiber Optic Receiver Application: Photodiode

Referred to Negative Supply

FIBER OPTIC SYSTEM NOISE PERFORMANCE

The AD8015 maintains 26.5 nA referred to input (RTI) to 100

MHz. Calculations below translate this specification into mini-

mum power level and bit error rate specifications for SONET

and FDDI systems. The dominant sources of noise are: 10 k

Ω

feedback resistor current noise, input bipolar transistor base

current noise, and input voltage noise.

The AD8015 has dielectrically isolated devices and bond pads

noise is negligible at lower frequencies, but can become the

critical to maintaining low noise levels at high frequencies. The

connection and should be left unconnected in an application.

ground plane or metal runs near Pins 1, 2, and 3 and to mini-

0.4 pF without the photodiode. Photodiodes used for SONET

or FDDI systems typically add 0.3 pF, resulting in roughly

0.7 pF total stray capacitance.

PIN CONFIGURATION

10k

Ω

5

6

7

8

4

3

2

1

AD8015

50

Ω

+1

NC

NC

–OUTPUT

+OUTPUT

G = 3

G = 30

NC = NO CONNECT

50

Ω

+1

– +

1.7V

METALIZATION PHOTOGRAPH

Dimensions shown in microns. Not to scale.

FIBER OPTIC RECEIVER APPLICATIONS

In a fiber optic receiver, the photodiode can be placed from the

converts the current from the photodiode to a differential volt-

below the positive supply. This node must be bypassed with a

capacitor (C1 in Figures 3 and 4 below) to the signal ground. If

large levels of power supply noise exist, then connecting C1 to

mum performance, choose C1 such that C1 > 1/(2

π × 1000 ×

frequency in Hz.

PHOTODIODE REFERRED TO POSITIVE SUPPLY

Figure 3 shows the AD8015 used in a circuit where the photo-

diode is referred to the positive supply. The back bias voltage on

the photodiode is

≈1.8 V. This method of referring the photo-

diode provides greater power supply noise immunity (PSRR)

than referring the photodiode to the negative supply. The signal

path is referred to the positive rail, and the photodiode capaci-

tance is not modulated by high frequency noise that may exist

on the negative rail.

OPTIONAL

+OUTPUT

–OUTPUT

973µ

998µ

838µ

813µ

NOTE:

FOR BEST PERFORMANCE ATTACH PACKAGE

MATERIAL AT BACK OF DIE IS SILICON. USE OF