AD8067

Data Sheet

Rev. B | Page 18 of 24

APPLICATIONS

WIDEBAND PHOTODIODE PREAMP

–

+

VOUT

VB

CF + CS

CD

CM

CM

RF

AD8067

RSH = 10

11

Ω

CS

IPHOTO

CF

RF

Figure 49. Wideband Photodiode Preamp

Figure 49 shows an I/V converter with an electrical model of a

photodiode.

The basic transfer function is

F

F

F

PHOTO

OUT

R

sC

R

I

V

+

×

=

1

where IPHOTO is the output current of the photodiode, and the

parallel combination of RF and CF sets the signal bandwidth.

The stable bandwidth attainable with this preamp is a function

of RF, the gain bandwidth product of the amplifier, and the total

capacitance at the amplifier’s summing junction, including CS

and the amplifier input capacitance. RF and the total capacitance

produce a pole in the amplifier’s loop transmission that can

result in peaking and instability. Adding CF creates a zero in the

loop transmission that compensates for the pole’s effect and

reduces the signal bandwidth. It can be shown that the signal

bandwidth resulting in a 45° phase margin (f(45)) is defined by

( )

S

F

45

C

R

GBP

f

×

×

=

π

2

GBP is the unit gain bandwidth product, RF is the feedback

resistance, and CS is the total capacitance at the amplifier

summing junction (amplifier + photodiode + board parasitics).

The value of CF that produces f(45) can be shown to be

GBP

R

C

C

F

S

F

×

×

=

π

2

The frequency response in this case shows about 2 dB of

peaking and 15% overshoot. Doubling CF and cutting the

bandwidth in half results in a flat frequency response, with

about 5% transient overshoot.

The preamp’s output noise over frequency is shown in Figure 50.

Table 6. RMS Noise Contributions of Photodiode Preamp

Contributor

Expression

RMS

Noise

(µV)1

RF × 2

57

1

2

f

R

kT

4

2

F

.

×

×

×

×

152

Amp to f1

1

f

VNOISE ×

4.3

Amp (f2 − f1)

(

)

1

f

–

2

f

C

C

2

C

C

C

V

F

D

F

M

S

NOISE

×

+

+

+

×

96

Amp (Past f2)

(

)

57

1

3

f

C

C

2

C

C

C

V

F

D

F

M

S

NOISE

.

×

×

+

+

+

×

684

RSS Total

708

1

RMS noise with RF = 50 kΩ, CS = 0.67 pF, CF = 0.33 pF, CM = 1.5 pF, and CD = 2.5 pF.

FREQUENCY – Hz

RF NOISE

f1

NOISE DUE TO AMPLIFIER

VEN

f2

1

2πRF(CF + CS + CM + 2C D)

f1 =

f2 =

1

2πRFCF

f3 =

GBP

(CS + CM + 2C D + CF)/CF

VEN (C F + CS + CM + 2C D)/CF

f3

Figure 50. Photodiode Voltage Noise Contributions

Figure 51 shows the AD8067 configured as a transimpedance

photodiode amplifier. The amplifier is used in conjunction with

a JDS uniphase photodiode detector. This amplifier has a

bandwidth of 9.6 MHz, as shown in Figure 52, and is verified by

the design equations shown in Figure 50.

AD8067

0.1

µF

10

µF

+5V

49.9k

Ω

50

Ω

49.9k

Ω

0.33pF

VOUT

0.1

µF

10

µF

–5V

EPM 605 LL

–5V

0.33pF

NOTES

ID @ –5V = 0.074nA

CD @ –5V = 0.690pF

RB @ 1550nm = –49dB

Figure 51. Photodiode Preamplifier