AD8067

Rev. 0 | Page 20 of 24

APPLICATIONS

Wideband Photodiode Preamp

–

+

AD8067

IPHOTO

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 I

parallel combination of R

The stable bandwidth attainable with this preamp is a function of

R

capacitance at the amplifier’s summing junction, including C

the amplifier input capacitance. R

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

in peaking and instability. Adding C

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 the expression:

()

S

F

45

C

R

π

GBP

f

×

×

=

2

GBP

is the unit gain bandwidth product, R

resistance, and C

junction (amplifier + photodiode + board parasitics).

The value of C

(45) can be shown to be:

GBP

R

π

C

C

F

S

F

×

×

=

2

The frequency response in this case will show about 2 dB of

peaking and 15% overshoot. Doubling C

bandwidth in half will result in a flat frequency response, with

about 5% transient overshoot.

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

Contributor

Expression

RMS

Noise

57

1

2

4

2

.

×

×

×

×

f

R

kT

F

152

Amp to f1

1

f

4.3

Amp (f2–f1)

(

)

1

2

2

f

–

f

C

C

C

C

C

V

F

D

F

M

S

noise

×

+

+

+

×

96

Amp (Past f2)

(

)

57

1

3

2

.

×

×

+

+

+

×

f

C

C

C

C

C

V

F

D

F

M

S

noise

684

RSS Total

708

Table 2. RMS Noise Contributions of Photodiode Preamp

FREQUENCY – Hz

RF NOISE

NOISE DUE TO AMPLIFIER

VEN

1

1

GBP

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.