Datasheet

AD8067 Data Sheet
Rev. B | Page 18 of 24
APPLICATIONS
WIDEBAND PHOTODIODE PREAMP
–
+
V
OUT
V
B
C
F
+ C
S
C
D
C
M
C
M
R
F
AD8067
R
SH
= 10
11
Ω
C
S
I
PHOTO
C
F
R
F
Figure 49. Wideband Photodiode Preamp
Figure 49 shows an I/V converter with an electrical model of a
photodiode.
The basic transfer function is
FF
F
PHOTO
OUT
RsC
RI
V
+
×
=
1
where I
PHOTO
is the output current of the photodiode, and the
parallel combination of R
F
and C
F
sets the signal bandwidth.
The stable bandwidth attainable with this preamp is a function
of R
F
, the gain bandwidth product of the amplifier, and the total
capacitance at the amplifier’s summing junction, including C
S
and the amplifier input capacitance. R
F
and the total capacitance
produce a pole in the amplifier’s loop transmission that can
result in peaking and instability. Adding C
F
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
CR
GBP
f
××
=
π2
GBP is the unit gain bandwidth product, R
F
is the feedback
resistance, and C
S
is the total capacitance at the amplifier
summing junction (amplifier + photodiode + board parasitics).
The value of C
F
that produces f
(45)
can be shown to be
GBPR
C
C
F
S
F
××
=
π2
The frequency response in this case shows about 2 dB of
peaking and 15% overshoot. Doubling C
F
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
R
F
× 2
5712fRkT42
F
.××××
152
Amp to f1
1fV
NOISE
×
4.3
Amp (f2 − f1)
( )
1f–2f
C
C2CCC
V
F
DFM
S
NOISE
×
+++
×
96
Amp (Past f2)
( )
5713f
C
C2CCC
V
F
DFM
S
NOISE
.××
+++
×
684
RSS Total
708
1
RMS noise with R
F
= 50 kΩ, C
S
= 0.67 pF, C
F
= 0.33 pF, C
M
= 1.5 pF, and C
D
= 2.5 pF.
FREQUENCY – Hz
VOLTAGE NOISE – nV/ Hz
RF NOISE
f
1
NOISE DUE TO AMPLIFIER
VEN
f
2
1
2
π
R
F
(C
F
+ C
S
+ C
M
+ 2C
D
)
f
1
=
f
2
=
1
2
πR
F
C
F
f
3
=
GBP
(C
S
+ C
M
+ 2C
D
+ C
F
)/C
F
VEN (C
F
+ C
S
+ C
M
+ 2C
D
)/C
F
f
3
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
0.1µF
10µF
+5V
49.9kΩ
50Ω
49.9kΩ
0.33pF
V
OUT
0.1µF
10µF
–5V
EPM 605 LL
–5V
0.33pF
NOTES
I
D
@ –5V = 0.074nA
C
D
@ –5V = 0.690pF
R
B
@ 1550nm = –49dB
Figure 51. Photodiode Preamplifier