AD8067 Analog Devices, AD8067 Datasheet - Page 18

• Manufacturer Part Number: AD8067
• Manufacturer: Analog Devices

Specifications of AD8067

• Vcc-vee: 5V to 24V
• Isy Per Amplifier: 7mA
• Packages: SOT
• -3db Bandwidth: 54MHz
• Slew Rate: 640V/µs
• Vos: 200µV
• Ib: 0.6pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 6.6nV/rtHz

AD8067
APPLICATIONS
WIDEBAND PHOTODIODE PREAMP
Figure 49 shows an I/V converter with an electrical model of a
photodiode.
The basic transfer function is
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
and the amplifier input capacitance. R
produce a pole in the amplifier’s loop transmission that can
result in peaking and instability. Adding C
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
GBP is the unit gain bandwidth product, R
resistance, and C
summing junction (amplifier + photodiode + board parasitics).
The value of C
The frequency response in this case shows about 2 dB of
peaking and 15% overshoot. Doubling C
bandwidth in half results in a flat frequency response, with
about 5% transient overshoot.
I
PHOTO
F
, the gain bandwidth product of the amplifier, and the total
V
C
f
(
OUT
F
45
PHOTO
=
V
)
B
=
=
π 2
I
is the output current of the photodiode, and the
1
π 2
PHOTO
F
Figure 49. Wideband Photodiode Preamp
+
×
C
that produces f (45) can be shown to be
S
×
S
sC
R
C
GBP
C
is the total capacitance at the amplifier
R
F
F
F
S
+ C
F
×
×
R
R
SH
×
GBP
R
S
F
C
F
= 10
S
F
and C
11
Ω
R
–
+
F
F
C
sets the signal bandwidth.
D
F
and the total capacitance
R
C
F
C
F
F
C
M
AD8067
and cutting the
F
M
F
creates a zero in the
is the feedback
V
OUT
Rev. A | Page 18 of 24
S
The preamp’s output noise over frequency is shown in Figure 50.
Table 6. RMS Noise Contributions of Photodiode Preamp
Contributor
R
Amp to f1
Amp (f2 − f1)
Amp (Past f2)
1
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.
RMS noise with R
F
× 2
NOTES
I
C
R
D
–5V
D
B
@ –5V = 0.074nA
VEN
@ –5V = 0.690pF
@ 1550nm = –49dB
f
1
RF NOISE
Figure 50. Photodiode Voltage Noise Contributions
EPM 605 LL
F
= 50 kΩ, C
Expression
RSS Total
f
f
f
NOISE DUE TO AMPLIFIER
V
V
V
1
2
3
0.33pF
2
NOISE
NOISE
NOISE
=
=
=
Figure 51. Photodiode Preamplifier
×
2
2
f
(C
2
π
π
4
R
R
S
kT
1
F
F
+ C
(C
×
×
×
C
S
F
= 0.67 pF, C
×
F
(
M
C
+ C
R
1 f
(
C
+ 2C
S
GBP
F
S
S
+
1
×
+ C
+
49.9kΩ
C
D
FREQUENCY – Hz
2 f
VEN (C
C
M
+ C
M
×
M
AD8067
C
F
+ 2C
+
1
= 0.33 pF, C
F
C
.
F
)/C
57
+
–5V
C
+5V
F
F
C
F
D
0.33pF
49.9kΩ
F
+ C
)
F
+
S
+
2
0.1μF
10μF
+ C
C
0.1μF
10μF
2
C
D
M
M
D
)
= 1.5 pF, and C
+ 2C
×
)
×
3 f
D
)/C
2 f
×
F
1
–
50Ω
.
57
1 f
D
f
= 2.5 pF.
3
V
RMS
Noise
(μV)
152
4.3
96
684
708
OUT
1