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參數資料
| 型號： | AD8034AR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Low Cost, 80 MHz FastFET ⑩ Op Amps |
| 中文描述： | DUAL OP-AMP, 3500 uV OFFSET-MAX, PDSO8 |
| 封裝： | MS-012AA, SOIC-8 |
| 文件頁數： | 18/20頁 |
| 文件大小： | 374K |
| 代理商： | AD8034AR |
REV. B
–18–
AD8033/AD8034
Wideband Photodiode Preamp
Figure 12 shows an I/V converter with an electrical model of a
photodiode.
The basic transfer function is
V
I
R
sC R
OUT
PHOTO
+
1
F
F
=
×
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 capaci-
tance 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 the
expression
f
f
R
C
CR
F
S
(
)
45
2
=
×
×
π
f
CR
is the amplifier crossover frequency.
R
F
is the feedback resistor.
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
C
C
R
f
F
S
F
CR
=
×
×
2
π
The frequency response in this case will show about 2 dB of
peaking and 15% overshoot. Doubling
C
F
and cutting the
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 13.
The pole in the loop transmission translates to a zero in the
amplifier’s noise gain, leading to an amplification of the input
voltage noise over frequency. The loop transmission zero
introduced by C
F
limits the amplification. The noise gain’s
bandwidth extends past the preamp signal bandwidth and
is eventually rolled off by the decreasing loop gain of the
amplifier. Keeping the input terminal impedances matched is
recommended to eliminate common-mode noise peaking
effects that will add to the output noise.
Integrating the square of the output voltage noise spectral density
over frequency and then taking the square root results in the
total rms output noise of the preamp.
C
S
R
SH
= 10
11
V
B
I
PHOTO
R
F
C
F
V
O
C
M
R
F
C
M
C
D
C
F
+ C
S
Figure 12. Wideband Photodiode Preamp
FREQUENCY – Hz
V
2 R
F
C
F
2 R
F
(C
F
+
C
S
+
C
M
+
2C
D
)
(C
S
+
C
M
+
2C
D
+
C
F
) /C
F
RF NOISE
VEN (C
F
+ C
S
+ C
M
+ 2C
D
) /C
F
f
3
f
2
f
3
=
VEN
f
1
f
2
=
f
1
=
1
1
f
CR
NOISE DUE TO AMPLIFIER
Figure 13. Photodiode Voltage Noise Contributions
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