Datasheet

AD8065/AD8066
Rev. J | Page 24 of 28
R
SH
= 10
11
Ω
V
O
R
F
C
F
C
M
R
F
C
M
C
D
C
F
+C
S
C
S
V
B
I
PHOTO
02916-E-058
Figure 58. Wideband Photodiode Preamp
INPUT-TO-OUTPUT COUPLING
To minimize capacitive coupling between the inputs and output,
the output signal traces should not be parallel with the inputs.
WIDEBAND PHOTODIODE PREAMP
Figure 58 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 0 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
CR
CR
f
f
××π
=
2
45
where f
CR
is the amplifier crossover frequency, R
F
is the feedback
resistor, 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
CR
F
S
F
fR
C
C
××π
=
2
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 59.
FREQUENCY (Hz)
VOLTAGE NOISE (nV/ Hz)
2πR
F
C
F
2πR
F
(C
F
+C
S
+C
M
+2C
D
)
(C
S
+C
M
+2C
D
+C
F
)/C
F
R
F
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
02916-E-059
Figure 59. Photodiode Voltage Noise Contributions
The pole in the loop transmission translates to a 0 in the
amplifier’s noise gain, leading to an amplification of the input
voltage noise over frequency. The loop transmission 0
introduced by C
F
limits the amplification. The noise gain
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, which adds to the output noise.
Integrating the square of the output voltage noise spectral
density over frequency and then taking the square root allows
users to obtain the total rms output noise of the preamp. Table 5
summarizes approximations for the amplifier and feedback and
source resistances. Noise components for an example preamp
with R
F
= 50 kΩ, C
S
= 15 pF, and C
F
= 2 pF (bandwidth of about
1.6 MHz) are also listed.