AD8067(Rev0) Ver la hoja de datos (PDF) - Analog Devices
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AD8067 Datasheet PDF : 24 Pages
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AD8067
APPLICATIONS
Wideband Photodiode Preamp
CF
RF
IPHOTO
VB
–
CS RSH = 1011Ω
CD
+
CF + CS
RF
CM
CM
AD8067
VOUT
Figure 49. Wideband Photodiode Preamp
Figure 49 shows an I/V converter with an electrical model of a
photodiode.
The basic transfer function is:
VOUT
=
IPHOTO × RF
1 + sCF RF
where IPHOTO is the output current of the photodiode, and the
parallel combination of RF and CF sets the signal bandwidth.
The stable bandwidth attainable with this preamp is a function of
RF, the gain bandwidth product of the amplifier, and the total
capacitance at the amplifier’s summing junction, including CS and
the amplifier input capacitance. RF and the total capacitance
produce a pole in the amplifier’s loop transmission that can result
in peaking and instability. Adding CF 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(45) =
GBP
2π × RF × CS
GBP is the unit gain bandwidth product, RF is the feedback
resistance, and CS is the total capacitance at the amplifier summing
junction (amplifier + photodiode + board parasitics).
The value of CF that produces f(45) can be shown to be:
CF =
CS
2π × RF × GBP
The frequency response in this case will show about 2 dB of
peaking and 15% overshoot. Doubling CF 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 50.
Contributor Expression
RF × 2
2 × 4kT × RF × f 2 ×1.57
RMS
Noise
(µV)1
152
Amp to f1
Vnoise × f 1
4.3
Amp (f2–f1)
Vnoise ×
(CS + CM + CF + 2CD ) ×
CF
f2–f1
96
Amp (Past f2)
Vnoise
×
(C S
+ CM + CF
CF
+ 2CD
)×
f 3×1.57
684
RSS Total
708
Table 2. RMS Noise Contributions of Photodiode Preamp
1 RMS noise with RF = 50 kΩ, CS = 0.67 pF, CF = 0.33 pF,
CM = 1.5 pF, and CD = 2.5 pF.
f1
=
2πRF(CF
+
1
CS +
CM
+
2C D)
f2
=
1
2πRFCF
f3 =
GBP
(CS + C M + 2C D + C F)/CF
RF NOISE
f1
VEN
f2
VEN (C F + C S + C M + 2C D)/CF
f3
NOISE DUE TO AMPLIFIER
FREQUENCY – Hz
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.
Rev. 0 | Page 20 of 24
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