AD8065ART-EBZ View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8065ART-EBZ Datasheet PDF : 29 Pages
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AD8065/AD8066
CF
RF
IPHOTO
RSH = 1011Ω
CS
CM
CD
CM
VO
VB
CF + CS
RF
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
VOUT
=
I PHOTO × 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 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
The frequency response in this case shows about 2 dB of
peaking and 15% overshoot. Doubling CF 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.
f1 =
1
2π RF (CF + CS + CM + 2CD)
1
f2 = 2π RFCF
f3 =
fCR
(CS + CM + 2CD + CF) / CF
RF NOISE
f1
VEN
f2
VEN (CF + CS + CM + 2CD)/CF
f3
NOISE DUE TO AMPLIFIER
FREQUENCY (Hz)
Figure 59. Photodiode Voltage Noise Contributions
f(45) =
fCR
2π × RF × CS
where fCR is the amplifier crossover frequency, RF is the feedback
resistor, 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 × fCR
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 CF 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 RF = 50 kΩ, CS = 15 pF, and CF = 2 pF (bandwidth of about
1.6 MHz) are also listed.
Rev. J | Page 24 of 28
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