ADA4817-1ACPZ-RL 查看數據表（PDF） - Analog Devices
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ADA4817-1ACPZ-RL Datasheet PDF : 28 Pages
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Data Sheet
APPLICATIONS INFORMATION
LOW DISTORTION PINOUT
The ADA4817-1/ADA4817-2 feature a new low distortion
pinout from Analog Devices. The new pinout provides two
advantages over the traditional pinout. The first advantage is
improved second harmonic distortion performance, which is
accomplished by the physical separation of the noninverting
input pin and the negative power supply pin. The second
advantage is the simplification of the layout due to the dedicated
feedback pin and easy routing of the gain set resistor back to
the inverting input pin. This allows a compact layout, which
helps to minimize parasitics and increase stability.
The designer does not need to use the dedicated feedback pin to
provide feedback for the ADA4817-1/ADA4817-2. The output
pin of the ADA4817-1/ADA4817-2 can still be used to provide
feedback to the inverting input of the ADA4817-1/ADA4817-2.
WIDEBAND PHOTODIODE PREAMP
The wide bandwidth and low noise of the ADA4817-1/
ADA4817-2 make it an ideal choice for transimpedance
amplifiers, such as those used for signal conditioning with
high speed photodiodes. Figure 50 shows an I/V converter
with an electrical model of a photodiode. The basic transfer
function is
VOUT
I PHOTO RF
1 sCF RF
(13)
where:
IPHOTO is the output current of the photodiode.
The parallel combination of RF and CF sets the signal bandwidth.
CF
RF
IPHOTO
RSH = 1011Ω
CS
CM
CD
CM
VB
Figure 50. Wideband Photodiode Preamp
VOUT
ADA4817-1/ADA4817-2
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 summing junction of the amplifier, including the
photodiode capacitance (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 compen-
sates for the effect of the pole and reduces the signal bandwidth.
It can be shown that the signal bandwidth obtained with a 45°
phase margin (f(45)) is defined by
f(45)
f CR
2 RF (CS CM CD )
(14)
where:
fCR is the amplifier crossover frequency.
RF is the feedback resistor.
CS is the source capacitance including the photodiode and the
board parasitic.
CM is the common-mode capacitance of the amplifier.
CD is the differential capacitance of the amplifier.
The value of CF that produces f(45) can be shown to be
CF
CS CM CD
2 RF fCR
(15)
The frequency response shows less peaking if bigger CF values
are used.
The preamplifier output noise over frequency is shown in
Figure 51.
f1 = 2
1
RF (CF + CS + CM + CD)
f2 = 2
1
RFCF
f3
=
(CF
+
CS
fCR
+ CM
+
CD)/CF
RF NOISE
f2
f1
VEN (CF + CS + CM + CD)/CF f3
VEN
NOISE DUE TO AMPLIFIER
FREQUENCY (Hz)
Figure 51. Photodiode Voltage Noise Contributions
Rev. B | Page 19 of 28
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