ADA4817-2 Analog Devices, ADA4817-2 Datasheet - Page 19

• Manufacturer Part Number: ADA4817-2
• Description: Low Noise, 1 GHz FastFET Op Amps
• Manufacturer: Analog Devices

Specifications of ADA4817-2

• -3db Bandwidth: 1.05GHz
• Slew Rate: 870V/µs
• Vos: 2mV
• Ib: 2pA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 4nV/rtHz
• Vcc-vee: 5V to 10V
• Isy Per Amplifier: 21mA
• Packages: CSP

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
where:
I
The parallel combination of R
PHOTO
I
PHOTO
V
is the output current of the photodiode.
OUT
V
=
B
I
1
PHOTO
C
Figure 50. Wideband Photodiode Preamp
+
S
sC
R
F
×
SH
R
R
F
= 10
F
11
Ω
F
and C
C
D
F
sets the signal bandwidth.
C
C
M
M
C
R
F
F
V
OUT
(13)
Rev. A | Page 19 of 28
The stable bandwidth attainable with this preamp is a function
of R
capacitance at the summing junction of the amplifier, including the
photodiode capacitance (C
R
loop transmission that can result in peaking and instability.
Adding C
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
where:
f
R
C
board parasitic.
C
C
The value of C
The frequency response shows less peaking if bigger C
are used.
The preamplifier output noise over frequency is shown in
Figure 51.
CR
F
F
S
M
D
is the feedback resistor.
is the source capacitance including the photodiode and the
and the total capacitance produce a pole in the amplifier’s
is the amplifier crossover frequency.
is the differential capacitance of the amplifier.
is the common-mode capacitance of the amplifier.
F
, the gain bandwidth product of the amplifier, and the total
C
f
(
F
45
)
=
F
=
VEN
Figure 51. Photodiode Voltage Noise Contributions
creates a zero in the loop transmission that compen-
f
C
2
1
R
2
π
S
F
F
π
(45)
NOISE
that produces f
+
×
×
NOISE DUE TO AMPLIFIER
C
R
) is defined by
R
F
f
M
f
f
f
2
F
1
2
3
×
=
=
=
+
×
2 R
2 R
(C
f
(
C
CR
C
F
FREQUENCY (Hz)
f
D
VEN (C
1
S
S
CR
+ C
) and the amplifier input capacitance.
F
F
C
ADA4817-1/ADA4817-2
(C
+
F
S
F
C
+ C
+ C
f
F
(45)
M
CR
+ C
1
M
S
+
can be shown to be
+ C
+ C
S
C
+ C
D
M
D
)/C
M
+ C
)
F
+ C
D
)
D
)/C
F
f
3
F
values
(14)
(15)