AD8065 AD8066 Analog Devices, AD8065 AD8066 Datasheet - Page 20

• Manufacturer Part Number: AD8065 AD8066
• Description: High Performance, 145 MHz Fast FET Op Amps
• Manufacturer: Analog Devices

AD8065/AD8066
Input-to-Output Coupling
The output signal traces should not be parallel with inputs in order
to minimize capacitive coupling between the inputs and output.
Wideband Photodiode Preamp
Figure 8 shows an I/V converter with an electrical model of a
photodiode. The basic transfer function is
where I
parallel combination of R
The stable bandwidth attainable with this preamp is a function
of R
capacitance at the amplifier’s summing junction, including C
the amplifier input capacitance. R
produce a pole in the amplifier’s loop transmission that can result
in peaking and instability. Adding C
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
where:
f
R
C
(amplifier + photodiode + board parasitics).
The value of C
CR
F
S
is the the feedback resistor.
is the the total capacitance at the amplifier summing junction
is the the amplifier crossover frequency.
F
, the gain bandwidth product of the amplifier, and the total
PHOTO
is the output current of the photodiode, and the
F
that produces f
C
V
f
(
IPHOTO
45
OUT
F
)
=
F
=
and C
=
2p
2
I
1
V
PHOTO
p
(45)
(45)
¥
B
+
F
¥
F
R
sC R
C
) is defined by the expression
set the signal bandwidth.
R
can be shown to be
f
and the total capacitance
F
CR
F
S
F
F
¥
creates a zero in the loop
¥
¥
R
f
C
F
C
CR
S
F
S
Figure 8. Wideband Photodiode Preamp
R
SH
= 10
C
F
11
S
+ C
and
S
–20–
The frequency response in this case will show about 2 dB of
peaking and 15% overshoot. Doubling C
width 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 9.
The pole in the loop transmission translates to a zero in the ampli-
fier’s noise gain, leading to an amplification of the input voltage
noise over frequency. The loop transmission zero introduced by
C
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 will add to the output noise.
F
R
limits the amplification. The noise gain’s bandwidth extends
F
Figure 9. Photodiode Voltage Noise Contributions
VEN
f
1
RF NOISE
C
D
NOISE DUE TO AMPLIFIER
C
C
f
f
f
f
M
M
1
2
3
2
=
=
=
C
R
2 R
2 R
(C
F
F
S
1
+ C
F
F
C
(C
F
M
VEN (C
F
+ 2C
FREQUENCY – Hz
+ C
f
CR
1
S
D
+ C
F
+ C
+ C
M
F
S
+ 2C
) /C
+ C
F
V
D
M
O
F
)
+ 2C
and cutting the band-
D
)/C
F
f
3
REV. B