AD8615/AD8616/AD8618
10
0
HIGH SPEED PHOTODIODE PREAMPLIFIER
The AD8615/AD8616/AD8618 are excellent choices for I-to-V
conversions. The very low input bias, low current noise, and
high unity-gain bandwidth of the parts make them suitable,
especially for high speed photodiode preamplifiers.
–10
–20
–30
–40
In high speed photodiode applications, the diode is operated in a
photoconductive mode (reverse biased). This lowers the junction
capacitance at the expense of an increase in the amount of dark
current that flows out of the diode.
The total input capacitance, C1, is the sum of the diode and op
amp input capacitances. This creates a feedback pole that causes
degradation of the phase margin, making the op amp unstable.
Therefore, it is necessary to use a capacitor in the feedback to
compensate for this pole.
0.1
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 46. Second-Order Butterworth, Low-Pass Filter Frequency Response
To get the maximum signal bandwidth, select
POWER DISSIPATION
C1
2πR2 fU
Although the AD8615/AD8616/AD8618 are capable of providing
load currents up to 150 mA, the usable output, load current,
and drive capability are limited to the maximum power dissipation
allowed by the device package.
C2 =
where fU is the unity-gain bandwidth of the amplifier.
C2
In any application, the absolute maximum junction temperature
for the AD8615/AD8616/AD8618 is 150°C. This should never
be exceeded because the device could suffer premature failure.
Accurately measuring power dissipation of an integrated circuit
is not always a straightforward exercise; Figure 47 is a design aid
for setting a safe output current drive level or selecting a heat
sink for the package options available on the AD8616.
1.5
R2
–2.5V
–
V–
I
R
C
C
IN
D
SH
D
V+
+
+2.5V
–V
BIAS
Figure 44. High Speed Photodiode Preamplifier
ACTIVE FILTERS
1.0
SOIC
The low input bias current and high unity-gain bandwidth of
the AD8616 make it an excellent choice for precision filter design.
MSOP
Figure 45 shows the implementation of a second-order, low-pass
filter. The Butterworth response has a corner frequency of 100 kHz
0.5
and a phase shift of 90°. The frequency response is shown in
Figure 46.
2nF
0
0
20
40
60
80
100
120
140
V
EE
TEMPERATURE (°C)
Figure 47. Maximum Power Dissipation vs. Ambient Temperature
V–
V+
1.1k
Ω
1.1kΩ
These thermal resistance curves were determined using the
AD8616 thermal resistance data for each package and a
maximum junction temperature of 150°C.
V
IN
1nF
V
CC
Figure 45. Second-Order, Low-Pass Filter
Rev. E | Page 13 of 20
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