Data Sheet AD8615/AD8616/AD8618
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.
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.
To get the maximum signal bandwidth, select
where f
U
is the unity-gain bandwidth of the amplifier.
V–
+2.5V
V+
–2.5V
R2
C2
C
IN
C
D
R
SH
I
D
–V
BIAS
–
+
04648-044
Figure 44. High Speed Photodiode Preamplifier
ACTIVE FILTERS
The low input bias current and high unity-gain bandwidth of
the AD8616 make it an excellent choice for precision filter design.
Figure 45 shows the implementation of a second-order, low-pass
filter. The Butterworth response has a corner frequency of 100 kHz
and a phase shift of 90°. The frequency response is shown in
Figure 46.
V–
V
CC
V+
V
EE
2nF
1nF
1.1k
Ω
1.1k
Ω
V
IN
04648-045
Figure 45. Second-Order, Low-Pass Filter
–40
–30
–20
–10
0
10
GAIN (dB)
10.1 10 100 1k 10k 100k 1M
FREQUENCY (Hz)
04648-046
Figure 46. Second-Order Butterworth, Low-Pass Filter Frequency Response
POWER DISSIPATION
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.
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.
POWER DISSIPATION (W)
TEMPERATURE (°C)
0
0
0.5
1.0
1.5
20
40 60 80
120100 140
SOIC
MSOP
04648-047
Figure 47. Maximum Power Dissipation vs. Ambient Temperature
These thermal resistance curves were determined using the
AD8616 thermal resistance data for each package and a
maximum junction temperature of 150°C.
Rev. G | Page 13 of 20
