AD8615/AD8616/AD8618

Rev. E | Page 13 of 20

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

U

f

2

R

1

C

2

C

π

=

2

V–

+2.5V

V+

–2.5V

R2

C2

–

+

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+

2nF

1nF

Figure 45. Second-Order, Low-Pass Filter

–40

–30

–20

–10

0

10

1

0.1

10

100

1k

10k

100k

1M

FREQUENCY (Hz)

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.

TEMPERATURE (°C)

0

0

0.5

1.0

1.5

20

40

60

80

120

100

140

SOIC

MSOP

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.