AD8615/AD8616/AD8618

Rev. C | 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 preamps.

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

1.1k

Ω

1.1k

Ω

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 is 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