AD8616/AD8618

Rev. A | Page 14 of 16

HIGH SPEED PHOTODIODE PREAMPLIFIER

The 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

capacitance and that of the op amp. This creates a feedback pole

and causes degradation of the phase margin, making the op

amp unstable. It is therefore 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 46. 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 47 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 48.

V–

V+

2nF

1nF

1.1k

Ω

1.1k

Ω

Figure 47. Second-Order Low-Pass Filter

–40

–30

–20

–10

0

10

1

0.1

10

100

1k

10k

100k

1M

FREQUENCY (Hz)

Figure 48. Second-Order Butterworth Low-Pass Filter Frequency Response

POWER DISSIPATION

Although the AD8616/AD8618 are capable of providing load

currents to 150 mA, the usable output load current drive

capability is limited to the maximum power dissipation allowed

by the device package used. In any application, the absolute

maximum junction temperature for the AD8616/AD8618 is

150°C; this should never be exceeded because the device could

suffer premature failure. Accurately measuring power dissipa-

tion of an integrated circuit is not always a straightforward

exercise; Figure 49 has been provided as 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 49. Maximum Power Dissipation vs. Ambient Temperature