Data Sheet

AD823A

Rev. | Page 17 of 20

log f

log f

fp

G = 1

fx

fu

OPEN-LOOP GAIN

OPEN-LOOP GAIN

(A) WITHOUT COMPENSATION

f

fp

G = 1

f

fx

fu

fz

fn

(B) WITH COMPENSATION

I TO V GAIN

–180°

–135°

–90°

–45°

0°

–135°

–90°

–45°

0°

45°

90°

Figure 43. Gain and Phase Plot of the Transimpedance Amplifier Design

The dominant sources of output noise in the wideband

photodiode preamp design are the input voltage noise of the

in Figure 43 shows the noise gain over frequencies for the

and it can be calculated by

(

)

F

F

S

u

N

C

C

C

f

f

+

=

(6)

Figure 44 shows the AD823A configured as a transimpedance

photodiode amplifier. The amplifier is used in conjunction with

a photodiode detector with input capacitance of 5 pF. Figure 45

is 1 µA p-p. The amplifier has a bandwidth of 2.2 MHz when it

completely eliminates the peaking. However, it reduces the

bandwidth to 1.2 MHz.

Table 8 shows the noise sources and total output noise for the

photodiode preamp, where the preamplifier is configured to

in this case.

AD823A

0.1µF

+5V

49.9kΩ

0.1µF

–5V

–5V

100

Ω

1.2pF

Figure 44. Photodiode Preamplifier

95

85

86

87

88

89

90

91

92

93

94

1k

10k

100k

1M

10M

FREQUENCY (Hz)

Figure 45. Photodiode Preamplifier Frequency Response

B