AD823AARZ-R7 datasheet(17/20 Pages) AD

17 / 20 page

Data Sheet

AD823A

Rev. | Page 17 of 20

log f

G = 1

G = R2C1s

OPEN-LOOP GAIN

(A) WITHOUT COMPENSATION

G = 1

G = 1 + CS/CF

(B) WITH COMPENSATION

I TO V GAIN

–180°

–135°

–90°

–45°

0°

–135°

–90°

–45°

0°

45°

90°

G = RFCS(s)

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

amplifier, VNOISE and the resistor noise due to RF. The gray curve

in Figure 43 shows the noise gain over frequencies for the

photodiode preamp. The noise bandwidth is at the frequency fN,

and it can be calculated by

(

)

(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

shows the transimpedance response of the AD823A when IPHOTO

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

is maximized for a 45° phase margin with CF = 1.2 pF. Note that

with the PCB parasitics added to CF, the peaking is only 0.5 dB

and the bandwidth is slightly reduced. Increasing CF to 2.7 pF

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

have a 45° phase margin for maximal bandwidth and fz = fx = fn

in this case.

AD823A

0.1µF

+5V

49.9kΩ

VOUT

0.1µF

–5V

100

Ω

1.2pF

Figure 44. Photodiode Preamplifier

10k

100k

10M

FREQUENCY (Hz)

IPHOTO = 1µA p-p

CF = 1.2pF

IPHOTO = 1µA p-p

CF = 2.7pF

Figure 45. Photodiode Preamplifier Frequency Response

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