ADA4622-2ACPZ-R7 datasheet(29/34 Pages) AD

29 / 34 page

Data Sheet

ADA4622-1/ADA4622-2

Rev. B | Page 29 of 34

Calculate the noise bandwidth at the fN frequency by

GBP

)

(





Figure 96 shows the ADA4622-1/ADA4622-2 configured as

a transimpedance photodiode amplifier. The amplifiers are

used in conjunction with a photodiode detector with an input

capacitance of 5 pF. Figure 97 shows the transimpedance response

of the ADA4622-1/ADA4622-2 when IPHOTO is 1 μA p-p. The

amplifiers have a bandwidth of 2 MHz when they are maximized

for a 45° phase margin with CF = 2 pF. Note that with the PCB

parasitics added to CF, the peaking is only 0.5 dB, and the

bandwidth is reduced slightly.

Increasing CF to 3 pF completely eliminates the peaking;

however, increasing CF to 3 pF reduces the bandwidth to 1 MHz.

Table 11 shows the noise sources and total output noise for the

photodiode preamp, where the preamp is configured to have a

45° phase margin for maximum bandwidth and fZ = fX = fN in

this case.

0.1µF

+5V

49.9kΩ

VOUT

0.1µF

–5V

100Ω

2pF

ADA4622-1/

ADA4622-2

Figure 96. Photodiode Preamplifier

100

–7

–6

–5

–4

–3

–2

–1

10k

100k

FREQUENCY (Hz)

10M

100M

2pF

3pF

Figure 97. Photodiode Preamplifier Frequency Response

Table 11. RMS Noise Contributions of the Photodiode Preamplifier

Contributor

Expression

RMS Noise (μV)1



4kT

50.8

VNOISE

NOISE











)

(

131.6

Root Sum Square (RSS) Total

NOISE

R 

141

1 RMS noise with RF = 50 kΩ, CS = 5 pF, CF = 2 pF, CM = 3.7 pF, and CD = 0.4 pF.

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