ADA4622-1 Datasheet, PDF (29/34 Pages) Analog Devices – 30 V, 8 MHz, Low Bias Current, Single-Supply, RRO, Precision Op Amps

Data Sheet
ADA4622-1/ADA4622-2
Calculate the noise bandwidth at the fN frequency by
2pF
f_N

(C _S
f _GBP
 C_F )
C_F
49.9kΩ
+5V
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.
–5V
0.1µF
ADA4622-1/
V_OUT
ADA4622-2
0.1µF
100Ω
–5V
Figure 96. Photodiode Preamplifier
3
2
1
2pF
0
–1
3pF
–2
–3
–4
–5
–6
–7
10
100
1k
10k
100k
1M
10M 100M
FREQUENCY (Hz)
Figure 97. Photodiode Preamplifier Frequency Response
Table 11. RMS Noise Contributions of the Photodiode Preamplifier
Contributor
Expression
RF
4kT

R_F

f_N

π
2
VNOISE
V_NOISE 
(C_S  C_M  C_F  C_D ) _
C_F
Root Sum Square (RSS) Total
R_F² V_NOISE²

2

f_N
RMS Noise (μV)¹
50.8
131.6
141
1 _{RMS noise with RF = 50 kΩ, CS = 5 pF, CF = 2 pF, CM = 3.7 pF, and CD = 0.4 pF.}
Rev. B | Page 29 of 34