AD823A 查看數據表（PDF） - Analog Devices
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AD823A Datasheet PDF : 20 Pages
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Data Sheet
OPEN-LOOP GAIN
OPEN-LOOP GAIN
AD823A
fx
G=1
G = R2C1s
fp
log f
fu
I TO V GAIN
G=1
fp
fx
fz
fn
G = RFCS(s)
G = 1 + CS/CF
f
fu
0°
90°
–45°
45°
–90°
–135°
–180°
0°
f
log f
–45°
–90°
(A) WITHOUT COMPENSATION
–135°
(B) WITH COMPENSATION
Figure 43. Gain and Phase Plot of the Transimpedance Amplifier Design
The dominant sources of output noise in the wideband
1.2pF
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
49.9kΩ
+5V
photodiode preamp. The noise bandwidth is at the frequency fN,
and it can be calculated by
0.1µF
–5V
AD823A
VOUT
fN
=
(CS
+
fu
CF
)
CF
(6)
0.1µF
100Ω
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.
–5V
Figure 44. Photodiode Preamplifier
95
94
93
92
IPHOTO = 1µA p-p
CF = 2.7pF
91
90
IPHOTO = 1µA p-p
89
CF = 1.2pF
88
87
86
Rev.B | Page 17 of 20
85
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 45. Photodiode Preamplifier Frequency Response
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