Data Sheet
AD823A
Rev. | Page 17 of 20
log f
fp
G = 1
G = R2C1s
fx
fu
OPEN-LOOP GAIN
(A) WITHOUT COMPENSATION
f
fp
G = 1
f
fx
fu
G = 1 + CS/CF
fz
fn
(B) WITH COMPENSATION
I TO V GAIN
PH
A
SE
(°
)
|A|
(
d
B)
|A
(
s)
|
–180°
–135°
–90°
–45°
0°
–135°
–90°
–45°
0°
45°
90°
G = RFCS(s)
09439-
400
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
i
n 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
(
)
F
S
u
N
C
f
+
=
(6)
photodiode amplifier. The amplifier is used in conjunction with
a photodiode detector with input capacitance of 5 pF
. Figure 45shows the transimpedance response of the AD823A when IPHOTO is 1 A 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
0.1F
+5V
49.9k
VOUT
0.1F
–5V
100
1.2pF
09439-
050
Figure 44. Photodiode Preamplifier
95
85
86
87
88
89
90
91
92
93
94
1k
10k
100k
1M
10M
T
RANS
IM
P
E
DANCE
G
AI
N
(
d
B)
FREQUENCY (Hz)
09439-
144
IPHOTO = 1A p-p
CF = 1.2pF
IPHOTO = 1A p-p
CF = 2.7pF
Figure 45. Photodiode Preamplifier Frequency Response
B