Datasheet

Data Sheet AD823A
Rev. | Page 17 of 20
log f
log f
fp
G = 1
G = R
2
C
1
s
fx
fu
OPEN-LOOP GAIN OPEN-LOOP GAIN
(A) WITHOUT COMPENSATION
f
fp
G = 1
f
fx
fu
G = 1 + C
S
/C
F
fz
fn
(B) WITH COMPENSATION
I TO V GAIN
PHASE (°) |A| (dB)
|A (s)|
–180°
–135°
–90°
–45°
0°
–135°
–90°
–45°
0°
45°
90°
G = R
F
C
S
(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, V
NOISE
and the resistor noise due to R
F
. The gray curve
in Figure 43 shows the noise gain over frequencies for the
photodiode preamp. The noise bandwidth is at the frequency f
N
,
and it can be calculated by
( )
FFS
u
N
CCC
f
f
+
=
(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 I
PHOTO
is 1 µA p-p. The amplifier has a bandwidth of 2.2 MHz when it
is maximized for a 45° phase margin with C
F
= 1.2 pF. Note that
with the PCB parasitics added to C
F
, the peaking is only 0.5 dB
and the bandwidth is slightly reduced. Increasing C
F
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 f
z
= f
x
= f
n
in this case.
AD823A
0.1µF
+5V
49.9kΩ
V
OUT
0.1µF
–5V
–5V
100Ω
1.2pF
09439-050
Figure 44. Photodiode Preamplifier
95
85
86
87
88
89
90
91
92
93
94
1k 10k 100k 1M 10M
TRANSIMPEDANCE GAIN (dB)
FREQUENCY (Hz)
09439-144
I
PHOTO
= 1µA p-p
C
F
= 1.2pF
I
PHOTO
= 1µA p-p
C
F
= 2.7pF
Figure 45. Photodiode Preamplifier Frequency Response
B