Datasheet

AD823A Data Sheet
Rev. B | Page 18 of 20
Table 8. RMS Noise Contributions of Photodiode Preamp
Contributor Expression (μV)
1
R
F
2
π

NF
fR4kT
55.17
V
NOISE

N
F
DFMS
NOISE
f
C
2CCCC
V 


2
π
138.5
RSS Total 149.1
1
RMS noise with R
F
= 50 kΩ, C
S
= 5 pF, C
F
= 1.2 pF, C
M
= 1.3 pF, and C
D
= 0.6 pF.
ACTIVE FILTER
The AD823A is an ideal candidate for an active filter because of
its low input bias current and its low input capacitance. Low
input bias current reduces dc error in the signal path while low
input capacitance improves the accuracy of the active filter.
As a general rule of thumb, the bandwidth of the amplifier should
be at least 10 times bigger than the cutoff frequency of the filter
implemented. Therefore, the AD823A is capable of implementing
active filters of up to 1.7 MHz.
0
9439-146
AD823A
R
T
49.9Ω
R2
1.12kΩ
R1
1.12kΩ
C1
200pF
+V
S
–V
S
V
OUT
V
IN
C2
100pF
Figure 46. Two-Pole Sallen-Key Active Filter
Figure 46 shows an example of a second-order Butterworth
filter, which is implemented by the Sallen-Key topology. This
structure can be duplicated to produce higher-order filters.
3
–36
–33
–30
–27
–24
–21
–18
–15
–12
–9
–6
–3
0
100 1k 10k 100k 10M1M
MAGNITUDE (dB)
FREQUENCY (Hz)
09439-147
Figure 47. Two-Pole Butterworth Active Filter Response
Figure 47 shows the two-pole Butterworth active filter’s response.
Note that it has a maximally flat pass band, a −3 dB bandwidth
of 1 MHz, and a 12 dB/octave roll-off in the stop band.
The cutoff frequency (f
c
) and the Q factor of the Butterworth
filter can be calculated by:
2121
2
1
CCRR
f
c


(7)

221
2121
CRR
CCRR
Q


(8)
Therefore, one can easily adjust the cutoff frequency by
appropriately factoring the resistor and capacitor values. For
example, a 100 kHz filter can be implemented by increasing the
values of R1 and R2 by 10 times. Note that the Q factor remains
the same in this case.