AD823A_12 Datasheet, PDF (18/20 Pages) Analog Devices – Wide Supply Dual, 17 MHz, Rail-to-Rail

AD823A
Data Sheet
Table 8. RMS Noise Contributions of Photodiode Preamp
Contributor
Expression
RF
4kT

R_F

f_N

π
2
VNOISE
  _{VNOISE }
C_S  C_M  C_F  2C_{D }
C_F
RSS Total
π
2

f_N
1 _{RMS noise with RF = 50 kΩ, CS = 5 pF, CF = 1.2 pF, CM = 1.3 pF, and CD = 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.
C1
200pF
R1
R2
+V_S
1.12kΩ 1.12kΩ
V_IN
R_T
49.9Ω
C2
100pF
AD823A
V_OUT
–V_S
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
0
–3
–6
–9
–12
–15
–18
–21
–24
–27
–30
–33
–36
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 47. Two-Pole Butterworth Active Filter Response
(μV)¹
55.17
138.5
149.1
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 (fc) and the Q factor of the Butterworth
filter can be calculated by:
1
f_c  _{2 R1R2C1C2}
(7)
  Q  R1R2C1C2
R₁  R₂  C₂
(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.
Rev. B | Page 18 of 20