AD820
LOW POWER 3-POLE SALLEN KEY LOW-PASS
FILTER
The high input impedance of the AD820 makes it a good
selection for active filters. High value resistors can be used to
construct low frequency filters with capacitors much less than
1 μF. The AD820 picoamp level input currents contribute
minimal dc errors.
Figure 46 shows an example, a 10 Hz 3-pole Sallen Key filter.
The high value used for R1 minimizes interaction with signal
source resistance. Pole placement in this version of the filter
minimizes the Q associated with the 2-pole section of the filter.
This eliminates any peaking of the noise contribution of
resistors R1, R2, and R3, thus minimizing the inherent output
voltage noise of the filter.
C2
0.022µF
+VS
R1
R2
R3
0.01µF
243kΩ 243kΩ 243kΩ
+
37
+
VIN
–
C1
0.022µF
C3
0.022µF
2
AD820 6
–
0.01µF
4
–VS
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
0.1
1
10
100
FREQUENCY (Hz)
Figure 46. 10 Hz Sallen Key Low-Pass Filter
+
VOUT
–
1k
Rev. E | Page 20 of 24