Datasheet
Data Sheet AD8625/AD8626/AD8627
Rev. F | Page 17 of 20
EIGHT-POLE SALLEN KEY LOW-PASS FILTER
The AD862x’s high input impedance and dc precision make it a
great selection for active filters. Due to the very low bias current
of the AD862x, high value resistors can be used to construct low
frequency filters. The AD862x’s picoamp-level input currents
contribute minimal dc errors. Figure 49 shows an example of a
10 Hz, 8-pole Sallen Key filter constructed using the AD862x.
Different numbers of the AD862x can be used depending on
the desired response, which is shown in Figure 48. 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 lower pole section of the filter. This
eliminates any peaking of the noise contribution of resistors in
the preceding sections, minimizing the inherent output voltage
noise of the filter.
03023-047
V2
V4
V3
V1
FREQUENCY (Hz)
VOLTAGE (V)
0.1
0
0.4
0.8
1.2
1 10 100 1k
Figure 48. Frequency Response Output at Different Stages
of the Low-Pass Filter
03023-048
V
IN
V
DD
V
EE
R1
162.3kΩ
R2
162.3k
Ω
3
2
11
1
4
U1
R3
25k
Ω
C2
96.19
µ
F
D
D
V3
R10
191.4k
Ω
R5
191.4k
Ω
U2
R4
25k
Ω
C4
69.14
µ
F
D
R11
286.5k
Ω
R7
286.5k
Ω
U3
R6
25k
Ω
C6
30.86
µ
F
D
R12
815.8k
Ω
R9
815.8k
Ω
U4
R8
25k
Ω
C8
3.805
µ
F
D
C1
100
µ
F
C3
1/4
AD8625
1/4
AD8625
1/4
AD8625
1/4
AD8625
100
µ
F
C5
100
µ
F
C7
100
µ
F
V1
V2
V3
V4
Figure 49. 10 Hz, 8-Pole Sallen Key Low-Pass Filter