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.

V2

V4

V3

V1

FREQUENCY (Hz)

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

R1

162.3k

Ω

R2

162.3k

Ω

3

2

11

1

4

U1

R3

25k

Ω

C2

96.19

D

D

V3

R10

191.4k

Ω

R5

191.4k

Ω

U2

R4

25k

Ω

C4

69.14

D

R11

286.5k

Ω

R7

286.5k

Ω

U3

R6

25k

Ω

C6

30.86

D

R12

815.8k

Ω

R9

815.8k

Ω

U4

R8

25k

Ω

C8

3.805

D

C1

100

C3

1/4

AD8625

1/4

AD8625

1/4

AD8625

1/4

AD8625

100

C5

100

C7

100

V1

V2

V3

V4

Figure 49. 10 Hz, 8-Pole Sallen Key Low-Pass Filter