AD8625 Analog Devices, AD8625 Datasheet - Page 17

• Manufacturer Part Number: AD8625
• Description: Precision, Low Power, Single-Supply, JFET Amplifier
• Manufacturer: Analog Devices

Specifications of AD8625

• -3db Bandwidth: 5MHz
• Slew Rate: 5V/µs
• Vos: 50µV
• Ib: 0.25pA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 16nV/rtHz
• Vcc-vee: 5V to 26V
• Isy Per Amplifier: 850µA
• Packages: SOIC,SOP

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.
V
IN
96.19
162.3k
V3
D
R1
μ
C2
F
Ω
D
R2
162.3k
Ω
3
2
100
C1
11
μ
4
F
25k
R3
AD8625
V
V
Ω
DD
EE
U1
1/4
1
V1
69.14
191.4k
R10
C4
μ
F
Ω
D
Figure 49. 10 Hz, 8-Pole Sallen Key Low-Pass Filter
R5
191.4k
Ω
100
C3
μ
F
25k
AD8625
R4
Rev. E | Page 17 of 20
1/4
Ω
U2
V2
30.86
286.5k
R11
μ
C6
F
Ω
D
R7
286.5k
1.2
0.8
0.4
Figure 48. Frequency Response Output at Different Stages
0
0.1
Ω
100
C5
μ
25k
AD8625
F
R6
1/4
Ω
U3
V3
1
V1
V2
V4
of the Low-Pass Filter
3.805
815.8k
AD8625/AD8626/AD8627
R12
FREQUENCY (Hz)
C8
μ
F
Ω
D
10
R9
815.8k
V3
Ω
100
C7
μ
25k
F
AD8625
R8
100
1/4
Ω
U4
V4
1k