5962F0723301VXC View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'5962F0723301VXC' PDF : 22 Pages View PDF
Active filtering
7
Active filtering
RHF310
Figure 27. Low-pass active filtering, Sallen-Key
C1
R1
IN
R2
+
C2
_
Rfb
RG
OUT
1 kΩ
AM00843
From the resistors Rfb and RG it is possible to directly calculate the gain of the filter in a
classic non-inverting amplification configuration.
AV
=
g
=
1 + R-----f--b-
Rg
The response of the system is assumed to be:
Tjω
=
V-----o----u---t--j-ω--
Vinjω
=
---------------------g----------------------
1
+
2
ζ
-j--ω---
ωc
+
(---j--ω----)--2-
ωc2
The cut-off frequency is not gain-dependent and so becomes:
ωc
=
------------------1------------------
R1R2C1C2
The damping factor is calculated using the following expression.
ζ
=
1--
2
ωc
(
C1
R1
+
C1R2
+
C2R1
–
C1R1g)
The higher the gain, the more sensitive the damping factor. When the gain is higher than 1,
it is preferable to use very stable resistor and capacitor values. In the case of R1=R2=R:
ζ = 2----C-----2----–----C-----1---R--R--------f-g--b--
2 C1C2
Due to a limited selection of capacitor values in comparison with the resistors, you can set
C1=C2=C, so that:
ζ = 2----R-----2----–----R-----1---R--R--------f-g--b--
2 R1R2
18/22
Doc ID 15577 Rev 3
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