RHF310 STMICROELECTRONICS [STMicroelectronics], RHF310 Datasheet - Page 18

• Manufacturer Part Number: RHF310
• Description: Rad-hard 400 ?A high-speed operational amplifier
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Active filtering
7
18/22
Active filtering
Figure 27. Low-pass active filtering, Sallen-Key
From the resistors R
classic non-inverting amplification configuration.
The response of the system is assumed to be:
The cut-off frequency is not gain-dependent and so becomes:
The damping factor is calculated using the following expression.
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:
Due to a limited selection of capacitor values in comparison with the resistors, you can set
C1=C2=C, so that:
IN
fb
and R
R1
ζ
G
=
T
Doc ID 15577 Rev 3
it is possible to directly calculate the gain of the filter in a
1
-- - ω
2
jω
=
c
R2
(
A
C
ω
Vout
----------------- -
V
Vin
1
c
ζ
ζ
R
=
=
1
=
=
jω
C1
R
jω
g
+
G
------------------------------------ -
2C
-------------------------------- -
2R
-------------------------------- -
C2
R1R2C1C2
C
=
=
2 C
2 R
1
2
2
1
R
-------------------------------------------
1
–
–
2
+
+
1
C
R
1
1
+
+
_
R
--------
R
2ζ
C
R
1
1
C
fb
R
--------
R
--------
g
R
2
R
2
----- -
ω
jω
2
fb
fb
g
g
R
g
R
c
fb
1
+
–
(
------------ -
ω
jω
C
c
1
)
2
2
R
1
g
)
OUT
AM00843
1 kΩ
RHF310