RHF330 STMICROELECTRONICS [STMicroelectronics], RHF330 Datasheet - Page 19

• Manufacturer Part Number: RHF330
• Description: Rad-hard 1 GHz low noise operational amplifier
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

RHF330
8
Active filtering
Figure 33. Low-pass active filtering, Sallen-Key
From the resistors R
inverting amplification configuration.
We assume the following expression is the response of the system.
The cut-off frequency is not gain-dependent and so becomes:
The damping factor is calculated by the following expression.
The higher the gain, the more sensitive the damping factor is. 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, we can set
C1=C2=C, so that:
IN
fb
and R
R1
G
=
Doc ID 15576 Rev 3
we can directly calculate the gain of the filter in a classic non-
T
1
-- -
2
j
c
=
R2
C
Vout
---------------- -
A
c
1
Vin
R
V
=
=
=
1
C1
R
=
j
+
------------------------------------ -
j
G
2R
-------------------------------- -
2C
-------------------------------- -
C2
R1R2C1C2
C
g
2 R
2 C
=
1
2
2
=
R
---------------------------------------- -
1
–
–
2
1
1
R
+
C
1
1
+
+
_
+
R
C
1
1
2
C
R
--------
R
--------
R
2
R
2
R
--------
R
2
fb
---- -
fb
j
g
g
fb
R
g
g
R
c
fb
+
1
–
----------- -
j
C
c
2
1
2
R
1
g
OUT
AM00840
100
Active filtering
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