TSH350_07 STMICROELECTRONICS [STMicroelectronics], TSH350_07 Datasheet - Page 18

• Manufacturer Part Number: TSH350_07
• Description: 550MHz low noise current feedback amplifier
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Active filtering
8
18/22
Active filtering
Figure 35. Low-pass active filtering, Sallen-Key
From the resistors R
inverting amplification configuration:
We assume the following expression as 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 some very stable resistor and capacitor values. In the case of
R1=R2=R:
Due to a limited selection of values of capacitors in comparison with resistors, we can set
C1=C2=C, so that:
IN
IN
fb
and R
R
R
1
1
G
=
we can directly calculate the gain of the filter in a classic non-
T
1
-- -
2
j
c
R
R
=
2
2
A
C
Vout
---------------- -
V
1
Vin
c
R
=
=
1
=
=
R
R
j
+
g
j
------------------------------------ -
G
G
2C
-------------------------------- -
2R
-------------------------------- -
C
C
R1R2C1C2
C
C
C
=
=
1
1
2 C
2 R
2
2
1
2
2
R
1
---------------------------------------- -
1
–
–
2
+
1
+
C
R
1
1
+
R
--------
+
+
R
2
C
R
_
_
1
1
C
fb
R
--------
R
--------
g
R
2
R
2
---- -
2
j
fb
fb
g
g
g
R
c
910
910
R
R
+
1
fb
fb
–
----------- -
j
C
c
2
1
2
R
1
g
OUT
OUT
100
100
TSH350