Sallen-Key Low-Pass Filter
3-5
Example Circuits
3.5
Sallen-Key Low-Pass Filter
Figure 3–4 shows area 200 equipped with a dual operational amplifier
configured as a second-order Sallen-Key low-pass filter using dual-power
supplies.
Basic setup is done by proper choice of resistors R and mR, and capacitors
C and nC. The transfer function is:
V
OUT
V
IN
+
1
1
*
ǒ
f
f
o
Ǔ
2
)
ǒ
j
Q
Ǔ
ǒ
f
f
o
Ǔ
Where:
fo
+
1
2
p
m n
Ǹ
RC
And
Q
+
m n
Ǹ
m
)
1
Figure 3–4. Sallen-Key Low-Pass Filter Wwith Dual Supply Using Area 200
C206
C207
C209
C210
V2+
V2–
V2+
GND2
V2–
Power Supply Bypass
+
–
14
1
4
3
2
V2+
V2–
R212
C215
R216
C211
R221
R220
R218
R219
R217
R215
C212
C213
A2OUT
1/2 Dual Op Amp
A201–
A202–
A203+
A204+
+
–
13
11
12
R206
C202
R207
C203
R204
R203
R205
R201
R202
R209
C204
C201
B2OUT
1/2 Dual Op Amp
B201–
B202–
B203+
B204+
R211
C
R
A
U202
R213
Voltage Reference
V2+
VREF2
U201A
U201B
R210
Jumper
+
–
mR
R
nC
Jumper
Jumper
Not Used
Not Used
Vout
Vin
Vin
=
1
1– (f/fo)2 + (j/Q)(f/fo)
fo =
1
2
π √
mn RC
Q =
√
mn
m+1
0.1
µ
F
10
µ
F
0.1
µ
F
10
µ
F
6
A2/SD
R214
A2 FLT
C214
9
B2/SD
R208
B2 FLT
C205
C208
