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:

VOUT
VIN

1

1f

fo
2j

Qf

fo

Where:

fo1

2mnRC

And

Qmn

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
RA
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
2π √mn RC
Q = √mn
m+1
0.1µF10µF
0.1µF10µF
6A2/SD
R214 A2 FLT
C214
9 B2/SD
R208 B2 FLT
C205
C208