Sallen-Key High-Pass Filter
3-6
Example Circuits
3.6
Sallen-Key High-Pass Filter
Figure 3–5 shows area 200 equipped with a dual operational amplifier
configured as a second-order Sallen-Key high-pass filter using single-supply
power input.
Basic setup is done by proper choice of resistors R and mR, and capacitors
C and nC. Note that capacitors should be used for components R201 and
R205, and a resistor for C201. The transfer function for the circuit as shown
is:
V
OUT
+
V
IN
ȧȧ
ȧ
ȡ
Ȣ
*
ǒ
f
f
o
Ǔ
2
1
)
ǒ
j
Q
Ǔ
ǒ
f
f
o
Ǔ
*
ǒ
f
f
o
Ǔ
2
ȧȧ
ȧ
ȣ
Ȥ
)
VREF2
Where:
f
o
+
1
2
p
m n
Ǹ
RC
And
Q
+
m n
Ǹ
n
)
1
The TL431 adjustable precision shunt regulator, configured as shown,
provides a low impedance reference for the circuit at about 1/2 V2+ in a 5 V
system. Another option is to adjust resistors R211 and R213 for the desired
VREF2 voltage. The formula for calculating VREF2 is:
VREF2
+
2.50 V
ǒ
R211
)
R213
R213
Ǔ
