OPA861
www.ti.com
SBOS338–AUGUST 2005
R3
R2
VIN
VOUT
BUF602
C1
C2
R1
R1M
R2M
RB1
RB2
RB3
R1S
R2S
R3S
Figure 43. Universal Active Filter
Transfer Function
3
0
50MHz Filter
The transfer function of the universal active filter of
Figure 43 is shown in Equation 7.
−
−
−
3
6
9
R
R
1M
1
s2C1R1M 2M ) sC1
)
−
−
−
−
−
−
−
−
−
−
−
−
−
12
15
18
21
24
27
30
33
36
39
42
45
48
R
3
R
R
VOUT
VIN
2
1
1MHz Filter
( )
F p +
+
R
R
1M
1
s2C1R1M 2M ) sC1
)
20MHz Filter
R
R
R
3S
2S
1S
(7)
For All Filters:
Filter Characteristics
Five filter types can be made with this structure:
∞
=
R2 = R3
R1 = R15 = R25 = 1/2 R35 = R
R1M = R2M = R0
C1 = C2 = C0
•
•
•
•
•
For a low-pass filter, set R2 = R3 = ∞,
For a high-pass filter, set R1 = R2 = ∞,
For a bandpass filter, set R1 = R3 = ∞,
For a band rejection filter, set R2 = ∞; R1 = R3,
For an all-pass filter, set R1 = R15; R2 = R25; and
R3 = R35.
10k
100k
1M
10M
100M
1G
Frequency (Hz)
Figure 44. Butterworth Low-Pass Filter with the
Universal Active Filter
A few designs for a low-pass filter are shown in
Figure 44 and Table 2.
The advantages of building active filters using a
Current Conveyor structure are:
•
The increase in output resistance of operational
amplifiers at high frequencies makes it difficult to
construct feedback filter structures (decrease in
stop-band attenuation).
All filter coefficients are represented by resist-
ances, making it possible to adjust the filter
frequency response without affecting the filter
coefficients.
Table 2. Component Values for Filters Shown In
Figure 44
fO
R
RO
CO
2nF
1MHz
20MHz
50MHz
150
150
150
100
100
100
•
112.5pF
55pF
21
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