OPA861IDBVT datasheet(21/28 Pages) TI

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OUT

BUF602

Transfer Function

F(p) +

OUT

s2C

1R1M

) sC1

)

s2C

1R1M

) sC1

)

(7)

Filter Characteristics

−3

−6

−9

−12

−15

−18

−21

−24

−27

−30

−33

−36

−39

−42

−45

−48

Frequency (Hz)

10k

100k

10M

100M

50MHz Filter

20MHz Filter

1MHz Filter

For All Filters:

2 = R3 = ∞

1 = R15 = R25 = 1/2 R35 = R

1M = R2M = R0

1 = C2 = C0

OPA861

SBOS338 – AUGUST 2005

Figure 43. Universal Active Filter

The transfer function of the universal active filter of

Figure 43 is shown in Equation 7.

Five filter types can be made with this structure:

•

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,

Figure 44. Butterworth Low-Pass Filter with the

•

For an all-pass filter, set R1 = R15; R2 = R25; and

Universal Active Filter

R3 = R35.

A few designs for a low-pass filter are shown in

The advantages of building active filters using a

Figure 44 and Table 2.

Current Conveyor structure are:

•

The increase in output resistance of operational

Table 2. Component Values for Filters Shown In

amplifiers at high frequencies makes it difficult to

Figure 44

construct feedback filter structures (decrease in

stop-band attenuation).

1MHz

150

100

2nF

•

All filter coefficients are represented by resist-

20MHz

150

100

112.5pF

ances, making it possible to adjust the filter

50MHz

150

100

55pF

frequency response without affecting the filter

coefficients.

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