LMV321 datasheet(17/27 Pages) NSC

17 / 27 page

Application Notes (Continued)

For minimum dc offset, V

+ =V−, the resistor values at both

inverting and non-inverting inputs should be equal, which

means

(8)

From Equation (1) and Equation (8), we obtain

(9)

(10)

The values of C

1 and C2 are normally close to or equal to

As a design example:

Require: A

LP =2,Q=1,fc= 1KHz

Start by selecting C

1 and C2. Choose a standard value that

is close to

From Equations (6), (7), (9), (10),

1=1

Ω

2=1

Ω

3=4

Ω

4=4

Ω

The above resistor values are normalized values with

n =

1rad/s and C

1 =C2 =Cn = 1F. To scale the normalized cut-off

frequency and resistances to the real values, two scaling

factors are introduced, frequency scaling factor (k

f) and im-

pedance scaling factor (k

m).

Scaled values:

2 =R1 = 15.9 k

Ω

3 =R4 = 63.6 k

Ω

1 =C2 = 0.01 µF

An adjustment to the scaling may be made in order to have

realistic values for resistors and capacitors. The actual value

used for each component is shown in the circuit.

4.4.3 2nd-order High Pass Filter

A 2nd-order high pass filter can be built by simply inter-

changing those frequency selective components (R

1,R 2,

1,C2) in the Sallen-Key 2nd-order active low pass filter. As

shown in Figure 14, resistors become capacitors, and ca-

pacitors become resistors. The resulted high pass filter has

the same corner frequency and the same maximum gain as

the previous 2nd-order low pass filter if the same compo-

nents are chosen.

4.4.4 State Variable Filter

A state variable filter requires three op amps. One conve-

nient way to build state variable filters is with a quad op amp,

such as the LMV324 (Figure 15).

This circuit can simultaneously represent a low-pass filter,

high-pass filter, and bandpass filter at three different outputs.

The equations for these functions are listed below. It is also

called "Bi-Quad" active filter as it can produce a transfer

function

which

quadratic

both

numerator

and

denominator.

10006083

FIGURE 14. Sallen-Key 2nd-Order Active High-Pass

Filter

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