LMC6035IMMSLASHNOPB datasheet(20/44 Pages) TI

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7.2.2 Low-Pass Active Filter

A common application for low voltage systems is active filters, in cordless and cellular phones for example. The

ultra low input bias currents (IB) of the LMC603x makes these op amps an excellent for low power active filter

applications, because the low input bias current allows the use of higher resistor values and lower capacitor

values. This reduces power consumption and space.

Figure 7-7 shows a low pass, active filter with a Butterworth (maximally flat) frequency response. The topology

is a Sallen and Key filter with unity gain. Note the normalized component values in parenthesis which are

obtainable from standard filter design handbooks. These values provide a 1Hz cutoff frequency, but can be

easily scaled for a desired cutoff frequency (fc). The bold component values of Figure 7-7 provide a cutoff

frequency of 3kHz. An example of the scaling procedure follows Figure 7-7.

–

**VIN

LMC603x

4.7nF

6.8nF

8.4k

VOUT

(0.7071F)*

(1 )*

(1.414F)*

* Normalized values

** Input requires dc offset

Figure 7-7. 2-Pole, 3kHz, Active, Sallen and Key, Low-Pass Filter With Butterworth Response

7.2.2.1 Low-Pass Frequency Scaling Procedure

The actual component values represented in bold of Figure 7-7 were obtained with the following scaling

procedure:

1. First determine the frequency scaling factor (FSF) for the desired cutoff frequency. Choosing fc at 3kHz,

provides the following FSF computation:

FSF= 2π×300kHz=18.84k

(1)

2. Then divide all of the normalized capacitor values by the FSF as follows (C1' and C2': prior to impedance

scaling):

C1′= C1normalized

FSF = 0.70718.84k=37.93×10−6F

(2)

C2′= C1normalized

FSF = 1.41418.84k=75.05×10−6F

(3)

3. Last, choose an impedance scaling factor (Z). This Z factor can be calculated from a standard value for C2.

Then Z can be used to determine the remaining component values as follows:

Z= C2′C2chosen=75.05×10−6F

6.8nF =8.4k

(4)

C1= C1′Z=37.93×10−6F

8.4k =4.52nF

(5)

R1= R1normalized×Z=1Ω ×8.4k=8.4kΩ

(6)

R2= R2normalized×Z=1Ω ×8.4k=8.4kΩ

(7)

4. A standard value of 8.45kΩ is chosen for R1 and R2.

LMC6035, LMC6036

SNOS875H – JANUARY 2000 – REVISED DECEMBER 2024

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