LM6035IBP datasheet(13/20 Pages) NSC

13 / 20 page

1.0 Application Notes (Continued)

Figure 4 shows the superior distortion performance of

LMC6035/6 over that of the benchmark op amp. The heavy

loading of the circuit causes the A

VOL of the benchmark part

to drop significantly which causes increased distortion.

1.2 APPLICATION CIRCUITS

1.2.1 Low-Pass Active Filter

A common application for low voltage systems would be

active filters, in cordless and cellular phones for example.

The ultra low input currents (I

IN) of the LMC6035/6 makes it

well suited for low power active filter applications, because it

allows the use of higher resistor values and lower capacitor

values. This reduces power consumption and space.

Figure 5 shows a low pass, active filter with a Butterworth

(maximally flat) frequency response. Its topology is a Sallen

and Key filter with unity gain. Note the normalized compo-

nent values in parenthesis which are obtainable from stan-

dard filter design handbooks. These values provide a 1Hz

cutoff frequency, but they can be easily scaled for a desired

cutoff frequency (f

c). The bold component values of Figure 5

provide a cutoff frequency of 3kHz. An example of the scal-

ing procedure follows Figure 5.

1.2.1.1 Low-Pass Frequency Scaling Procedure

The actual component values represented in bold of Figure 5

were obtained with the following scaling procedure:

First determine the frequency scaling factor (FSF) for

the desired cutoff frequency. Choosing f

c at 3kHz, pro-

vides the following FSF computation:

FSF=2

π x 3kHz

(desired cutoff freq.) = 18.84 x 10

Then divide all of the normalized capacitor values by the

FSF as follows:

C1’ = C

(Normalized)/FSF

C1’ =

0.707/18.84 x 10

3 = 37.93 x 10−6

C2’ = 1.414/18.84

x10

3 = 75.05 x 10−6

(C1’ and C2’: prior to impedance

scaling)

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 compo-

nent values as follows:

Z = C2’/C2

(chosen) = 75.05 x 10

−6/6.8nF = 8.4k

C1 = C1’/Z = 37.93 x 10

−6 /8.4k = 4.52nF

(Standard capacitor value chosen for C1 is 4.7nF )R1 =

(normalized) x Z=1

Ω x 8.4k = 8.4kΩ

R2=R2

(normalized)

x Z=1

Ω x 8.4k = 8.4kΩ

(Standard value chosen for R1 and R2 is 8.45k

Ω )

1.2.2 High Pass Active Filter

The previous low-pass filter circuit of Figure 5 converts to a

high-pass active filter per Figure 6.

01283047

FIGURE 4. THD+Noise Performance of LMC6035 and

“Benchmark” per Circuit of Figure 1

01283048

FIGURE 5. 2-Pole, 3kHz, Active, Sallen and Key,

Lowpass Filter with Butterworth Response

01283049

FIGURE 6. 2 Pole, 300Hz, Sallen and Key,

High-Pass Filter

LM6035IBP Datasheet(PDF) 13 Page - National Semiconductor (TI)

LM6035IBP Datasheet(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL