LMV821 datasheet(13/24 Pages) NSC

13 / 24 page

2.4 Dual Amplifier Active Filters (DAAFs)

The LMV822/24 bring economy and performance to DAAFs.

The low-pass and the high-pass filters of

Figure 10 and Fig-

ure 11 (respectively), offer one key feature: excellent sensi-

tivity performance. Good sensitivity is when deviations in

component values cause relatively small deviations in a fil-

ter’s parameter such as cutoff frequency (Fc). Single ampli-

fier active filters like the Sallen-Key provide relatively poor

sensitivity performance that sometimes cause problems for

high production runs; their parameters are much more likely

to deviate out of specification than a DAAF would. The

DAAFs of

Figure 10 and Figure 11 are well suited for high

volume production.

Table 1 provides sensitivity measurements for a 10 M

Ω load

condition. The left column shows the passive components

for the 3 kHz low-pass DAAF. The third column shows the

components for the 300 Hz high-pass DAAF. Their respec-

tive sensitivity measurements are shown to the right of each

component column. Their values consists of the percent

change in cutoff frequency (Fc) divided by the percent

change in component value. The lower the sensitivity value,

the better the performance.

Each resistor value was changed by about 10 percent, and

this measured change was divided into the measured

change in Fc. A positive or negative sign in front of the mea-

sured value, represents the direction Fc changes relative to

components’ direction of change. For example, a sensitivity

value of negative 1.2, means that for a 1 percent increase in

component value, Fc decreases by 1.2 percent.

Note that this information provides insight on how to fine

tune the cutoff frequency, if necessary. It should be also

noted that R

4 and R5 of each circuit also caused variations in

DS100128-39

FIGURE 8. Amplitude Modulator Circuit

mod

carrier

DS100128-40

FIGURE 9. Output signal per the Circuit of

Figure 8

DS100128-36

FIGURE 10. Dual Amplifier, 3 kHz Low-Pass Active

Filter with a Butterworth Response and a Pass Band

Gain of Times Two

DS100128-37

FIGURE 11. Dual Amplifier, 300 Hz High-Pass Active

Filter with a Butterworth Response and a Pass Band

Gain of Times Two

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