DS100128-39
FIGURE 8. Amplitude Modulator Circuit
DS100128-36
FIGURE 10. Dual Amplifier, 3 kHz Low-Pass Active
Filter with a Butterworth Response and a Pass Band
Gain of Times Two
f
mod
f
carrier
DS100128-40
FIGURE 9. Output signal per the Circuit of
Figure 8
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.
DS100128-37
FIGURE 11. Dual Amplifier, 300 Hz High-Pass Active
Filter with a Butterworth Response and a Pass Band
Gain of Times Two
Table 1 provides sensitivity measurements for a 10 MΩ 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 R
5
of each circuit also caused variations in
13
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