Datasheet
LMV821-N, LMV822-N, LMV822-N-Q1
LMV824-N, LMV824-N-Q1
www.ti.com
SNOS032G –AUGUST 1999–REVISED NOVEMBER 2013
300 Hz High-Pass Active Filter with a Butterworth Response and a Pass Band Gain of Times Two
Figure 42. Dual Amplifier
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 respective 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 measured 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 the pass band gain. Increasing R
4
by ten percent,
increased the gain by 0.4 dB, while increasing R
5
by ten percent, decreased the gain by 0.4 dB.
Table 1.
Component Sensitivity Component Sensitivity
(LPF) (LPF) (HPF) (HPF)
R
a
-1.2 C
a
-0.7
C
1
-0.1 R
b
-1.0
R
2
-1.1 R
1
+0.1
R
3
+0.7 C
2
-0.1
C
3
-1.5 R
3
+0.1
R
4
-0.6 R
4
-0.1
R
5
+0.6 R
5
+0.1
Active filters are also sensitive to an op amp's parameters -Gain and Bandwidth, in particular. The LMV822/24
provide a large gain and wide bandwidth. And DAAFs make excellent use of these feature specifications.
Single Amplifier versions require a large open-loop to closed-loop gain ratio - approximately 50 to 1, at the Fc of
the filter response. Figure 43 shows an impressive photograph of a network analyzer measurement (hp3577A).
The measurement was taken from a 300 kHz version of Figure 41. At 300 kHz, the open-loop to closed-loop gain
ratio @ Fc is about 5 to 1. This is 10 times lower than the 50 to 1 “rule of thumb” for Single Amplifier Active
Filters.
Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LMV821-N LMV822-N LMV822-N-Q1 LMV824-N LMV824-N-Q1