LMV821 Datasheet, PDF (14/24 Pages) National Semiconductor (TI) – Low Voltage, Low Power, R-to-R Output, 5 MHz Op Amps

the pass band gain. Increasing R₄ by ten percent, increased
the gain by 0.4 dB, while increasing R₅ by ten percent, de-
creased the gain by 0.4 dB.
Component (LPF)
R_a
C₁
R₂
R₃
C₃
R₄
R₅
Sensitivity (LPF)
-1.2
-0.1
-1.1
+0.7
-1.5
-0.6
+0.6
TABLE 1.
Component (HPF)
C_a
R_b
R₁
C₂
R₃
R₄
R₅
Sensitivity (HPF)
-0.7
-1.0
+0.1
-0.1
+0.1
-0.1
+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 excel-
lent 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 12 shows an impressive photo-
graph of a network analyzer measurement (hp3577A). The
measurement was taken from a 300kHz version of Figure
10. 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.
To simplify the design process, certain components are set
equal to each other. Refer to Figure 10 and Figure 11. These
equal component values help to simplify the design equa-
tions as follows:
To illustrate the design process/implementation, a 3 kHz,
Butterworth response, low-pass filter DAAF (Figure 10) is
designed as follows:
1. Choose C₁ = C₃ = C = 1 nF
2. Choose R₄ = R₅ = 1 kΩ
3. Calculate R_a and R₂ for the desired Fc as follows:
DS100128-92
FIGURE 12. 300 kHz, Low-Pass Filter, Butterworth
Response as Measured by the HP3577A Network
Analyzer
In addition to performance, DAAFs are relatively easy to de-
sign and implement. The design equations for the low-pass
and high-pass DAAFs are shown below. The first two equa-
tion calculate the Fc and the circuit Quality Factor (Q) for the
LPF (Figure 10). The second two equations calculate the Fc
and Q for the HPF (Figure 11).
4. Calculate R₃ for the desired Q. The desired Q for a Butter-
worth (Maximally Flat) response is 0.707 (45 degrees into
the s-plane). R₃ calculates as follows:
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Notice that R₃ could also be calculated as 0.707 of R_a or R_2.
The circuit was implemented and its cutoff frequency mea-
sured. The cutoff frequency measured at 2.92 kHz.
The circuit also showed good repeatability. Ten different
LMV822 samples were placed in the circuit. The correspond-
ing change in the cutoff frequency was less than a percent.
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