LMC272 Datasheet, PDF (13/16 Pages) National Semiconductor (TI) – CMOS Dual Low Cost Rail to Rail Output Operational Amplifier

Application Information (Continued)
Ω
R1
270
1000
TABLE 1. Equivalent Input Noise Voltage, e_n, for Two Different Values of R1
mA
nV/√Hz
I_C(Q1, 3)
1.85
e_n (100 Hz)
3.2
e_n (1 kHz)
2.0
0.50
5.3
2.4
e_n (10 kHz)
1.7
1.9
Single Supply Twin-T Notch Filter with “Q” Adjustment
Here is another application for the LMC272. This is a single
supply notch filter set for 60 Hz using the component values
shown, but the frequency can be changed using the equa-
tions below. The main feature of this circuit is its ability to ad-
just the filter selectivity (Q) using RPOT. You can trade off
notch depth for Q. Table 2 shows data for two different set-
tings. The LMC272 lends itself nicely to general purpose ap-
plications like this because it is very well behaved and easy
to use. This filter can operate from 2.7V to 15V supplies.
Component value matching is important to achieve good re-
sults. Here R4 is used to set the input to within the common
mode range of the device to allow maximum swing on the
non-inverting input (pin 3). Since R1, R2, and R4 form a volt-
age divider at low frequencies, C4 is added to introduce a
high frequency attenuation in conjunction with C1, and C3.
R5 and R6 were picked to set the pass band gain to 0 dB.
R = R1 = R2 = 2R3
C = C1 = C3 = C2/2
DS012867-3
TABLE 2. Filter Selectivity (Q) vs Notch Depth
(dB)
Q
Notch Depth
0.3
40
6
17
13
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