TABLE 1. Equivalent Input Noise Voltage, e

Ω

mA

nV/

R1

I

e

e

e

270

1.85

3.2

2.0

1.7

1000

0.50

5.3

2.4

1.9

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

TABLE 2. Filter Selectivity (Q) vs Notch Depth

(dB)

Q

Notch Depth

0.3

40

617

Single Supply Twin-T Notch Filter with “Q” Adjustment

DS012867-3

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