10 M:

-

+

VOUT

10 M:

10 M:

VIN

10 M:

10 M:

270 pF

270 pF

270 pF

CR2032 Coin Cell

225 mAh = 5 circuits @ 9.5 yrs.

60 Hz Twin T Notch Filter

AV = 2 V/V

10 M:

270 pF

VBATT

VBATT = 3V o 2V @ end of life

Remote Sensor

Signal

+

60 Hz

To ADC

Signal × 2

(No 60 Hz)

12

TLV522

SNOSD27 – MAY 2016

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Product Folder Links: TLV522

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Copyright © 2016, Texas Instruments Incorporated

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The TLV522 is a ultra-low power operational amplifier that provides 8 kHz bandwidth with only 490 nA quiescent

current, and near precision offset and drift specifications at a low cost. These rail-to-rail input and output

amplifiers are specifically designed for battery-powered applications. The input common-mode voltage range

extends to the power-supply rails and the output swings to within millivolts of the rails, maintaining a wide

dynamic range.

8.2 Typical Application: 60 Hz Twin "T" Notch Filter

Figure 24. 60 Hz Notch Filter

8.2.1 Design Requirements

Small signals from transducers in remote and distributed sensing applications commonly suffer strong 60 Hz

interference from AC power lines. The circuit of Figure 24 notches out the 60 Hz and provides a gain AV = 2 for

the sensor signal represented by a 1 kHz sine wave. Similar stages may be cascaded to remove 2nd and 3rd

harmonics of 60 Hz. Thanks to the nA power consumption of the TLV522, even 5 such circuits can run for 9.5

years from a small CR2032 lithium cell. These batteries have a nominal voltage of 3 V and an end of life voltage

of 2 V. With an operating voltage from 1.7 V to 5.5 V the TLV522 can function over this voltage range.

8.2.2 Detailed Design Procedure

The notch frequency is set by:

F0 = 1 / 2πRC.

(2)

To achieve a 60 Hz notch use R = 10 M

Ω and C = 270 pF. If eliminating 50 Hz noise, which is common in

European systems, use R = 11.8 M

Ω and C = 270 pF.

The Twin T Notch Filter works by having two separate paths from VIN to the amplifier’s input. A low frequency

path through the series input resistors and another separate high frequency path through the series input

capacitors. However, at frequencies around the notch frequency, the two paths have opposing phase angles and

the two signals will tend to cancel at the amplifier’s input.