7.2 Typical Applications

7.2.1 60Hz Twin T-Notch Filter

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)

Figure 7-3. 60Hz Notch Filter

7.2.1.1 Design Requirements

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

interference from ac power lines. The circuit of Figure 7-3 notches out the 60Hz and provides a gain AV = 2 for

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

harmonics of 60Hz. Thanks to the nA power consumption of the LPV521, even five such circuits can run for 9.5

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

of 2V. With an operating voltage from 1.6V to 5.5V, the LPV521 can function over this voltage range.

7.2.1.2 Detailed Design Procedure

The notch frequency is set by F0 = 1 / 2πRC. To achieve a 60Hz notch, use R = 10MΩ and C = 270pF. If

eliminating 50Hz noise, which is common in European systems, use R = 11.8MΩ and C = 270pF.

The twin T notch filter works by having two separate paths from VIN to the amplifier input. A low-frequency

path through resistors R-R and another separate high-frequency path through capacitors C-C. However, at

frequencies around the notch frequency, the two paths have opposing phase angles and the two signals tend to

cancel at the amplifier input.

To ensure that the target center frequency is achieved, and to maximize the notch depth (Q factor), balance

the filter as much as possible. To obtain circuit balance, while overcoming limitations of available standard

resistor and capacitor values, use passives in parallel to achieve the 2C and R/2 circuit requirements for the filter

components that connect to ground.

To ensure that passive component values stay as expected, clean the board with alcohol, rinse with deionized

water, and air dry. Ensure that the board remains in a relatively low humidity environment to minimize moisture

that can increase the conductivity of board components. Also large resistors come with considerable parasitic

stray capacitance; the effects can be reduced by cutting out the ground plane below components of concern.

Use Large resistors in the feedback network to minimize battery drain. When designing with large resistors,

consider the resistor thermal noise, op-amp current noise, as well as op-amp voltage noise in the noise analysis

of the circuit. The noise analysis for the circuit in Figure 7-3 can be done over a bandwidth of 5kHz, which takes

the conservative approach of overestimating the bandwidth (LPV521 typical GBW/AV is less). The total noise

at the output is approximately 800µVPP, which is excellent considering the total consumption of the circuit is

only 540nA. The dominant noise terms are op-amp voltage noise (550µVPP), current noise through the feedback

network (430µVPP), and current noise through the notch filter network (280µVPP). Thus, the total circuit noise is

less than ½ LSB of a 10-bit system with a 2V reference, which is 1mV.

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LPV521

SNOSB14E – AUGUST 2009 – REVISED JULY 2024

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