LPV521_15 Datasheet, PDF (22/33 Pages) Texas Instruments – LPV521 NanoPower, 1.8-V, RRIO, CMOS Input, Operational Amplifier

LPV521
SNOSB14D – AUGUST 2009 – REVISED DECEMBER 2014
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Typical Applications (continued)
8.2.1.1 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 64 notches out the 60 Hz and provides a gain A_V = 2 for
the sensor signal represented by a 1-kHz sine wave. Similar stages may be cascaded to remove 2^nd and 3^rd
harmonics of 60 Hz. Thanks to the nA power consumption of the LPV521, 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 1.6 V to 5.5 V the LPV521 can function over this voltage range.
8.2.1.2 Detailed Design Procedure
The notch frequency is set by F₀ = 1 / 2πRC. To achieve a 60-Hz notch use R = 10 MΩ and C = 270 pF. If
eliminating 50-Hz noise, which is common in European systems, use R = 11.8 MΩ and C = 270 pF.
The Twin T Notch Filter works by having two separate paths from V_IN to the amplifier’s input. A low frequency
path through the resistors R - R and another separate high frequency path through the capacitors C - C.
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.
To ensure that the target center frequency is achieved and to maximize the notch depth (Q factor) the filter
needs to be as balanced 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 make sure passive component values stay as expected clean board with alcohol, rinse with deionized water,
and air dry. Make sure board remains in a relatively low humidity environment to minimize moisture which may
increase the conductivity of board components. Also large resistors come with considerable parasitic stray
capacitance which effects can be reduced by cutting out the ground plane below components of concern.
Large resistors are used in the feedback network to minimize battery drain. When designing with large resistors,
resistor thermal noise, op amp current noise, as well as op amp voltage noise, must be considered in the noise
analysis of the circuit. The noise analysis for the circuit in Figure 64 can be done over a bandwidth of 5 kHz,
which takes the conservative approach of overestimating the bandwidth (LPV521 typical GBW/A_V is lower). The
total noise at the output is approximately 800 µVpp, which is excellent considering the total consumption of the
circuit is only 540 nA. The dominant noise terms are op amp voltage noise (550 µVpp), current noise through the
feedback network (430 µVpp), and current noise through the notch filter network (280 µVpp). Thus the total
circuit's noise is below ½ LSB of a 10 bit system with a 2-V reference, which is 1 mV.
8.2.1.3 Application Curve
Figure 65. 60-Hz Notch Filter Waveform
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