LPV521

SNOSB14C – AUGUST 2009 – REVISED FEBRUARY 2013

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APPLICATION INFORMATION

The LPV521 is fabricated with Texas Instruments' state-of-the-art VIP50 process. This proprietary process

dramatically improves the performance of Texas Instruments' low-power and low-voltage operational amplifiers.

The following sections showcase the advantages of the VIP50 process and highlight circuits which enable ultra-

low power consumption.

60 HZ TWIN T NOTCH FILTER

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

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 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.

eliminating 50 Hz noise, which is common in European systems, use R = 11.8 M

Ω and C = 270 pF.

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 62 can be done over a bandwidth of 5 kHz,

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 1/2 LSB of a 10 bit system with a 2 V reference, which is 1 mV.

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