LPV542 Datasheet, PDF (17/29 Pages) Texas Instruments – LPV542 Dual Nanopower 1.8 V, 490nA, RRIO CMOS Operational Amplifier

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LPV542
SNOSCX9 – MARCH 2015
Typical Application: 60 Hz Twin "T" Notch Filter (continued)
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 39 can be done over a bandwidth of 2 kHz,
which takes the conservative approach of overestimating the bandwidth (LPV542 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 900 nA. The dominant noise terms are op amp voltage noise , 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.
8.2.3 Application Curve
Figure 40. 60 Hz Notch Filter Waveform
8.3 Do's and Don'ts
Do properly bypass the power supplies.
Do add series resistance to the output when driving capacitive loads, particularly cables, Muxes and ADC inputs.
Do add series current limiting resistors and external schottky clamp diodes if input voltage is expected to exceed
the supplies. Limit the current to 1 mA or less (1 KΩ per volt).
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