LPV542

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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,

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 K

Ω per volt).

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