In environments where a known, narrow-frequency band must be attenuated, a series LC filter network can be added between the filter pins, as shown in Figure 9-2. The connection adds to the internal resistor network, and results in a RLC filter network that is also commonly known as bandstop filter.

Use Equation 3 to calculate the middle frequency of the filter:

In general, the width of the stopband is between one and two decades, meaning that the highest attenuation is between 10 to 100 around the middle frequency. This indication is commonly defined as the quality factor (Q factor) of the filter and is calculated by:

The resistor is given by the internal resistor of 6 kΩ; therefore, the damping factor of the filter can further be affected of the series resistance of the inductor L_FL. For stable operation of the filter, choose an inductor in the range of 100 µH.