EMC Question of the Week: September 25, 2023

Unidentified low-pass filter between a noise source and a high-impedance load

The best low-pass filter option for a high-impedance (capacitive) input is usually a 

  1. series inductor
  2. series resistor
  3. shunt capacitor
  4. series ferrite


The best answer is “b.” While all of the options listed nominally provide low-pass filtering to a capacitive load, a series resistor is almost always the best option. The series resistor creates a first-order RC filter that works reliably over a fairly wide bandwidth. 

A series inductor on a capacitive load forms a 2nd-order filter that nominally provides greater attenuation above the cut-off frequency. However, LC filters can ring near their resonant frequency if there is not sufficient loss in the source. This ringing can distort the signal and amplify noise. Another issue with inductors is that their inter-winding capacitance can resonate with circuit loop inductances and further limit the effective bandwidth of the filter. 

Shunt capacitors can form a first-order RC filter with the source resistance, but most sources have a low equivalent series resistance requiring the use of large-valued capacitors. The connection inductance of these capacitors limits their effectiveness at frequencies above cut-off. Capacitors can also introduce new resonances, draw more current, tend to cost more, and can impact the long-term reliability of the circuit. 

Ferrites should not be used with capacitive loads. At best, a ferrite introduces a series resistance in the same way as a series resistor. But compared to series resistors, ferrites have a more limited bandwidth. Ferrites also tend to look inductive at some frequencies, introducing the possibility that they may resonate with capacitive loads. In addition, ferrites can saturate in high currents, tend to cost more, and are physically larger than comparable resistors. 

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