Filter Selection

Selection of the proper filter for your application is key to achieving the best overall performance in your system.  At times it can be difficult to know which filter to specify with all the cirucit types and body styles available.  Below is a general overview of the various filter configurations and the circuit types they are generally best suited for.

"C" Circuit 

The "C" is the most common EMI filter.  It is a three terminal feed-thru capacitor, used to attenuate high frequency signals.  "C" circuits, unlike inductor bearing circuits, do not suffer performance drops due to current saturation situations. The "C" is the most common (and thus more likely to be in stock), most reliable, lowest cost type.

 

"L" Circuit

The "L" filter consists of one inductive element and one capacitor.  It is most commonly used in circuits where there is a high impedance loada low impedance source (L1), or where the circuit has a high impedance source and low impedance load (L2).  One disadvantage of miniature "L" filters that engineers need to be aware of is that the inductive element consists of a ferrite bead that will saturate and lose its effectiveness if more than 250mA of current is applied across the filter.

 

"Pi" Circuit

The "Pi" filter consists of two capacitors and one inductive element.  It presents a low impedance to both the source and load in a circuit.  Because of the additional capacitor element, it provides better high frequency attenuation than the "C" and "L"filters of the same capacitance.  "Pi" filters are not recommended for swithching applications due to the possibility of "ringing".  Again, ferrite inductors lose their effectiveness if more than 250mA current is passed through.  In such cases, filter performance drops to that of the base capacitance.  An engineer should evaluate a higher capacitance "C" filter as both an economic and performance substitute for a miniature "Pi" filter.

 

"T" Circuit

The "T" filter consists of two inductive elements and one capacitor.  This filter presents a high impedance to both the source and load of the circuit.  It has similar filter performance to "Pi" filter, without the possibility of ringing, and can be used in switching applications.

 

 

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