# Circuit Guidelines

Get PDF versionInstec EMI/RFI Filters are available in the following circuit configurations:

**C-Filter: **The C filter is a three terminal feed-through capacitor. It is used to attenuate high frequency signals. Instec filters employ multi-layer ceramic discoidal capacitors. These offer the best attenuation due to a circumferential metallization’s with infinite paths to ground. MLCC discoidal cap designs offer superior performance over chip capacitor and tubular capacitor designs. In the case of the latter, the discoidal cap is much more robust and in the case of the former, the discoidal has the advantage of circumferential metallization's. This filter should be avoided for use in circuits where large voltage transients occur in favor of a circuit which includes inductive elements.

**L-Filter:** The L filter is a three terminal filter consisting of one inductive element and one capacitive element. Instec’s design of L filters also employ the MLCC discoidal capacitor for the capacitive element providing the same superior performance described in the C-Filter description above. The inductive element may be a ferrite bead or a toroid, depending on the Insertion Loss requirements, the current rating, and the space available. L filters can offer high or low impedance depending on their orientation in the circuit. These filters are most commonly used in applications where there is a high impedance source and a low impedance load (L1) or where there is a low impedance source and a high impedance load (L2).

**PI-Filter:** The PI filter contains two capacitive element separated by an inductive element. Instec’s design of PI filters also employ the MLCC discoidal capacitor for the capacitive element providing the same superior performance described in the C-Filter description above. The inductive element may be a ferrite bead or a toroid, depending on the Insertion Loss requirements, the current rating, and the space available. PI filters present low impedance to both the source and the load and provide better high frequency performance that the C or L configurations. Due to the possibility of resonance, PI filters are not recommended for switching applications.

**T-Filter: **The T filter consists of two inductive elements separate by a capacitive element. Instec’s design of T filters also employ the MLCC discoidal capacitor for the capacitive element providing the same superior performance described in the C-Filter description above. The inductive element may be a ferrite bead or a toroid, depending on the Insertion Loss requirements, the current rating, and the space available. T filters present high impedance input from either side and can be sued in applications where transient conditions may occur. It has similar filtering performance to a PI circuit without the resonance characteristic of a PI filter. They may therefore be used in switching applications.

**T-T-Filter:** The T-T filter, or multi-element filter, consists of three inductive elements separated by two capacitive elements. Instec’s design of T-T filters also employ the MLCC discoidal capacitor for the capacitive element providing the same superior performance described in the C-Filter description above. The inductive element may be a ferrite bead or a toroid, depending on the Insertion Loss requirements, the current rating, and the space available. T-T filters are designed for optimum insertion loss in circuits with relatively low source and load impedances, as well as applications where a high degree of filtering is required.

## Measuring the Effectiveness of a Filter

The industry standard measurement of the effectiveness of a filter is called Insertion Loss or Attenuation. Insertion Loss (IL) is defined as the ratio of voltage (A1) across the circuit load without the filter and the voltage (A2) across the load with the filter in place. The IL is dependent on the source and load impedance which the filter is subjected to. The typical IL readings used by Instec are based on MIL-STD-202. Measurements are defined for a matched 50? system. IL is measured in decibels (dB) and defined by the following formula: