Contacts and electrical · Updated 2026-09-28
EMI Filter Connectors: C vs π Filters and Choosing Capacitance
A filter connector puts a small low-pass filter on each contact, right where cables enter the enclosure. High-frequency noise is shunted to the shell before it reaches the electronics, and noise from inside cannot leave on the cable.
Filter circuits
| Circuit | Build | Behaviour |
|---|---|---|
| C | Capacitor from contact to shell | Simple, good for high-impedance sources and loads |
| π | Capacitor – ferrite inductor – capacitor | Steeper attenuation; preserves signal shape on high-impedance lines |
| L (CL / LC) | Capacitor plus ferrite | Used where source or load impedance is low on one side |
Choosing capacitance
Higher capacitance lowers the cut-off frequency and attenuates more at a given frequency. Examples from our planar filter arrays (25 °C, no load, 50 Ω):
| Circuit | Capacitance | 3 dB cut-off | Loss at 100 MHz |
|---|---|---|---|
| C | 470 pF | 12 MHz | 10 dB |
| C | 4700 pF | 2 MHz | 30 dB |
| C | 100 nF | 0.1 MHz | 40 dB |
| π | 4700 pF | 0.8 MHz | 41 dB |
| π | 100 nF | 0.1 MHz | 55 dB |
The cut-off must stay well above your signal frequency, or the filter will distort the wanted signal. Different contacts in one connector can have different values — or none.
Precautions
- DC filter connectors must not be used on AC supply lines.
- Tell us about voltage spikes, surges and inductive loads such as relays and motors.
- Do not test filter contacts with a megger or AC hipot above the rated values — the capacitors can be damaged.
- Use a conductive panel gasket so the filter has a low-impedance path to the enclosure.
Available versions
We build filter connectors in 38999 Series III and Series I types, with working-voltage classes of 50, 100, 200 and 400 VDC and marine-grade construction.
