Smiths Connectors is now Smiths Interconnect

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EMI filter connectors use multi-layer ceramic capacitor arrays together with inductive materials to realize robust, high performance low pass filter networks. Where required, protection can be provided against high energy transients from direct lightning strikes, standard EMP, high altitude EMP (HEMP), non-nuclear EMP (NNEMP) and electromagnetic interferences (EMI).

Features:

  • C, L and Pi filter styles available
  • Filtering technology (planar arrays, discoidal) to suit application
  • Unfiltered (high speed) and ground lines
  • Transient protection compliant with RTCA D160F Section 22 levels 3, 4 and 5
  • Can be combined with high energy TVS for high transient protection


Electromagnetic Interference (EMI) is broadly defined as any unwanted electrical or electromagnetic energy that causes undesirable responses, degraded performance, or failure in electronic equipment.  EMI is broadly categorized as narrowband EMI (ex: GSM phone interference) and broadband EMI (ex: high voltage power lines).  In order to protect modern electronic systems from the abundance of EMI sources in today’s environments, a combination of approaches are implemented: shielding, filtering, and reduction of EMI at the source (if possible).

The easiest and most common method of protecting electronics involves encapsulating the device in a faraday cage (enclosure of conducting material).  This allows for the device to be protected from outside EMI as well as containing EMI generated from the device from interfacing with other electronic devices.  The problem with using this approach exclusively is that most electronic systems require signal and power lines.  Inserting these through the faraday cage would provide a path for the EMI to leak through.  This issue is resolved through the use of a filter connector that contains EMI on each side of the faraday cage.

Grounded Conductive Enclosure

EMI Filter Terms and Definitions

  • Attenuation (dB): The reduction of power density of a signal as it propagates along any transmission path. 
  • Attenuation Slope (dB/oct): A measurement of attenuation in relation to frequency.
  • Insertion Loss (dB): The loss of power delivered to a load as the result of a filter. 
  • Cutoff Frequency (Hz): The boundary at which the frequency response within a system begins to be attenuated rather than passing through.  Typically defined as the point where 3 dB of insertion loss is achieved.
  • Bode Plot (dB vs Hz): A graph of the transfer function for a linear, time-invariant system to show a systems frequency response across a range of frequencies.
Ideal Low Pass Filter
  • 3 dB point: 1 rad/sec -> .159 Hz
  • Attenuation slope: -20 dB/decade
  • Insertion loss at 100 rad/sec:-40 dB
Angular Frequency

EMI Filter Capacitor Technologies

Tubular Capacitors:
  • Capacitor takes the form of a cylinder with axial leadsTubular Capacitors
  • Tubular capacitors were the first to be incorporated into EMI filters
  • Not available in higher capacitance values
  • Fail quickly in extreme shock and vibration environments
  • Cannot withstand higher voltages due to think wall thickness
  • Tubular capacitors still have a significant market share, but are being phased out by other technologies
Chip Capacitors:
  • These capacitors mount of the surface of the PCB
  • Much greater range of capacitance values than tubular capacitorsChip Capacitors
  • Can be removed and replaced from the PCB to modify the capacitor value with minimal cost
  • Less susceptible to shock and vibration than tubular capacitors
  • Has a resonance frequency of 120 MHz, may be problematic for some applications
  • Low cost and easily to source
Planar Capacitor Array:
  • This capacitor is constructed out of a ceramic disk
  • Disk can contain different capacitor values for each pin, unfiltered (feed through) pins, and ground pins
  • Best technology to use in high shock and vibration environments
  • Lowest rate of failure
  • No resonance frequency
  • Can handle up to 1000 VDC working voltage
  • Can handle mixed capacitance values within a 10:1 range
    • Example: 1000 pF to – 10,000 pF
Planar Array

EMI Filter Configurations

Filter Configurations

  • High source or load impedance >100 Ohms
  • Low source or load impedance < 10 Ohms
Pi FilterLC Filter