Turn Down the Noise
Filtered Connectors Provide Solutions to Reduce EMI/ESD Distortion
By Bob Hult, Bishop & Associates Inc.

It’s a noisy world out there. We are all familiar with how difficult it is to concentrate when background noise levels rise. The ability to pick out words and understand a conversation can become a real challenge in a noisy restaurant. The same principles apply to electronic digital channels, where the objective is to be able to identify a pattern of ones and zeros among a rising tide of random electronic noise.

As system speeds have increased, the voltage levels of data signals have decreased. Similar to hearing a whisper at a football game, the discrimination of lower signal levels embedded in high levels of noise (signal to noise ratio) has a major influence on the overall performance of electronic systems.

The ability of an electronic device to operate properly at its rated specification within its intended environment is known as electromagnetic compatibility (EMC), and is the goal of every system designer. Achieving EMC in an electronic device involves addressing emission of harmful energy and managing susceptibility or immunity, which is the ability of a device to resist the negative effects of environmental electromagnetic interference (EMI).

Equipment that ranges from avionic radar to automotive control systems may be packed in tight spaces, putting strong sources of EMI in close proximity to sensitive receivers.

Isolating circuits from noise can be accomplished by providing a barrier between noise sources and the system. A metallic shield applied to the exterior of a cable or enclosure can either reflect or absorb this energy, reducing it to acceptable levels, and can serve as a primary defense against EMI. The effectiveness of the shield is highly dependent on the choice of materials, the size of any apertures, and the grounding design.

Cost constraints and physical packaging requirements of the system often dictate the level of protection afforded the circuitry. Both radiated and conducted EMI have the ability to turn a clean square wave into mush. A successful design will mitigate EMI without major impact in manufacturability or overall system cost.

The internal and external environment of typical electronic products provides plenty of opportunity for noise to corrupt the digital stream. Electromagnetic interference can come from inside the equipment enclosure, such as unwanted signals generated by adjacent PCB traces or wires. Some discrete components can also be a source of EMI. Physical separation, the use of properly grounded shielding partitions, and good printed circuit board design practices can minimize the effect of these internal EMI sources. Containing internally generated EMI is essential to ensure that it does not interfere with the operation of adjacent devices.

Unwanted noise can be induced in electronic circuits via exterior radiated or conducted paths. Our world is filled with a host of radiated EMI that include such diverse sources as radio, television, cellular phones, garage door openers, computer clock lines, power transmission cables, and florescent lightning, as well as the starting and stopping of nearby electric motors and switches. The most common solutions to minimizing the effects of radiated EMI include external shielding, proper ground plane design, and the use of differential signaling. The metallic or metallized plastic box that typically surrounds electronic devices is known as a Faraday Cage, and can provide effectively shield sensitive circuitry from external radiated EMI.

Unshielded copper cables passing through external electric and magnetic fields will have noise impressed on the signals they are carrying and can enter the box via power inlet and input/output connectors. One of the most effective solutions to minimizing conducted signal distortion from external EMI is the insertion of a filter in the circuit.

Some applications that require relatively low levels of filtering may utilize ferrite rings surrounding the cable. Ferrite has the ability to absorb RF energy and dissipate it as heat. Many personal computer peripheral, DC power, and video cables feature the familiar lump near the ends of the cable. External ferrite filters are a very cost-effective solution that is easy to implement.

Applications that require a greater degree of noise isolation utilize a variety of discrete filter technologies, but most involve adding capacitance between the signal line and ground. As the frequency increases, higher order frequencies are shunted to ground.

These filters are broadly identified as low pass filters, meaning that they allow lower frequencies to pass through the circuit with minimal reduction in signal strength, while attenuating signals with higher frequency. A filter element achieves this by essentially becoming a variable resistor connected between the signal lines and ground, whose resistive value is inversely proportional to the frequency of the signal it is conducting. As the frequency increases, the resistance of the filter decreases, providing a lower impedance path to ground.

Individual chip capacitors can be placed on the printed circuit board to filter incoming lines, but the fact that these higher order frequencies have entered the EMI-shielded enclosure provides an opportunity for them to radiate to sensitive internal circuits. The most effective solution would be to prevent EMI from entering the box, and is achieved by locating the filter within the bulkhead connector.

One relatively low-cost option is the use of ferrite blocks attached to the back end of standard I/O connectors. These were popular within the personal computer market, where price pressures don’t permit higher performance solutions. They may be used to provide a bit of extra margin when a particular device is close to an emission limit. In some cases, a small block of ferrite material, providing low levels of common mode noise rejection, replaces the PCB pin organizer and consumes the same amount of PCB space as an unfiltered connector.

Where a greater degree of filtering is required, capacitance can be introduced to individual connector contacts using chip or thick film capacitors mounted on a substrate. These configurations have the advantage of requiring less space within the connector body, a significant advantage when working with high-density, small centerline connectors. Greater capacitive values can be achieved using this technique.

Filtered connectors may utilize additional filtering elements, including tubular and planar capacitors in a variety of configurations, with added inductance to achieve a specific filtering bandwidth profile. In some cases, small ferrite blocks are an additional component in the filter network.

A key advantage of a filtered connector is the fact that the connector insert includes a ground plane, which effectively closes the mounting hole that would be open to radiated EMI using a standard interface. The chassis-attached connector shell and insert provides a low impedance path to ground and prevents EMI from radiating in or out of the shielded enclosure.

Filtered connectors have been adapted over the years from just about every available interface, but some have found applications in select markets and are considered de facto standards. The D-subminiature connector utilizes a variety of filter technologies, including individual chip capacitors and inductors, and remains one of the most common interfaces for commercial and military applications.

The military and aerospace connector market has traditionally been the most fertile for the use of filtered connectors. The mission-critical nature of equipment in this market segment makes the filtered connector a key element in system designs.

Standard military connectors are modified with custom filter assemblies to provide the required isolation for specific applications.

Common filtered circular connectors include Mil-C-38999, 26482, 83723, as well as the military version of the D-subminiature Mil-C-24308.

The avionics industry has used filtered ARINC 404/600 connectors for many years, in a large variety of configurations.

Widespread use of filtered connectors has been limited by their relatively high cost. The custom nature of filtered connectors tends to result in a low-volume, high-mix product line. Filtered connectors often consist of hand-assembled customized filter elements that may require precision soldering and extensive performance testing, all contributing to a high per-piece price. The highly-fragmented market for filtered connectors is shared by several large suppliers, such as Airborn, Amphenol, Radiall, Sabritec, and Spectrum Control, along with approximately 60 smaller players. Suppliers have improved their assembly and test verification processes to reduce the long lead times, but the combination of cost, limited design flexibility, and long lead times remain barriers to broad market usage.

The control of electrostatic discharge voltages is another area where protection devices integrated into a connector may offer the most effective solution.

Signal speeds are going up, while chip feature geometry and logic voltage levels are going down. Smaller transistors densely packed on a chip typically increase the sensitivity of the device to high voltage surges. A high voltage spike can corrupt a high-speed data stream, or worse, permanently damage the device.

Manufacturers of electronic products have always faced the destructive effects of sudden user-generated voltage surges, and have utilized a combination of defenses at the system, PCB, and chip levels to combat the problem. The profusion of portable devices exposes a greater range of electronic products to unwanted spikes in voltage.

The generation of electrostatic charge is the culprit and occurs as the result of simple mechanical action. The amount of charge created between two surfaces depends on the properties of the two materials, degree of friction, speed of separation, as well as relative humidity. Typical static voltages generated with common activities include: 

• Walking across carpet, 10,000 volts

• Walking across tile, 5,000 volts

• Working at bench, 1,500 volts

• Removing ICs from a protective tube, 700 volts

A typical ESD event is characterized by an extremely rapid high voltage, low power spike. Anyone who has drawn a quarter-inch arc between a wall switch and their finger has experienced the power of an electrostatic discharge. This mild sting to the finger would prove deadly to an integrated circuit. Protecting a system from electrostatic discharge (ESD) is becoming a critical element in electronic product design.

Integrated circuits of 20 years ago were relatively large and slow, making it easier for designers to add the necessary ESD protection circuitry on the periphery of an IC. New devices can be 1/20^th the size of their predecessors, leaving little space available for ESD protection.

The problem will become even more pronounced as IC manufacturers head toward devices based on 32 nanometer and smaller geometries.

This IC size reduction directly impacts the dielectric materials chosen to fabricate chips. These materials have specific limits of resistance to increasing voltages, and to a large degree affect the spacing between conductors. If these limits are exceeded, the dielectric can be punctured, and voltage breakdown occurs. Circuit damage can take the form of circuit splatter/fusing and shorting, reduced breakdown voltage, increased leakage current, or complete circuit failure.

Another potential risk of an ESD event is electrical overstress (EOS). Applying a voltage that exceeds the device rating can result in latent damage that may not result in circuit failure until some future point in time. An ESD hit can weaken a device that may shorten its service life or make it more susceptible to subsequent voltage spikes in the future. This type of latent failure mechanism is extremely hard to detect or predict. Product burn-in and stress testing can often force the failure to occur, but the reduced yield adds cost. Recognition of both ESD and EOS requires proper analysis and identification of their respective root causes to eliminate potential failures.

Ensuring that static discharges cannot damage a device in an application is the responsibility of the I/O design engineer. A properly grounded case is often sufficient to shunt an ESD pulse around sensitive circuitry, but I/O ports can provide a direct path to internal components.

The most effective location for protection is at the closest point to where the voltage pulse enters the equipment.

An effective device should be capable of clamping an incoming voltage surge to a level within the operating range of sensitive components. Since ESD events are extremely fast pulses, a clamping transient voltage suppression device must be capable of responding quickly to voltage spikes of several thousand volts. They must also consume little valuable PCB and IC real estate, add minimal cost, and be applied using conventional automated placement equipment.

A variety of discrete components, including ceramic capacitors, zener diodes, metal oxide varistors, or transorbs, can be used to limit the voltage delivered to a circuit during an ESD event. Each has its own advantages and shortcomings. Ceramic capacitors are used to block high voltages, but can degrade high-speed signals. Zener diodes can be used, but may be limited in high-speed performance and clamping voltages. Multilayer varistors work well in high-voltage applications, but their high capacitance can degrade high-speed signals and may exhibit low DC breakdown. Many of these devices are available in surface-mount packages, making them easy to install very close to I/O connectors. Few of these solutions can respond to sub-nanosecond ESD events.

A more effective solution is to block high-voltage spikes at the external interface, insuring that voltage spikes never enter the equipment enclosure. The connector is typically grounded to the chassis of the equipment, providing an ideal low impedance ground path. Incorporating voltage suppression within the connector is also a more efficient utilization of PCB space. Several connector manufacturers who specialize in EMI-protected connectors also offer connectors that incorporate ESD clamping features.



Sabritec offers military circular connectors that attach a diode matrix to each contact position, shunting voltage spikes to ground.




Other common I/O interfaces, such as Universal Serial Bus (USB), can include EMI filtering as well as ESD protection. Suppliers, such as Spectrum Control, manufacture broad product lines of circular and rectangular connectors that incorporate EMI/ESD elements.

Filtered and/or ESD protected connectors are often incorporated late in the equipment design process. Engineers do all they can to anticipate challenges in achieving electromagnetic and electrostatic compatibility in a new product, but may not discover until the final qualification test process that they have a problem. At that point, a mad scramble to find a solution may make the replacement of a standard connector with a filtered equivalent the best solution. The cost of a signal conditioning connector becomes a secondary consideration to meeting the product release schedule.

Bishop & Associates Comments: 

1. In the universe of electronic connectors, filtered and ESD protected connectors occupy a relatively small niche.

2. Military and avionic applications have become the primary users of filtered connectors, as absolute reliability in difficult environments is the highest priority consideration beyond cost. Manufacturers such as Amphenol, Sabritec, Jerrik, and Airborn focus on these applications.

3. The introduction of emission standards in computing equipment stimulated interest in filtered connectors, but cost pressures forced designers in this segment to find alternative solutions.

4. Incorporating EMI filters and/or ESD protection insures the integrity of the shielded enclosure while saving valuable PCB space in the box.

5. EMI filter technology is considered a relatively mature science, with a few incremental improvements in packaging density, mechanical durability and voltage isolation.

6. Relatively new film-based ESD protective devices have improved the ability to protect sensitive devices from higher static voltages, while introducing little capacitance to the circuit.

7. Elastomeric gaskets featuring integrated capacitors that can be applied to the face of standard connectors offer the potential of quickly adding EMI filtering at the end of the design cycle.

8. The proliferation of portable devices, especially in the consumer medical equipment market, may offer new applications for the use of filtered and ESD protected connectors.

Robert Hult
Director of Product Technology, Bishop & Associates Inc.
Robert Hult has been in the connector industry for more than 36 years. Hult began his career as a sales engineer for Amphenol. He joined AMP in 1972 and served in several management positions through 1996. In 1997, Hult joined Foxconn as group marketing manager for Intel in Chandler, Arizona, U.S. Prior to joining Bishop & Associates, Hult was the regional application engineering manager for Tyco Electronics.

Hult graduated in 1968 from Bradley University with a bachelor of science degree in electronics technology and a minor in business.