Interface filtering will be essential if the cable that will be connected to an interface is not screened. Even if it is, a modicum of filtering can reduce the need for high quality screening - for instance, if the screened connection has to use a pigtail construction. It can use a combination of

parallel capacitors to the interface ground or chassis, to provide a low impedance "bypass" function

series chokes or resistors, to provide a high impedance to the interference in either direction

parallel capacitors to circuit 0V, to protect inputs or reduce the HF bandwidth of outputs.

If I/O connections carry only low bandwidth signals it is possible to filter them using simple RC low-pass networks. This is not possible with high-speed data links, but you can attenuate common-mode currents entering or leaving the equipment without affecting the signal frequencies by using a discrete common-mode choke arrangement. Such units are available commercially or can be custom designed. If they are mounted on a PCB, some degradation in the high frequency response is to be expected because of stray capacitance feedthrough and the method of choke construction. C-M chokes are only effective when faced with low impedances and are therefore often implemented with low value parallel capacitors, which have an increasing effect at RF but do not significantly restrict circuit bandwidth.

Capacitors on the outside of series impedance (choke or resistors) must be decoupled to the clean I/O ground, not to the circuit 0V, which is often the prime carrier of interference. This defines a preferential route to ground for both incoming and outgoing interference. The clean ground may be (in order of preference) the screened case metalwork, a grounding plate for the I/O connectors or a designated ground plane area on the PCB. Three terminal capacitors are often a good choice for these components since the circuit configuration lends itself to their use.

Capacitors inboard of the CM choke may be useful for increased performance, especially if the circuit impedance is too high for the choke to be effective on its own. In this case the inboard capacitors should be returned to circuit 0V. The capacitor value should be set first by the required bandwidth of the wanted signal, related to the impedance of the signal circuit; even if this results in a few hundred pF or less, a small capacitor can still be helpful for RF immunity and emissions control in the region of hundreds of MHz.

A further limitation on the use of capacitors occurs in differential circuits, when the imbalance of capacitor values may become significant. Imbalance will worsen the high frequency common mode rejection of the circuit, leading to unwanted conversion of common mode signals to differential. Typical tolerances on capacitor values may be ±20% which can give unacceptable conversion. But the X2Y capacitor construction can provide a significant improvement in this respect.

Voltage rating

Using filter capacitors to chassis introduces the question of the voltage withstand rating required of them. Often, the maximum voltage between the circuit and the chassis metalwork, when the two are isolated, is undefined. Depending on application, it may reach only a few volts, or possibly several hundred. Two approaches exist if the maximum value is unknown:

use capacitors with a high breakdown voltage - 500V or even 1kV is possible in larger surface mount sizes, although 3-terminal parts tend to be limited to 100 or 200V;

if the peak voltage will be transient (e.g. from a lightning surge or EFT burst) then consider putting a transient voltage suppressor in parallel with the capacitor(s) for protection. Remember though, with respect to the EFT burst, that even if the specification voltage is e.g. 1kV, this comes from a source impedance of at least 50 ohms and each spike lasts only for a few tens of nanoseconds; so if the capacitor is, say 10nF it will only see around 100V (possibly a lot less, depending on other circuit and stray impedances) and can be rated accordingly.

Filtered connectors

Filtered connectors are designed to be interchangeable with standard multi-way connectors but each pin incorporates a RF filter. Various designs of filter may be used and a single connector may include more than one filter type. These types may consist of:

a shunt capacitor

a shunt capacitor and a series inductor

two shunt capacitors and a series inductor between them (a pi-configuration)

The capacitor values range from about 100 pF to 0.1 µF and the inductors a few µH. The effective range of filtering depends mainly on the capacitance value. For the lower values the range may be 100 MHz - 1 GHz whereas for higher values the range may extend down to 10 MHz or even 1 MHz. Typical insertion loss values as measured in a 50Ω test system are 2-30dB up to 100MHz, and 20-50dB from 100MHz to 1GHz.

The components, which are incorporated in filter pins, may be affected by temperature, current and/or voltage. The capacitors are usually constructed with high-permittivity ceramics, and at high temperatures, or with a voltage applied, the capacitance can be reduced significantly. The inductors are provided by passing the central conductor through a very thin ferrite tube, and if an appreciable current flows the permeability of the ferrite and the inductance value are both reduced. These effects on the components all result in a decrease in the insertion loss at frequencies in the lower part of the stop band.

The capacitors all return to the connector shell which must therefore be very well bonded to its chassis, otherwise a high-pass filter is formed between the individual pins, with potential degradation for both circuit operation and EMC performance.

Connector EMI Suppression Plates

Some manufacturers offer EMI suppression plates, as shown in the picture(Courtesy:Fair-rite). These plates can be positioned close to the connector, allowing signal pins/wires to pass through them, thus providing some degree of noise attenuation. It's akin to placing a cable ferrite core on a bundle of signal wires. One notable advantage is that it eliminates the need for an external core on the cable bundle, resulting in a smaller form factor. However, it's worth noting that the insertion loss of such plates is relatively low. Another valuable application is in dampening structural resonance, often caused when retrofitting a new IC chip to replace an older one in an existing product. Case study 2 in this article provides a noteworthy example.

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