Connectors between interface circuits and cables

The main role of the connector is to provide a good interconnection to the cable or interface circuit and to ensure good grounding. Selected a bad connector may be the effect of the front-stage filter circuit will be ruined, the connector to consider impedance matching, pin definition, ground contact characteristics. Connector selection should also consider ESD issues, if the connector is a plastic package, to ensure that the surface gap to the internal metal conductor between the air gap is sufficient. Sometimes installed on the circuit board interface filter circuit has a problem is after the filter circuit after the signal line in the chassis longer, easy to sense again on the interfering signals, the formation of a new common-mode current, resulting in cable radiation.

There are two sources of re-induced signals, one is the electromagnetic waves inside the chassis that will be induced on the cable, and the other is the interfering signals in front of the filtering circuit that will be directly coupled to the cable ports through parasitic capacitance. The solution to this problem is to minimize the length of wire exposed to the chassis after filtering. Connectors with filtering are ideal devices for solving this problem.

Filtered connectors have a low-pass filter on each pin that filters out the common mode current from the pins. These filtered connectors are often the same shape and size as regular connectors and can be used as a direct replacement for regular connectors. Since the connector is installed at the port where the cable enters the chassis, the filtered wire will not induct any more interfering signals. If a filtered connector is selected, it is important to ensure that the filtered connector has good grounding characteristics. Especially for filtered connectors with bypass capacitors (which most of them contain), since most of the interference in the signal line is bypassed to ground, there will be a large interference current flowing at the contact point between the filter and ground. If the contact impedance of the filter to ground is large, a large voltage drop across this impedance will result, leading to serious EMC problems. The following points are the basic principles for selecting connectors: (1) Interface signal connectors are recommended to use connectors with shielded housings, especially for high-frequency signal connectors. (2) the connector's metal shell should be maintained with the chassis to maintain good electrical continuity, for the ability to 360 ° around the connector, it must be 360 ° around the connection, and usually connected to the impedance to be less than 1 mΩ. (3) for the connector can not be 360 ° around the connection, it is recommended to use the shell surrounded by upwardly reeded connector, and the reed must be of sufficient size and performance (elasticity), in order to maintain a good electrical connection with the chassis. (4) filtering connector on the product EMC performance is often a great help, but its cost is relatively high, usually in the use of on-board filtering, cable shielding and other methods to solve the problem, it does not use filtering connectors. Filtered connectors are usually used in some special circumstances, such as strict military standard requirements, harsh industrial environments, small batch applications and the use of some special circumstances (such as structural size limitations, etc.). (5) The shielding of the shielded wire should be as far as possible with the connector shell to maintain a 360 ° connection. For the interface can not do this, there are usually other corresponding measures to ensure the EMC performance of the interface. (6) If the connector is mounted on a circuit board and is connected to the chassis via a ground on the board, take care to provide a clean ground for the connector that is separate from the signal ground on the board, connected only by a single point, and maintains a good lap to the chassis.

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