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Foil shield vs Braid shield

September 5th, 2015

If you use shielded differential cable to transmits signals, you have two choices to of shielding, one that uses an aluminum foil with a central wire passing inside it or a braided shield in which the braided shield wire is used as shield.

Braid and Foil in  Differential Cable

Braid Foil

The basic purpose of shielding is to prevent the external noise from entering the cable.

Differential Signal Integrity Implication

In differential signaling we aim at keeping the things symmetrical, so that if any noise source enters the cable system, it is symmetrically coupled to both the wires and are canceled at the receiver. In the foil shield, the central wire running through the cable system is typically closer to one of the the two wires. Also, since the central wire is closer, most of the return current flows through it. Since the central wire is closer to one of the two differential wires, it creates an imbalance and here is a source of noise. The common mode rejection ratio is better in case of braided shield.

If you are looking for a higher CMRR, use the braided shield in place of the Foil shield.

Shielding as well as balancing is needed to achieve higher CMRR. In a typical system design requiring 80 dB of CMM, shielding can give 30 dB of CMRR while balancing and give 50 dB of CMRR.

The shielding acts as a Faraday cage, preventing external electrical disturbances enter the cables itself. If you have any cabling inside your system you would better to use a shielded cable for EMI tests.

Unshielded cables could be used for transporting Power Signals. Care should be taken though to ensure that the noise coupled on the power rails do not enter the cable. Shielding a cable adds to the cost of the cable simply because additional material is required. Sheilded cable also makes them bulkier and require more space. These are less flexible than their non sheilded variation.

The foild shielding provide 100% EMI protection and is less expensive as the foil requires less "meat". It is also lighter in weight.

 

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