A Practical Guide To Cable Selection - Texas …

1 A Practical Guide To Cable SelectionLiterature Number: SNLA164A Practical Guide To INTRODUCTIONThis application note provides an overview of the variousconsiderations necessary for selecting suitable copper multi-conductor or twisted pair cables for use with standard inter-face devices. It is important that a Cable is well matched tothe application; as well as, that the various Cable selectiontrade-offs are considered for a cost effective system types, constructions, and characteristics are coveredand then related to the various device TYPES OF CABLESThe two most basic Cable categories are flat and round (seeFigure 1andFigure 2).

2 Both flat and round cables are avail-able in multiconductor or twisted pair configurations andeach with or without shielding. Shielding of various types isalso available in both cases. Flat cables have carefully con-trolled conductor spacing making them suitable for mass ter-mination. Round cables are suited for long Cable runs orwhere flexibility and compactness are cables are available for basic single-ended, , unbalanced applications. Twisted pair cables are avail-able for differential, , balanced applications (Figure 3).

3 Note that a coaxial Cable , a single insulated conductor withan overall shield; is, in this context, a multiconductor cablewith only one conductor (the shield serving the dual purposeof signal return path and signal containment). In a similarsense, a two conductor multiconductor Cable , since it istwisted, is equivalent to a single twisted pair is a registered trademark of Elf is a registered trademark of Dupont de Nemours 1. Drawing of Flat Cable , Cross-SectionAN011937-2 FIGURE 2.

4 Drawing of Round Cable , Cross-SectionNational SemiconductorApplication Note 916 David HessJohn GoldieOctober 1993A Practical Guide to Cable SelectionAN-916 1998 National Semiconductor is common and preferable to use a twisted pair Cable forsingle-ended applications. Some higher speed or longer dis-tance single-ended applications provide a separate returnconductor for each data and timing circuit, helping to reducecrosstalk. Note that single-ended applications cannot fullyutilize the special capability of a twisted pair.

5 Multiconductorcable should not be used for differential applications wheretwisted pairs are is important to recognize that a twisted pair serves a funda-mental electrical purpose. A twisted pair maintains, along itslength, the balance necessary for and thus the commonmode rejection sought in differential applications. The de-gree of physical symmetry achieved in constructing a twistedpair of insulated conductors determines how well it is bal-anced electrically. The double helix configuration of the pairproduces symmetrical parasitics for each conductor.

6 Sym-metrical parasitics assure that induced noise signals areequal, or common , to both conductors. This is the primarymeans of reducing crosstalk between various circuits withina Cable . Flat cables constructed with twisted pairs are alsoavailable to achieve improved crosstalk of Flat CablesBy providing a means for mass termination, flat cables arerelatively inexpensive to terminate. Connectors are availablein configurations with insulation displacement contactsaligned for flat Cable termination.

7 The contacts are simulta-neously pressed through the insulation onto all of the con-ductors of a flat Cable . The Cable conductor to connectorcontact alignment is critical. The two industry standard con-ductor centerline spacings are and inches. Con-trolling this parameter is a primary concern in producing flatcable and somewhat limits the range of cables electricalcharacteristics of Round CablesRound Cable flexibility is not limited to a single plane, as inthe case of flat Cable .

8 For long Cable runs, especially in-stalled in conduit or raceway, flat Cable is impractical. Theflexibility of round cables is the result of having the individualelements, single conductors or twisted pairs, cabled ; thatis, they are laid at a pitch angle relative to the axis of thecable, forming a helix. The greater the pitch angle thegreater the degree of flexibility. Color coding is usually pro-vided as the means of identifying the individual conductorsaiding the process of individually terminating each conduc-tor.

9 A round Cable is simpler to manufacture with a can be reduced with thicker insulation walls,since there are no inherent conductor spacing than the case of simple, flat, strait, unshielded multi-conductor cables; round cables have less cross-sectionalarea for a given number of conductors. More cross-sectionalarea is required for a shield or jacket on a flat Cable CONSTRUCTIONO verall ConstructionConductorsStandard subminiature D-style connectors are designed toaccept conductor sizes ranging typically from 22 AWG downto 26 AWG.

10 Stranded tinned copper is normally provides a considerable improvement in flexibilityand protection from conductor breaks due to repeated flex-ing, , improved flex life. Tin coating on the strands im-proves environmental resistance by preventing corrosion ofthe conductor. The tin coating also makes the conductormore suitable for soldering. Standard connector pins aresized to accept the stranded conductor s increased diametercompared to solid conductor or non-tinned conductors are not recommended foruse with some connectors.