Data Acquisition

age at the receiving end. In the presence of noise, this affects the signal-to-noise (S/N)ratio and thus the quality of the signal received.As the frequency (or data transfer rate) increases, the other characteristics of the cable,such as capacitance and series inductance, become important. Inductance and capacitanceare factors that depend on the construction of the cable and on the type of insulationmaterial.The resistance, inductance and capacitance are distributed along the length of the cable.At high frequencies they combine to present the effects of a low pass filter. The simplifiedelectrical single-line diagram of a cable shows these electrical parametersdistributed along the length of the cable and can be seen in Figure 3.41. Note, however,that a more complex model would also need to include a minor conductance factor (theinverse of resistance) in parallel across the cable.Figure 3.41Main parameters of a cableTo derive the optimum performance from a cable, the correct type and size must beused. The following general rules apply to most applications:• Low data transfer rates: use low-frequency cables (for example, twistedpaircables)• High data transfer rates: use high-frequency cables (for example, coaxialcables, optical fiber; though there are some new types of twisted-pair cablesthat give very good high-frequency performance)• High noise environment: use shielded copper or optic fiber cables3.7.1 Twisted-pair cablesTwisted-pair cables are the most economical solution for data transmission (differentialcircuit). They allow for transmission rates of up to 100 Mbps on communication links ofup to 300 m (or even longer distances but with lower data transfer rates). Some new typesof twisted-pair cables (e.g., ‘Twistlan’) are suitable for up to 100 Mbps. Twisted-paircables can be STP (shielded twisted-pair) or UTP (unshielded twisted-pair).Twisted-pair cables are made from two identical insulated conductors that are twistedtogether along their length a specified number of times per meter, typically 40 twists permeter (12 twists per foot). The wires are twisted to reduce the effect of electromagneticand electrostatic induction. An earth screen, or shield, is often placed around them as wellto reduce the capacitance-induced noise, and an insulating PVC sheath usually providesmechanical protection. As the cross-sectional area of the conductor affects IR loss, heavierconductor sizes are recommended for long distances. The capacitance of a twistedpairis low at about 15 to 50 pF/m, allowing a reasonable bandwidth and an achievableslew rate.